vaadin-framework/server/src/main/java/com/vaadin/server/VaadinService.java

/*
 * Copyright 2000-2021 Vaadin Ltd.
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not
 * use this file except in compliance with the License. You may obtain a copy of
 * the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations under
 * the License.
 */

package com.vaadin.server;

import static java.nio.charset.StandardCharsets.UTF_8;

import java.io.BufferedWriter;
import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.io.Serializable;
import java.lang.reflect.Constructor;
import java.net.MalformedURLException;
import java.net.URL;
import java.nio.charset.StandardCharsets;
import java.security.MessageDigest;
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Locale;
import java.util.Map;
import java.util.ServiceLoader;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.logging.Level;
import java.util.logging.Logger;

import javax.portlet.Portlet;
import javax.portlet.PortletContext;
import javax.servlet.Servlet;
import javax.servlet.ServletContext;
import javax.servlet.http.HttpServletResponse;

import com.vaadin.annotations.PreserveOnRefresh;
import com.vaadin.server.VaadinSession.FutureAccess;
import com.vaadin.server.VaadinSession.State;
import com.vaadin.server.communication.AtmospherePushConnection;
import com.vaadin.server.communication.FileUploadHandler;
import com.vaadin.server.communication.HeartbeatHandler;
import com.vaadin.server.communication.PublishedFileHandler;
import com.vaadin.server.communication.SessionRequestHandler;
import com.vaadin.server.communication.UidlRequestHandler;
import com.vaadin.shared.ApplicationConstants;
import com.vaadin.shared.JsonConstants;
import com.vaadin.shared.Registration;
import com.vaadin.shared.ui.ui.UIConstants;
import com.vaadin.ui.UI;
import com.vaadin.util.CurrentInstance;

import elemental.json.Json;
import elemental.json.JsonException;
import elemental.json.JsonObject;
import elemental.json.impl.JsonUtil;

/**
 * Provide deployment specific settings that are required outside terminal
 * specific code.
 *
 * @author Vaadin Ltd.
 *
 * @since 7.0
 */
public abstract class VaadinService implements Serializable {
    /**
     * Attribute name for telling
     * {@link VaadinSession#valueUnbound(javax.servlet.http.HttpSessionBindingEvent)}
     * that it should not close a {@link VaadinSession} even though it gets
     * unbound. If a {@code VaadinSession} has an attribute with this name and
     * the attribute value is {@link Boolean#TRUE}, that session will not be
     * closed when it is unbound from the underlying session.
     */
    // Use the old name.reinitializing value for backwards compatibility
    static final String PRESERVE_UNBOUND_SESSION_ATTRIBUTE = VaadinService.class
            .getName() + ".reinitializing";

    /**
     * @deprecated As of 7.1.1, use {@link #PRESERVE_UNBOUND_SESSION_ATTRIBUTE}
     *             instead
     */
    @Deprecated
    static final String REINITIALIZING_SESSION_MARKER = PRESERVE_UNBOUND_SESSION_ATTRIBUTE;

    /**
     * @deprecated As of 7.0. Only supported for {@link LegacyApplication}.
     */
    @Deprecated
    public static final String URL_PARAMETER_RESTART_APPLICATION = "restartApplication";

    /**
     * @deprecated As of 7.0. Only supported for {@link LegacyApplication}.
     */
    @Deprecated
    public static final String URL_PARAMETER_CLOSE_APPLICATION = "closeApplication";

    private static final String REQUEST_START_TIME_ATTRIBUTE = "requestStartTime";

    /**
     * Should never be used directly, always use
     * {@link #getDeploymentConfiguration()}
     */
    private final DeploymentConfiguration deploymentConfiguration;

    /*
     * Can't use EventRouter for these listeners since it's not thread safe. One
     * option would be to use an EventRouter instance guarded with a lock, but
     * then we would needlessly hold a "global" lock while invoking potentially
     * slow listener implementations.
     */
    private final Set<ServiceDestroyListener> serviceDestroyListeners = Collections
            .newSetFromMap(new ConcurrentHashMap<>());

    private final List<SessionInitListener> sessionInitListeners = new CopyOnWriteArrayList<>();

    private final List<SessionDestroyListener> sessionDestroyListeners = new CopyOnWriteArrayList<>();

    private SystemMessagesProvider systemMessagesProvider = DefaultSystemMessagesProvider
            .get();

    private ClassLoader classLoader;

    private Iterable<RequestHandler> requestHandlers;
    private Iterable<DependencyFilter> dependencyFilters;
    private ConnectorIdGenerator connectorIdGenerator;

    private Boolean atmosphereAvailable = null;

    /**
     * Keeps track of whether a warning about missing push support has already
     * been logged. This is used to avoid spamming the log with the same message
     * every time a new UI is bootstrapped.
     */
    private boolean pushWarningEmitted = false;

    /**
     * Has {@link #init()} been run?
     */
    private boolean initialized = false;

    /**
     * Creates a new vaadin service based on a deployment configuration.
     *
     * @param deploymentConfiguration
     *            the deployment configuration for the service
     */
    public VaadinService(DeploymentConfiguration deploymentConfiguration) {
        this.deploymentConfiguration = deploymentConfiguration;

        final String classLoaderName = getDeploymentConfiguration()
                .getClassLoaderName();
        if (classLoaderName != null) {
            try {
                final Class<?> classLoaderClass = getClass().getClassLoader()
                        .loadClass(classLoaderName);
                final Constructor<?> c = classLoaderClass
                        .getConstructor(ClassLoader.class);
                setClassLoader((ClassLoader) c
                        .newInstance(getClass().getClassLoader()));
            } catch (final Exception e) {
                throw new RuntimeException(
                        "Could not find specified class loader: "
                                + classLoaderName,
                        e);
            }
        }

        if (getClassLoader() == null) {
            setDefaultClassLoader();
        }
    }

    /**
     * Creates a service. This method is for use by dependency injection
     * frameworks etc. and must be followed by a call to
     * {@link #setClassLoader(ClassLoader)} or {@link #setDefaultClassLoader()}
     * before use. Furthermore {@link #getDeploymentConfiguration()} should be
     * overridden (or otherwise intercepted) so it does not return
     * <code>null</code>.
     *
     * @since 8.2
     */
    protected VaadinService() {
        deploymentConfiguration = null;
    }

    /**
     * Initializes this service. The service should be initialized before it is
     * used.
     *
     * @since 7.1
     * @throws ServiceException
     *             if a problem occurs when creating the service
     */
    public void init() throws ServiceException {
        List<RequestHandler> handlers = createRequestHandlers();

        ServiceInitEvent event = new ServiceInitEvent(this);

        Iterator<VaadinServiceInitListener> initListeners = getServiceInitListeners();
        while (initListeners.hasNext()) {
            initListeners.next().serviceInit(event);
        }

        handlers.addAll(event.getAddedRequestHandlers());

        Collections.reverse(handlers);

        requestHandlers = Collections.unmodifiableCollection(handlers);

        dependencyFilters = Collections.unmodifiableCollection(
                initDependencyFilters(event.getAddedDependencyFilters()));

        connectorIdGenerator = initConnectorIdGenerator(
                event.getAddedConnectorIdGenerators());
        assert connectorIdGenerator != null;

        initialized = true;
    }

    /**
     * Gets all available service init listeners. A custom Vaadin service
     * implementation can override this method to discover init listeners in
     * some other way in addition to the default implementation that uses
     * {@link ServiceLoader}. This could for example be used to allow defining
     * an init listener as an OSGi service or as a Spring bean.
     *
     * @since 8.0
     *
     * @return an iterator of available service init listeners
     */
    protected Iterator<VaadinServiceInitListener> getServiceInitListeners() {
        ServiceLoader<VaadinServiceInitListener> loader = ServiceLoader
                .load(VaadinServiceInitListener.class, getClassLoader());
        return loader.iterator();
    }

    /**
     * Called during initialization to add the request handlers for the service.
     * Note that the returned list will be reversed so the last handler will be
     * called first. This enables overriding this method and using add on the
     * returned list to add a custom request handler which overrides any
     * predefined handler.
     *
     * @return The list of request handlers used by this service.
     * @throws ServiceException
     *             if a problem occurs when creating the request handlers
     */
    protected List<RequestHandler> createRequestHandlers()
            throws ServiceException {
        List<RequestHandler> handlers = new ArrayList<>();
        handlers.add(new SessionRequestHandler());
        handlers.add(new PublishedFileHandler());
        handlers.add(new HeartbeatHandler());
        handlers.add(new FileUploadHandler());
        handlers.add(new UidlRequestHandler());
        handlers.add(new UnsupportedBrowserHandler());
        handlers.add(new ConnectorResourceHandler());

        return handlers;
    }

    /**
     * Return the URL from where static files, e.g. the widgetset and the theme,
     * are served. In a standard configuration the VAADIN folder inside the
     * returned folder is what is used for widgetsets and themes.
     *
     * The returned folder is usually the same as the context path and
     * independent of e.g. the servlet mapping.
     *
     * @param request
     *            the request for which the location should be determined
     *
     * @return The location of static resources (should contain the VAADIN
     *         directory). Never ends with a slash (/).
     */
    public abstract String getStaticFileLocation(VaadinRequest request);

    /**
     * Gets the widgetset that is configured for this deployment, e.g. from a
     * parameter in web.xml.
     *
     * @param request
     *            the request for which a widgetset is required
     * @return the name of the widgetset
     */
    public abstract String getConfiguredWidgetset(VaadinRequest request);

    /**
     * Gets the theme that is configured for this deployment, e.g. from a portal
     * parameter or just some sensible default value.
     *
     * @param request
     *            the request for which a theme is required
     * @return the name of the theme
     */
    public abstract String getConfiguredTheme(VaadinRequest request);

    /**
     * Checks whether the UI will be rendered on its own in the browser or
     * whether it will be included into some other context. A standalone UI may
     * do things that might interfere with other parts of a page, e.g. changing
     * the page title and requesting focus upon loading.
     *
     * @param request
     *            the request for which the UI is loaded
     * @return a boolean indicating whether the UI should be standalone
     */
    public abstract boolean isStandalone(VaadinRequest request);

    /**
     * Gets the class loader to use for loading classes loaded by name, e.g.
     * custom UI classes. This is by default the class loader that was used to
     * load the Servlet or Portlet class to which this service belongs.
     *
     * @return the class loader to use, or <code>null</code>
     *
     * @see #setClassLoader(ClassLoader)
     */
    public ClassLoader getClassLoader() {
        return classLoader;
    }

    /**
     * Sets the class loader to use for loading classes loaded by name, e.g.
     * custom UI classes. Invokers of this method should be careful to not break
     * any existing class loader hierarchy, e.g. by ensuring that a class loader
     * set for this service delegates to the previously set class loader if the
     * class is not found.
     *
     * @param classLoader
     *            the new class loader to set, not <code>null</code>.
     *
     * @see #getClassLoader()
     */
    public void setClassLoader(ClassLoader classLoader) {
        if (classLoader == null) {
            throw new IllegalArgumentException(
                    "Can not set class loader to null");
        }
        this.classLoader = classLoader;
    }

    /**
     * Returns the MIME type of the specified file, or null if the MIME type is
     * not known. The MIME type is determined by the configuration of the
     * container, and may be specified in a deployment descriptor. Common MIME
     * types are "text/html" and "image/gif".
     *
     * @param resourceName
     *            a String specifying the name of a file
     * @return a String specifying the file's MIME type
     *
     * @see ServletContext#getMimeType(String)
     * @see PortletContext#getMimeType(String)
     */
    public abstract String getMimeType(String resourceName);

    /**
     * Gets the deployment configuration. Should be overridden (or otherwise
     * intercepted) if the no-arg constructor is used in order to prevent NPEs.
     *
     * @return the deployment configuration
     */
    public DeploymentConfiguration getDeploymentConfiguration() {
        return deploymentConfiguration;
    }

    /**
     * Sets the system messages provider to use for getting system messages to
     * display to users of this service.
     *
     * @see #getSystemMessagesProvider()
     *
     * @param systemMessagesProvider
     *            the system messages provider; <code>null</code> is not
     *            allowed.
     */
    public void setSystemMessagesProvider(
            SystemMessagesProvider systemMessagesProvider) {
        if (systemMessagesProvider == null) {
            throw new IllegalArgumentException(
                    "SystemMessagesProvider can not be null.");
        }
        this.systemMessagesProvider = systemMessagesProvider;
    }

    /**
     * Gets the system messages provider currently defined for this service.
     * <p>
     * By default, the {@link DefaultSystemMessagesProvider} which always
     * provides the built-in default {@link SystemMessages} is used.
     * </p>
     *
     * @see #setSystemMessagesProvider(SystemMessagesProvider)
     * @see SystemMessagesProvider
     * @see SystemMessages
     *
     * @return the system messages provider; not <code>null</code>
     */
    public SystemMessagesProvider getSystemMessagesProvider() {
        return systemMessagesProvider;
    }

    /**
     * Gets the system message to use for a specific locale. This method may
     * also be implemented to use information from current instances of various
     * objects, which means that this method might return different values for
     * the same locale under different circumstances.
     *
     * @param locale
     *            the desired locale for the system messages
     * @param request
     * @return the system messages to use
     */
    public SystemMessages getSystemMessages(Locale locale,
            VaadinRequest request) {
        SystemMessagesInfo systemMessagesInfo = new SystemMessagesInfo();
        systemMessagesInfo.setLocale(locale);
        systemMessagesInfo.setService(this);
        systemMessagesInfo.setRequest(request);
        return getSystemMessagesProvider()
                .getSystemMessages(systemMessagesInfo);
    }

    /**
     * Returns the context base directory.
     *
     * Typically an application is deployed in a such way that is has an
     * application directory. For web applications this directory is the root
     * directory of the web applications. In some cases applications might not
     * have an application directory (for example web applications running
     * inside a war).
     *
     * @return The application base directory or null if the application has no
     *         base directory.
     */
    public abstract File getBaseDirectory();

    /**
     * Adds a listener that gets notified when a new Vaadin service session is
     * initialized for this service.
     * <p>
     * Because of the way different service instances share the same session,
     * the listener is not necessarily notified immediately when the session is
     * created but only when the first request for that session is handled by
     * this service.
     *
     * @see #removeSessionInitListener(SessionInitListener)
     * @see SessionInitListener
     *
     * @param listener
     *            the Vaadin service session initialization listener
     * @return a registration object for removing the listener
     * @since 8.0
     */
    public Registration addSessionInitListener(SessionInitListener listener) {
        sessionInitListeners.add(listener);
        return () -> sessionInitListeners.remove(listener);
    }

    /**
     * Removes a Vaadin service session initialization listener from this
     * service.
     *
     * @see #addSessionInitListener(SessionInitListener)
     *
     * @param listener
     *            the Vaadin service session initialization listener to remove.
     * @deprecated use the {@link Registration} object returned by
     *             {@link #addSessionInitListener(SessionInitListener)} to
     *             remove the listener
     */
    @Deprecated
    public void removeSessionInitListener(SessionInitListener listener) {
        sessionInitListeners.remove(listener);
    }

    /**
     * Adds a listener that gets notified when a Vaadin service session that has
     * been initialized for this service is destroyed.
     * <p>
     * The session being destroyed is locked and its UIs have been removed when
     * the listeners are called.
     *
     * @see #addSessionInitListener(SessionInitListener)
     *
     * @param listener
     *            the vaadin service session destroy listener
     * @return a registration object for removing the listener
     * @since 8.0
     */
    public Registration addSessionDestroyListener(
            SessionDestroyListener listener) {
        sessionDestroyListeners.add(listener);
        return () -> sessionDestroyListeners.remove(listener);
    }

    /**
     * Handles destruction of the given session. Internally ensures proper
     * locking is done.
     *
     * @param vaadinSession
     *            The session to destroy
     */
    public void fireSessionDestroy(VaadinSession vaadinSession) {
        final VaadinSession session = vaadinSession;
        session.access(() -> {
            if (session.getState() == State.CLOSED) {
                return;
            }
            if (session.getState() == State.OPEN) {
                closeSession(session);
            }
            List<UI> uis = new ArrayList<>(session.getUIs());
            for (final UI ui : uis) {
                ui.accessSynchronously(() -> {
                    /*
                     * close() called here for consistency so that it is always
                     * called before a UI is removed. UI.isClosing() is thus
                     * always true in UI.detach() and associated detach
                     * listeners.
                     */
                    if (!ui.isClosing()) {
                        ui.close();
                    }
                    session.removeUI(ui);
                });
            }
            SessionDestroyEvent event = new SessionDestroyEvent(
                    VaadinService.this, session);
            for (SessionDestroyListener listener : sessionDestroyListeners) {
                try {
                    listener.sessionDestroy(event);
                } catch (Exception e) {
                    /*
                     * for now, use the session error handler; in the future,
                     * could have an API for using some other handler for
                     * session init and destroy listeners
                     */
                    session.getErrorHandler().error(new ErrorEvent(e));
                }
            }
            session.setState(State.CLOSED);
        });
    }

    /**
     * Removes a Vaadin service session destroy listener from this service.
     *
     * @see #addSessionDestroyListener(SessionDestroyListener)
     *
     * @param listener
     *            the vaadin service session destroy listener
     * @deprecated use the {@link Registration} object returned by
     *             {@link #addSessionDestroyListener(SessionDestroyListener)} to
     *             remove the listener
     */
    @Deprecated
    public void removeSessionDestroyListener(SessionDestroyListener listener) {
        sessionDestroyListeners.remove(listener);
    }

    /**
     * Attempts to find a Vaadin service session associated with this request.
     * <p>
     * Handles locking of the session internally to avoid creation of duplicate
     * sessions by two threads simultaneously.
     * </p>
     *
     * @param request
     *            the request to get a vaadin service session for.
     *
     * @see VaadinSession
     *
     * @return the vaadin service session for the request, or <code>null</code>
     *         if no session is found and this is a request for which a new
     *         session shouldn't be created.
     */
    public VaadinSession findVaadinSession(VaadinRequest request)
            throws ServiceException, SessionExpiredException {
        VaadinSession vaadinSession = findOrCreateVaadinSession(request);
        if (vaadinSession == null) {
            return null;
        }

        VaadinSession.setCurrent(vaadinSession);
        request.setAttribute(VaadinSession.class.getName(), vaadinSession);

        return vaadinSession;
    }

    /**
     * Associates the given lock with this service and the given wrapped
     * session. This method should not be called more than once when the lock is
     * initialized for the session.
     *
     * @see #getSessionLock(WrappedSession)
     * @param wrappedSession
     *            The wrapped session the lock is associated with
     * @param lock
     *            The lock object
     */
    protected void setSessionLock(WrappedSession wrappedSession, Lock lock) {
        if (wrappedSession == null) {
            throw new IllegalArgumentException(
                    "Can't set a lock for a null session");
        }
        Object currentSessionLock = wrappedSession
                .getAttribute(getLockAttributeName());
        assert (currentSessionLock == null
                || currentSessionLock == lock) : "Changing the lock for a session is not allowed";

        wrappedSession.setAttribute(getLockAttributeName(), lock);
    }

    /**
     * Returns the name used to store the lock in the HTTP session.
     *
     * @return The attribute name for the lock
     */
    protected String getLockAttributeName() {
        return getServiceName() + ".lock";
    }

    /**
     * Gets the lock instance used to lock the VaadinSession associated with the
     * given wrapped session.
     * <p>
     * This method uses the wrapped session instead of VaadinSession to be able
     * to lock even before the VaadinSession has been initialized.
     * </p>
     *
     * @param wrappedSession
     *            The wrapped session
     * @return A lock instance used for locking access to the wrapped session
     */
    protected Lock getSessionLock(WrappedSession wrappedSession) {
        Object lock = wrappedSession.getAttribute(getLockAttributeName());

        if (lock instanceof ReentrantLock) {
            return (ReentrantLock) lock;
        }

        if (lock == null) {
            return null;
        }

        throw new RuntimeException(
                "Something else than a ReentrantLock was stored in the "
                        + getLockAttributeName() + " in the session");
    }

    /**
     * Locks the given session for this service instance. Typically you want to
     * call {@link VaadinSession#lock()} instead of this method.
     * <p>
     * Note: The method and its signature has been changed to return lock 
     * instance in Vaadin 8.14.0. If you have overriden this method, you need
     * to update your implementation.
     * <p>
     * Note: Overriding this method is not recommended, for custom lock storage
     * strategy override {@link #getSessionLock(WrappedSession)} and
     * {@link #setSessionLock(WrappedSession,Lock)} instead.
     * 
     * @param wrappedSession
     *            The session to lock
     * @return Lock instance
     * 
     * @throws IllegalStateException
     *             if the session is invalidated before it can be locked
     */
    protected Lock lockSession(WrappedSession wrappedSession) {
        Lock lock = getSessionLock(wrappedSession);
        if (lock == null) {
            /*
             * No lock found in the session attribute. Ensure only one lock is
             * created and used by everybody by doing double checked locking.
             * Assumes there is a memory barrier for the attribute (i.e. that
             * the CPU flushes its caches and reads the value directly from main
             * memory).
             */
            synchronized (VaadinService.class) {
                lock = getSessionLock(wrappedSession);
                if (lock == null) {
                    lock = new ReentrantLock();
                    setSessionLock(wrappedSession, lock);
                }
            }
        }
        lock.lock();

        try {
            // Someone might have invalidated the session between fetching the
            // lock and acquiring it. Guard for this by calling a method that's
            // specified to throw IllegalStateException if invalidated
            // (#12282)
            wrappedSession.getAttribute(getLockAttributeName());
        } catch (IllegalStateException e) {
            lock.unlock();
            throw e;
        }
        return lock;
    }

    /**
     * Releases the lock for the given session for this service instance.
     * Typically you want to call {@link VaadinSession#unlock()} instead of this
     * method.
     * <p>
     * Note: The method and its signature has been changed to get lock instance
     * as parameter in Vaadin 8.14.0. If you have overriden this method, you need
     * to update your implementation.
     * <p>
     * Note: Overriding this method is not recommended, for custom lock storage 
     * strategy override {@link #getSessionLock(WrappedSession)} and
     * {@link #setSessionLock(WrappedSession,Lock)} instead.
     *
     * @param wrappedSession
     *            The session to unlock, used only with assert
     * @param lock
     *            Lock instance to unlock
     */
    protected void unlockSession(WrappedSession wrappedSession, Lock lock) {
        assert ((ReentrantLock) lock).isHeldByCurrentThread() : "Trying to unlock the session but it has not been locked by this thread";
        lock.unlock();
    }

    private VaadinSession findOrCreateVaadinSession(VaadinRequest request)
            throws SessionExpiredException, ServiceException {
        boolean requestCanCreateSession = requestCanCreateSession(request);
        WrappedSession wrappedSession = getWrappedSession(request,
                requestCanCreateSession);

        final Lock lock;
        try {
            lock = lockSession(wrappedSession);
        } catch (IllegalStateException e) {
            throw new SessionExpiredException();
        }

        try {
            return doFindOrCreateVaadinSession(request,
                    requestCanCreateSession);
        } finally {
            unlockSession(wrappedSession, lock);
        }

    }

    /**
     * Finds or creates a Vaadin session. Assumes necessary synchronization has
     * been done by the caller to ensure this is not called simultaneously by
     * several threads.
     *
     * @param request
     * @param requestCanCreateSession
     * @return
     * @throws SessionExpiredException
     * @throws ServiceException
     */
    private VaadinSession doFindOrCreateVaadinSession(VaadinRequest request,
            boolean requestCanCreateSession)
            throws SessionExpiredException, ServiceException {
        assert ((ReentrantLock) getSessionLock(request.getWrappedSession()))
                .isHeldByCurrentThread() : "Session has not been locked by this thread";

        /* Find an existing session for this request. */
        VaadinSession session = getExistingSession(request,
                requestCanCreateSession);

        if (session != null) {
            /*
             * There is an existing session. We can use this as long as the user
             * not specifically requested to close or restart it.
             */

            final boolean restartApplication = hasParameter(request,
                    URL_PARAMETER_RESTART_APPLICATION)
                    && !hasParameter(request,
                            BootstrapHandler.IGNORE_RESTART_PARAM);
            final boolean closeApplication = hasParameter(request,
                    URL_PARAMETER_CLOSE_APPLICATION);

            if (closeApplication) {
                closeSession(session, request.getWrappedSession(false));
                return null;
            } else if (restartApplication) {
                closeSession(session, request.getWrappedSession(false));
                return createAndRegisterSession(request);
            } else {
                return session;
            }
        }

        // No existing session was found

        if (requestCanCreateSession) {
            /*
             * If the request is such that it should create a new session if one
             * as not found, we do that.
             */
            return createAndRegisterSession(request);
        } else {
            /*
             * The session was not found and a new one should not be created.
             * Assume the session has expired.
             */
            throw new SessionExpiredException();
        }

    }

    private static boolean hasParameter(VaadinRequest request,
            String parameterName) {
        return request.getParameter(parameterName) != null;
    }

    /**
     * Creates and registers a new VaadinSession for this service. Assumes
     * proper locking has been taken care of by the caller.
     *
     *
     * @param request
     *            The request which triggered session creation.
     * @return A new VaadinSession instance
     * @throws ServiceException
     */
    private VaadinSession createAndRegisterSession(VaadinRequest request)
            throws ServiceException {
        assert ((ReentrantLock) getSessionLock(request.getWrappedSession()))
                .isHeldByCurrentThread() : "Session has not been locked by this thread";

        VaadinSession session = createVaadinSession(request);

        VaadinSession.setCurrent(session);

        storeSession(session, request.getWrappedSession());

        // Initial WebBrowser data comes from the request
        session.getBrowser().updateRequestDetails(request);

        // Initial locale comes from the request
        Locale locale = request.getLocale();
        session.setLocale(locale);
        session.setConfiguration(getDeploymentConfiguration());
        session.setCommunicationManager(
                new LegacyCommunicationManager(session));

        ServletPortletHelper.initDefaultUIProvider(session, this);
        onVaadinSessionStarted(request, session);

        return session;
    }

    /**
     * Get the base URL that should be used for sending requests back to this
     * service.
     * <p>
     * This is only used to support legacy cases.
     *
     * @param request
     * @return
     * @throws MalformedURLException
     *
     * @deprecated As of 7.0. Only used to support {@link LegacyApplication}.
     */
    @Deprecated
    protected URL getApplicationUrl(VaadinRequest request)
            throws MalformedURLException {
        return null;
    }

    /**
     * Creates a new Vaadin session for this service and request.
     *
     * @param request
     *            The request for which to create a VaadinSession
     * @return A new VaadinSession
     * @throws ServiceException
     *
     */
    protected VaadinSession createVaadinSession(VaadinRequest request)
            throws ServiceException {
        return new VaadinSession(this);
    }

    private void onVaadinSessionStarted(VaadinRequest request,
            VaadinSession session) throws ServiceException {
        SessionInitEvent event = new SessionInitEvent(this, session, request);
        for (SessionInitListener listener : sessionInitListeners) {
            try {
                listener.sessionInit(event);
            } catch (Exception e) {
                /*
                 * for now, use the session error handler; in the future, could
                 * have an API for using some other handler for session init and
                 * destroy listeners
                 */
                session.getErrorHandler().error(new ErrorEvent(e));
            }
        }

        ServletPortletHelper.checkUiProviders(session, this);
    }

    private void closeSession(VaadinSession vaadinSession,
            WrappedSession session) {
        if (vaadinSession == null) {
            return;
        }

        if (session != null) {
            removeSession(session);
        }
    }

    protected VaadinSession getExistingSession(VaadinRequest request,
            boolean allowSessionCreation) throws SessionExpiredException {

        final WrappedSession session = getWrappedSession(request,
                allowSessionCreation);

        VaadinSession vaadinSession = loadSession(session);

        if (vaadinSession == null) {
            return null;
        }

        return vaadinSession;
    }

    /**
     * Retrieves the wrapped session for the request.
     *
     * @param request
     *            The request for which to retrieve a session
     * @param requestCanCreateSession
     *            true to create a new session if one currently does not exist
     * @return The retrieved (or created) wrapped session
     * @throws SessionExpiredException
     *             If the request is not associated to a session and new session
     *             creation is not allowed
     */
    private WrappedSession getWrappedSession(VaadinRequest request,
            boolean requestCanCreateSession) throws SessionExpiredException {
        final WrappedSession session = request
                .getWrappedSession(requestCanCreateSession);
        if (session == null) {
            throw new SessionExpiredException();
        }
        return session;
    }

    /**
     * Checks whether it's valid to create a new service session as a result of
     * the given request.
     *
     * @param request
     *            the request
     * @return <code>true</code> if it's valid to create a new service session
     *         for the request; else <code>false</code>
     */
    protected abstract boolean requestCanCreateSession(VaadinRequest request);

    /**
     * Gets the currently used Vaadin service. The current service is
     * automatically defined when processing requests related to the service
     * (see {@link ThreadLocal}) and in {@link VaadinSession#access(Runnable)}
     * and {@link UI#access(Runnable)}. In other cases, (e.g. from background
     * threads, the current service is not automatically defined.
     *
     * @return the current Vaadin service instance if available, otherwise
     *         <code>null</code>
     *
     * @see #setCurrentInstances(VaadinRequest, VaadinResponse)
     */
    public static VaadinService getCurrent() {
        return CurrentInstance.get(VaadinService.class);
    }

    /**
     * Sets the this Vaadin service as the current service and also sets the
     * current Vaadin request and Vaadin response. This method is used by the
     * framework to set the current instances when a request related to the
     * service is processed and they are cleared when the request has been
     * processed.
     * <p>
     * The application developer can also use this method to define the current
     * instances outside the normal request handling, e.g. when initiating
     * custom background threads.
     * </p>
     *
     * @param request
     *            the Vaadin request to set as the current request, or
     *            <code>null</code> if no request should be set.
     * @param response
     *            the Vaadin response to set as the current response, or
     *            <code>null</code> if no response should be set.
     *
     * @see #getCurrent()
     * @see #getCurrentRequest()
     * @see #getCurrentResponse()
     */
    public void setCurrentInstances(VaadinRequest request,
            VaadinResponse response) {
        setCurrent(this);
        CurrentInstance.set(VaadinRequest.class, request);
        CurrentInstance.set(VaadinResponse.class, response);
    }

    /**
     * Sets the given Vaadin service as the current service.
     *
     * @param service
     */
    public static void setCurrent(VaadinService service) {
        CurrentInstance.set(VaadinService.class, service);
    }

    /**
     * Gets the currently processed Vaadin request. The current request is
     * automatically defined when the request is started. The current request
     * can not be used in e.g. background threads because of the way server
     * implementations reuse request instances.
     *
     * @return the current Vaadin request instance if available, otherwise
     *         <code>null</code>
     *
     * @see #setCurrentInstances(VaadinRequest, VaadinResponse)
     */
    public static VaadinRequest getCurrentRequest() {
        return VaadinRequest.getCurrent();
    }

    /**
     * Gets the currently processed Vaadin response. The current response is
     * automatically defined when the request is started. The current response
     * can not be used in e.g. background threads because of the way server
     * implementations reuse response instances.
     *
     * @return the current Vaadin response instance if available, otherwise
     *         <code>null</code>
     *
     * @see #setCurrentInstances(VaadinRequest, VaadinResponse)
     */
    public static VaadinResponse getCurrentResponse() {
        return VaadinResponse.getCurrent();
    }

    /**
     * Gets a unique name for this service. The name should be unique among
     * different services of the same type but the same for corresponding
     * instances running in different JVMs in a cluster. This is typically based
     * on e.g. the configured servlet's or portlet's name.
     *
     * @return the unique name of this service instance.
     */
    public abstract String getServiceName();

    /**
     * Finds the {@link UI} that belongs to the provided request. This is
     * generally only supported for UIDL requests as other request types are not
     * related to any particular UI or have the UI information encoded in a
     * non-standard way. The returned UI is also set as the current UI (
     * {@link UI#setCurrent(UI)}).
     *
     * @param request
     *            the request for which a UI is desired
     * @return the UI belonging to the request or null if no UI is found
     *
     */
    public UI findUI(VaadinRequest request) {
        // getForSession asserts that the lock is held
        VaadinSession session = loadSession(request.getWrappedSession());

        // Get UI id from the request
        String uiIdString = request.getParameter(UIConstants.UI_ID_PARAMETER);
        UI ui = null;
        if (uiIdString != null && session != null) {
            int uiId = Integer.parseInt(uiIdString);
            ui = session.getUIById(uiId);
        }

        UI.setCurrent(ui);
        return ui;
    }

    /**
     * Check if the given UI should be associated with the
     * <code>window.name</code> so that it can be re-used if the browser window
     * is reloaded. This is typically determined by the UI provider which
     * typically checks the @{@link PreserveOnRefresh} annotation but UI
     * providers and ultimately VaadinService implementations may choose to
     * override the defaults.
     *
     * @param provider
     *            the UI provider responsible for the UI
     * @param event
     *            the UI create event with details about the UI
     *
     * @return <code>true</code> if the UI should be preserved on refresh;
     *         <code>false</code> if a new UI instance should be initialized on
     *         refreshed.
     */
    public boolean preserveUIOnRefresh(UIProvider provider,
            UICreateEvent event) {
        return provider.isPreservedOnRefresh(event);
    }

    /**
     * Discards the current session and creates a new session with the same
     * contents. The purpose of this is to introduce a new session key in order
     * to avoid session fixation attacks.
     * <p>
     * Please note that this method makes certain assumptions about how data is
     * stored in the underlying session and may thus not be compatible with some
     * environments.
     *
     * @param request
     *            The Vaadin request for which the session should be
     *            reinitialized
     */
    public static void reinitializeSession(VaadinRequest request) {
        WrappedSession oldSession = request.getWrappedSession();

        // Stores all attributes (security key, reference to this context
        // instance) so they can be added to the new session
        Set<String> attributeNames = oldSession.getAttributeNames();
        Map<String, Object> attrs = new HashMap<>(attributeNames.size() * 2);
        for (String name : attributeNames) {
            Object value = oldSession.getAttribute(name);
            if (value instanceof VaadinSession) {
                // set flag to avoid cleanup
                VaadinSession serviceSession = (VaadinSession) value;
                serviceSession.setAttribute(PRESERVE_UNBOUND_SESSION_ATTRIBUTE,
                        Boolean.TRUE);
            }
            attrs.put(name, value);
        }

        // Invalidate the current session
        oldSession.invalidate();

        // Create a new session
        WrappedSession newSession = request.getWrappedSession();

        // Restores all attributes (security key, reference to this context
        // instance)
        for (String name : attrs.keySet()) {
            Object value = attrs.get(name);
            newSession.setAttribute(name, value);

            // Ensure VaadinServiceSession knows where it's stored
            if (value instanceof VaadinSession) {
                VaadinSession serviceSession = (VaadinSession) value;
                VaadinService service = serviceSession.getService();
                // Use the same lock instance in the new session
                service.setSessionLock(newSession,
                        serviceSession.getLockInstance());

                service.storeSession(serviceSession, newSession);
                serviceSession.setAttribute(PRESERVE_UNBOUND_SESSION_ATTRIBUTE,
                        null);
            }
        }

    }

    /**
     *
     * Finds the given theme resource from the web content folder or using the
     * class loader and returns a stream for it.
     *
     * @param ui
     *            The ui for which to find the resource
     * @param themeName
     *            The name of the theme
     * @param resource
     *            The name of the resource, e.g. "layouts/mycustomlayout.html"
     * @return A stream for the resource or null if the resource was not found
     * @throws IOException
     *             if a problem occurred while finding or opening the resource
     */
    public abstract InputStream getThemeResourceAsStream(UI ui,
            String themeName, String resource) throws IOException;

    /**
     * Creates and returns a unique ID for the DIV where the UI is to be
     * rendered.
     *
     * @param session
     *            The service session to which the bootstrapped UI will belong.
     * @param request
     *            The request for which a div id is needed
     * @param uiClass
     *            The class of the UI that will be bootstrapped
     *
     * @return the id to use in the DOM
     */
    public abstract String getMainDivId(VaadinSession session,
            VaadinRequest request, Class<? extends UI> uiClass);

    /**
     * Sets the given session to be closed and all its UI state to be discarded
     * at the end of the current request, or at the end of the next request if
     * there is no ongoing one.
     * <p>
     * After the session has been discarded, any UIs that have been left open
     * will give a Session Expired error and a new session will be created for
     * serving new UIs.
     * <p>
     * To avoid causing out of sync errors, you should typically redirect to
     * some other page using {@link Page#setLocation(String)} to make the
     * browser unload the invalidated UI.
     *
     * @see SystemMessages#getSessionExpiredCaption()
     *
     * @param session
     *            the session to close
     */
    public void closeSession(VaadinSession session) {
        session.close();
    }

    /**
     * Closes inactive UIs in the given session, removes closed UIs from the
     * session, and closes the session if it is itself inactive. This operation
     * should not be performed without first acquiring the session lock. By
     * default called at the end of each request, after sending the response.
     *
     * @param session
     *            the session to clean up
     *
     * @since 8.10
     */
    public void cleanupSession(VaadinSession session) {
        if (isSessionActive(session)) {
            closeInactiveUIs(session);
            removeClosedUIs(session);
        } else {
            if (session.getState() == State.OPEN) {
                closeSession(session);
                if (session.getSession() != null) {
                    getLogger().log(Level.FINE, "Closing inactive session {0}",
                            session.getSession().getId());
                }
            }
            if (session.getSession() != null) {
                /*
                 * If the VaadinSession has no WrappedSession then it has
                 * already been removed from the HttpSession and we do not have
                 * to do it again
                 */
                removeSession(session.getSession());
            }

            /*
             * The session was destroyed during this request and therefore no
             * destroy event has yet been sent
             */
            fireSessionDestroy(session);
        }
    }

    /**
     * Removes those UIs from the given session for which {@link UI#isClosing()
     * isClosing} yields true.
     *
     * @param session
     */
    private void removeClosedUIs(final VaadinSession session) {
        List<UI> uis = new ArrayList<>(session.getUIs());
        for (final UI ui : uis) {
            if (ui.isClosing()) {
                ui.accessSynchronously(() -> {
                    getLogger().log(Level.FINER, "Removing closed UI {0}",
                            ui.getUIId());
                    session.removeUI(ui);
                });
            }
        }
    }

    /**
     * Closes those UIs in the given session for which {@link #isUIActive}
     * yields false.
     *
     * @since 7.0.0
     */
    private void closeInactiveUIs(VaadinSession session) {
        final String sessionId = session.getSession().getId();
        for (final UI ui : session.getUIs()) {
            if (!isUIActive(ui) && !ui.isClosing()) {
                ui.accessSynchronously(() -> {
                    getLogger().log(Level.FINE,
                            "Closing inactive UI #{0} in session {1}",
                            new Object[] { ui.getUIId(), sessionId });
                    ui.close();
                });
            }
        }
    }

    /**
     * Returns the number of seconds that must pass without a valid heartbeat or
     * UIDL request being received from a UI before that UI is removed from its
     * session. This is a lower bound; it might take longer to close an inactive
     * UI. Returns a negative number if heartbeat is disabled and timeout never
     * occurs.
     *
     * @see DeploymentConfiguration#getHeartbeatInterval()
     *
     * @since 7.0.0
     *
     * @return The heartbeat timeout in seconds or a negative number if timeout
     *         never occurs.
     */
    private int getHeartbeatTimeout() {
        // Permit three missed heartbeats before closing the UI
        return (int) (getDeploymentConfiguration().getHeartbeatInterval()
                * (3.1));
    }

    /**
     * Returns the number of seconds that must pass without a valid UIDL request
     * being received for the given session before the session is closed, even
     * though heartbeat requests are received. This is a lower bound; it might
     * take longer to close an inactive session.
     * <p>
     * Returns a negative number if there is no timeout. In this case heartbeat
     * requests suffice to keep the session alive, but it will still eventually
     * expire in the regular manner if there are no requests at all (see
     * {@link WrappedSession#getMaxInactiveInterval()}).
     *
     * @see DeploymentConfiguration#isCloseIdleSessions()
     * @see #getHeartbeatTimeout()
     *
     * @since 7.0.0
     *
     * @return The UIDL request timeout in seconds, or a negative number if
     *         timeout never occurs.
     */
    private int getUidlRequestTimeout(VaadinSession session) {
        return getDeploymentConfiguration().isCloseIdleSessions()
                ? session.getSession().getMaxInactiveInterval()
                : -1;
    }

    /**
     * Returns whether the given UI is active (the client-side actively
     * communicates with the server) or whether it can be removed from the
     * session and eventually collected.
     * <p>
     * A UI is active if and only if its {@link UI#isClosing() isClosing}
     * returns false and {@link #getHeartbeatTimeout() getHeartbeatTimeout} is
     * negative or has not yet expired.
     *
     * @since 8.1
     *
     * @param ui
     *            The UI whose status to check
     *
     * @return true if the UI is active, false if it could be removed.
     */
    public boolean isUIActive(UI ui) {
        if (ui.isClosing()) {
            return false;
        }

        // Check for long running tasks
        Lock lockInstance = ui.getSession().getLockInstance();
        if (lockInstance instanceof ReentrantLock) {
            if (((ReentrantLock) lockInstance).hasQueuedThreads()) {
                /*
                 * Someone is trying to access the session. Leaving all UIs
                 * alive for now. A possible kill decision will be made at a
                 * later time when the session access has ended.
                 */
                return true;
            }
        }

        // Check timeout
        long now = System.currentTimeMillis();
        int timeout = 1000 * getHeartbeatTimeout();
        return timeout < 0 || now - ui.getLastHeartbeatTimestamp() < timeout;
    }

    /**
     * Returns whether the given session is active or whether it can be closed.
     * <p>
     * A session is active if and only if its {@link VaadinSession#getState()}
     * returns {@link State#OPEN} and
     * {@link #getUidlRequestTimeout(VaadinSession) getUidlRequestTimeout} is
     * negative or has not yet expired.
     *
     * @param session
     *            The session whose status to check
     *
     * @return true if the session is active, false if it could be closed.
     */
    private boolean isSessionActive(VaadinSession session) {
        if (session.getState() != State.OPEN || session.getSession() == null) {
            return false;
        } else {
            long now = System.currentTimeMillis();
            int timeout = 1000 * getUidlRequestTimeout(session);
            return timeout < 0
                    || now - session.getLastRequestTimestamp() < timeout;
        }
    }

    private static final Logger getLogger() {
        return Logger.getLogger(VaadinService.class.getName());
    }

    /**
     * Called before the framework starts handling a request.
     *
     * @param request
     *            The request
     * @param response
     *            The response
     */
    public void requestStart(VaadinRequest request, VaadinResponse response) {
        if (!initialized) {
            throw new IllegalStateException(
                    "Can not process requests before init() has been called");
        }
        setCurrentInstances(request, response);
        request.setAttribute(REQUEST_START_TIME_ATTRIBUTE, System.nanoTime());
    }

    /**
     * Called after the framework has handled a request and the response has
     * been written.
     *
     * @param request
     *            The request object
     * @param response
     *            The response object
     * @param session
     *            The session which was used during the request or null if the
     *            request did not use a session
     */
    public void requestEnd(VaadinRequest request, VaadinResponse response,
            VaadinSession session) {
        if (session != null) {
            assert VaadinSession.getCurrent() == session;
            session.lock();
            try {
                cleanupSession(session);
                final long duration = (System.nanoTime() - (Long) request
                        .getAttribute(REQUEST_START_TIME_ATTRIBUTE)) / 1000000;
                session.setLastRequestDuration(duration);
            } finally {
                session.unlock();
            }
        }
        CurrentInstance.clearAll();
    }

    /**
     * Returns the request handlers that are registered with this service. The
     * iteration order of the returned collection is the same as the order in
     * which the request handlers will be invoked when a request is handled.
     *
     * @return a collection of request handlers in the order they are invoked
     *
     * @see #createRequestHandlers()
     *
     * @since 7.1
     */
    public Iterable<RequestHandler> getRequestHandlers() {
        return requestHandlers;
    }

    /**
     * Updates the list of resource dependency filters to use for the
     * application.
     * <p>
     * The filters can freely update the dependencies in any way they see fit
     * (bundle, rewrite, merge).
     * <p>
     * The framework collects filters from the {@link SessionInitEvent} where
     * session init listeners can add them. This method is called with the
     * combined list to optionally modify it, and the result is then stored by
     * the caller as the final list to use.
     * <p>
     * The filters are called in the order the session init listeners are
     * called, which is undefined. If you need a specific order, you can
     * override this method and alter the order.
     *
     * @since 8.1
     * @param sessionInitFilters
     *            a list of dependency filters collected from the session init
     *            event
     * @return the list of dependency filters to use for filtering resources,
     *         not null
     * @throws ServiceException
     *             if something went wrong while determining the filters
     *
     */
    protected List<DependencyFilter> initDependencyFilters(
            List<DependencyFilter> sessionInitFilters) throws ServiceException {
        assert sessionInitFilters != null;

        return sessionInitFilters;
    }

    /**
     * Determines the connector id generator to use for the application.
     * <p>
     * The connector id generator creates a unique id for each connector
     * attached to a UI.
     * <p>
     * The framework collects generators from the {@link SessionInitEvent} where
     * session init listeners can add them. This method is called with the
     * combined list to determine one generator to use.
     * <p>
     * If the list is empty, a default implementation based on
     * {@link VaadinSession#getNextConnectorId()} is used. If the list contains
     * one item, it is used. If there are multiple generators in the list, an
     * exception is thrown.
     *
     * @since 8.1
     * @param addedConnectorIdGenerators
     *            a list of connector id generators collected from the session
     *            init event, not <code>null</code>
     * @return the connector id generator to use, not <code>null</code>
     *
     * @throws ServiceException
     *             if something went wrong while determining the filters, e.g.
     *             if there are multiple implementations to choose from
     *
     */
    protected ConnectorIdGenerator initConnectorIdGenerator(
            List<ConnectorIdGenerator> addedConnectorIdGenerators)
            throws ServiceException {
        assert addedConnectorIdGenerators != null;

        switch (addedConnectorIdGenerators.size()) {
        case 0:
            return ConnectorIdGenerator::generateDefaultConnectorId;
        case 1:
            return addedConnectorIdGenerators.get(0);
        default:
            throw new ServiceException(
                    "Cannot start application since there are multiple connector id generators. Remove redundant implementations from the classpath or override VaadinService.initConenctorIdGenerator to explicitly select one to use. The found generators are: "
                            + addedConnectorIdGenerators);
        }
    }

    /**
     * Gets the filters which all resource dependencies are passed through
     * before being sent to the client for loading.
     *
     * @see #initDependencyFilters(List)
     *
     * @since 8.1
     * @return the dependency filters to pass resources dependencies through
     *         before loading
     */
    public Iterable<DependencyFilter> getDependencyFilters() {
        if (dependencyFilters == null) {
            return Collections.emptyList();
        }
        return dependencyFilters;
    }

    /**
     * Handles the incoming request and writes the response into the response
     * object. Uses {@link #getRequestHandlers()} for handling the request.
     * <p>
     * If a session expiration is detected during request handling then each
     * {@link RequestHandler request handler} has an opportunity to handle the
     * expiration event if it implements {@link SessionExpiredHandler}. If no
     * request handler handles session expiration a default expiration message
     * will be written.
     * </p>
     *
     * @param request
     *            The incoming request
     * @param response
     *            The outgoing response
     * @throws ServiceException
     *             Any exception that occurs during response handling will be
     *             wrapped in a ServiceException
     */
    public void handleRequest(VaadinRequest request, VaadinResponse response)
            throws ServiceException {
        requestStart(request, response);

        VaadinSession vaadinSession = null;
        try {
            // Find out the service session this request is related to
            vaadinSession = findVaadinSession(request);
            if (vaadinSession == null) {
                return;
            }

            for (RequestHandler handler : getRequestHandlers()) {
                if (handler.handleRequest(vaadinSession, request, response)) {
                    return;
                }
            }

            // Request not handled by any RequestHandler
            response.sendError(HttpServletResponse.SC_NOT_FOUND,
                    "Request was not handled by any registered handler.");

        } catch (final SessionExpiredException e) {
            handleSessionExpired(request, response);
        } catch (final Throwable e) {
            handleExceptionDuringRequest(request, response, vaadinSession, e);
        } finally {
            requestEnd(request, response, vaadinSession);
        }
    }

    private void handleExceptionDuringRequest(VaadinRequest request,
            VaadinResponse response, VaadinSession vaadinSession, Throwable t)
            throws ServiceException {
        if (vaadinSession != null) {
            vaadinSession.lock();
        }
        try {
            ErrorHandler errorHandler = ErrorEvent
                    .findErrorHandler(vaadinSession);

            if (errorHandler != null) {
                errorHandler.error(new ErrorEvent(t));
            }

            // if this was an UIDL request, send UIDL back to the client
            if (ServletPortletHelper.isUIDLRequest(request)) {
                SystemMessages ci = getSystemMessages(ServletPortletHelper
                        .findLocale(null, vaadinSession, request), request);
                try {
                    writeUncachedStringResponse(response,
                            JsonConstants.JSON_CONTENT_TYPE,
                            createCriticalNotificationJSON(
                                    ci.getInternalErrorCaption(),
                                    ci.getInternalErrorMessage(), null,
                                    ci.getInternalErrorURL()));
                } catch (IOException e) {
                    // An exception occurred while writing the response. Log
                    // it and continue handling only the original error.
                    getLogger().log(Level.WARNING,
                            "Failed to write critical notification response to the client",
                            e);
                }
            } else {
                // Re-throw other exceptions
                throw new ServiceException(t);
            }
        } finally {
            if (vaadinSession != null) {
                vaadinSession.unlock();
            }
        }

    }

    /**
     * Writes the given string as a response using the given content type.
     *
     * @param response
     *            The response reference
     * @param contentType
     *            The content type of the response
     * @param responseString
     *            The actual response
     * @throws IOException
     *             If an error occurred while writing the response
     */
    public void writeStringResponse(VaadinResponse response, String contentType,
            String responseString) throws IOException {

        response.setContentType(contentType);

        final OutputStream out = response.getOutputStream();
        try (PrintWriter outWriter = new PrintWriter(
                new BufferedWriter(new OutputStreamWriter(out, UTF_8)))) {
            outWriter.print(responseString);
        }
    }

    /**
     * Writes the given string as a response with headers to prevent caching and
     * using the given content type.
     *
     * @param response
     *            The response reference
     * @param contentType
     *            The content type of the response
     * @param responseString
     *            The actual response
     * @throws IOException
     *             If an error occurred while writing the response
     * @since 8.3.2
     */
    public void writeUncachedStringResponse(VaadinResponse response,
            String contentType, String responseString) throws IOException {
        // Response might contain sensitive information, so prevent all forms of
        // caching
        response.setNoCacheHeaders();

        writeStringResponse(response, contentType, responseString);
    }

    /**
     * Called when the session has expired and the request handling is therefore
     * aborted.
     *
     * @param request
     *            The request
     * @param response
     *            The response
     * @throws ServiceException
     *             Thrown if there was any problem handling the expiration of
     *             the session
     */
    protected void handleSessionExpired(VaadinRequest request,
            VaadinResponse response) throws ServiceException {
        for (RequestHandler handler : getRequestHandlers()) {
            if (handler instanceof SessionExpiredHandler) {
                try {
                    if (((SessionExpiredHandler) handler)
                            .handleSessionExpired(request, response)) {
                        return;
                    }
                } catch (IOException e) {
                    throw new ServiceException(
                            "Handling of session expired failed", e);
                }
            }
        }

        // No request handlers handled the request. Write a normal HTTP response

        try {
            // If there is a URL, try to redirect there
            SystemMessages systemMessages = getSystemMessages(
                    ServletPortletHelper.findLocale(null, null, request),
                    request);
            String sessionExpiredURL = systemMessages.getSessionExpiredURL();
            if (sessionExpiredURL != null
                    && (response instanceof VaadinServletResponse)) {
                ((VaadinServletResponse) response)
                        .sendRedirect(sessionExpiredURL);
            } else {
                /*
                 * Session expired as a result of a standard http request and we
                 * have nowhere to redirect. Reloading would likely cause an
                 * endless loop. This can at least happen if refreshing a
                 * resource when the session has expired.
                 */

                // Ensure that the browser does not cache expired responses.
                // iOS 6 Safari requires this (#3226)
                response.setHeader("Cache-Control", "no-cache");
                // If Content-Type is not set, browsers assume text/html and may
                // complain about the empty response body (#4167)
                response.setHeader("Content-Type", "text/plain");

                response.sendError(HttpServletResponse.SC_FORBIDDEN,
                        "Session expired");
            }
        } catch (IOException e) {
            throw new ServiceException(e);
        }
    }

    /**
     * Creates a JSON message which, when sent to client as-is, will cause a
     * critical error to be shown with the given details.
     *
     * @param caption
     *            The caption of the error or null to omit
     * @param message
     *            The error message or null to omit
     * @param details
     *            Additional error details or null to omit
     * @param url
     *            A url to redirect to. If no other details are given then the
     *            user will be immediately redirected to this URL. Otherwise the
     *            message will be shown and the browser will redirect to the
     *            given URL only after the user acknowledges the message. If
     *            null then the browser will refresh the current page.
     * @return A JSON string to be sent to the client
     */
    public static String createCriticalNotificationJSON(String caption,
            String message, String details, String url) {
        String returnString = "";
        try {
            JsonObject appError = Json.createObject();
            putValueOrJsonNull(appError, "caption", caption);
            putValueOrJsonNull(appError, "url", url);
            putValueOrJsonNull(appError, "message", message);
            putValueOrJsonNull(appError, "details", details);

            JsonObject meta = Json.createObject();
            meta.put("appError", appError);

            JsonObject json = Json.createObject();
            json.put("changes", Json.createObject());
            json.put("resources", Json.createObject());
            json.put("locales", Json.createObject());
            json.put("meta", meta);
            json.put(ApplicationConstants.SERVER_SYNC_ID, -1);
            returnString = JsonUtil.stringify(json);
        } catch (JsonException e) {
            getLogger().log(Level.WARNING,
                    "Error creating critical notification JSON message", e);
        }

        return "for(;;);[" + returnString + "]";
    }

    private static void putValueOrJsonNull(JsonObject json, String key,
            String value) {
        if (value == null) {
            json.put(key, Json.createNull());
        } else {
            json.put(key, value);
        }
    }

    /**
     * @deprecated As of 7.0. Will likely change or be removed in a future
     *             version
     */
    @Deprecated
    public void criticalNotification(VaadinRequest request,
            VaadinResponse response, String caption, String message,
            String details, String url) throws IOException {
        writeUncachedStringResponse(response, JsonConstants.JSON_CONTENT_TYPE,
                createCriticalNotificationJSON(caption, message, details, url));
    }

    /**
     * Enables push if push support is available and push has not yet been
     * enabled.
     *
     * If push support is not available, a warning explaining the situation will
     * be logged at least the first time this method is invoked.
     *
     * @return <code>true</code> if push can be used; <code>false</code> if push
     *         is not available.
     */
    public boolean ensurePushAvailable() {
        if (isAtmosphereAvailable()) {
            return true;
        } else {
            if (!pushWarningEmitted) {
                pushWarningEmitted = true;
                getLogger().log(Level.WARNING,
                        Constants.ATMOSPHERE_MISSING_ERROR);
            }
            return false;
        }
    }

    private boolean checkAtmosphereSupport() {
        String rawVersion = AtmospherePushConnection.getAtmosphereVersion();
        if (rawVersion == null) {
            return false;
        }

        if (!Constants.REQUIRED_ATMOSPHERE_RUNTIME_VERSION.equals(rawVersion)) {
            getLogger().log(Level.WARNING,
                    Constants.INVALID_ATMOSPHERE_VERSION_WARNING,
                    new Object[] {
                            Constants.REQUIRED_ATMOSPHERE_RUNTIME_VERSION,
                            rawVersion });
        }
        return true;
    }

    /**
     * Checks whether Atmosphere is available for use.
     *
     * @since 7.6
     * @return true if Atmosphere is available, false otherwise
     */
    protected boolean isAtmosphereAvailable() {
        if (atmosphereAvailable == null) {
            atmosphereAvailable = checkAtmosphereSupport();
        }
        return atmosphereAvailable;
    }

    /**
     * Checks that another {@link VaadinSession} instance is not locked. This is
     * internally used by {@link VaadinSession#accessSynchronously(Runnable)}
     * and {@link UI#accessSynchronously(Runnable)} to help avoid causing
     * deadlocks.
     *
     * @since 7.1
     * @param session
     *            the session that is being locked
     * @throws IllegalStateException
     *             if the current thread holds the lock for another session
     */
    public static void verifyNoOtherSessionLocked(VaadinSession session) {
        if (isOtherSessionLocked(session)) {
            throw new IllegalStateException(
                    "Can't access session while another session is locked by the same thread. This restriction is intended to help avoid deadlocks.");
        }
    }

    /**
     * Checks whether there might be some {@link VaadinSession} other than the
     * provided one for which the current thread holds a lock. This method might
     * not detect all cases where some other session is locked, but it should
     * cover the most typical situations.
     *
     * @since 7.2
     * @param session
     *            the session that is expected to be locked
     * @return <code>true</code> if another session is also locked by the
     *         current thread; <code>false</code> if no such session was found
     */
    public static boolean isOtherSessionLocked(VaadinSession session) {
        VaadinSession otherSession = VaadinSession.getCurrent();
        if (otherSession == null || otherSession == session) {
            return false;
        }
        return otherSession.hasLock();
    }

    /**
     * Verifies that the given CSRF token (aka double submit cookie) is valid
     * for the given session. This is used to protect against Cross Site Request
     * Forgery attacks.
     * <p>
     * This protection is enabled by default, but it might need to be disabled
     * to allow a certain type of testing. For these cases, the check can be
     * disabled by setting the init parameter
     * <code>disable-xsrf-protection</code> to <code>true</code>.
     *
     * @see DeploymentConfiguration#isXsrfProtectionEnabled()
     *
     * @since 7.1
     *
     * @param session
     *            the vaadin session for which the check should be done
     * @param requestToken
     *            the CSRF token provided in the request
     * @return <code>true</code> if the token is valid or if the protection is
     *         disabled; <code>false</code> if protection is enabled and the
     *         token is invalid
     */
    public static boolean isCsrfTokenValid(VaadinSession session,
            String requestToken) {

        if (session.getService().getDeploymentConfiguration()
                .isXsrfProtectionEnabled()) {
            String sessionToken = session.getCsrfToken();

            if (sessionToken == null || !MessageDigest.isEqual(
                    sessionToken.getBytes(StandardCharsets.UTF_8),
                    requestToken.getBytes(StandardCharsets.UTF_8))) {
                return false;
            }
        }
        return true;
    }

    /**
     * Implementation for {@link VaadinSession#access(Runnable)}. This method is
     * implemented here instead of in {@link VaadinSession} to enable overriding
     * the implementation without using a custom subclass of VaadinSession.
     *
     * @since 7.1
     * @see VaadinSession#access(Runnable)
     *
     * @param session
     *            the vaadin session to access
     * @param runnable
     *            the runnable to run with the session locked
     *
     * @return a future that can be used to check for task completion and to
     *         cancel the task
     */
    public Future<Void> accessSession(VaadinSession session,
            Runnable runnable) {
        FutureAccess future = new FutureAccess(session, runnable);
        session.getPendingAccessQueue().add(future);

        ensureAccessQueuePurged(session);

        return future;
    }

    /**
     * Makes sure the pending access queue is purged for the provided session.
     * If the session is currently locked by the current thread or some other
     * thread, the queue will be purged when the session is unlocked. If the
     * lock is not held by any thread, it is acquired and the queue is purged
     * right away.
     *
     * @since 7.1.2
     * @param session
     *            the session for which the access queue should be purged
     */
    public void ensureAccessQueuePurged(VaadinSession session) {
        /*
         * If no thread is currently holding the lock, pending changes for UIs
         * with automatic push would not be processed and pushed until the next
         * time there is a request or someone does an explicit push call.
         *
         * To remedy this, we try to get the lock at this point. If the lock is
         * currently held by another thread, we just back out as the queue will
         * get purged once it is released. If the lock is held by the current
         * thread, we just release it knowing that the queue gets purged once
         * the lock is ultimately released. If the lock is not held by any
         * thread and we acquire it, we just release it again to purge the queue
         * right away.
         */
        try {
            // tryLock() would be shorter, but it does not guarantee fairness
            if (session.getLockInstance().tryLock(0, TimeUnit.SECONDS)) {
                // unlock triggers runPendingAccessTasks
                session.unlock();
            }
        } catch (InterruptedException e) {
            // Just ignore
        }
    }

    /**
     * Purges the queue of pending access invocations enqueued with
     * {@link VaadinSession#access(Runnable)}.
     * <p>
     * This method is automatically run by the framework at appropriate
     * situations and is not intended to be used by application developers.
     *
     * @param session
     *            the vaadin session to purge the queue for
     * @since 7.1
     */
    public void runPendingAccessTasks(VaadinSession session) {
        assert session.hasLock();

        if (session.getPendingAccessQueue().isEmpty()) {
            return;
        }

        FutureAccess pendingAccess;

        // Dump all current instances, not only the ones dumped by setCurrent
        Map<Class<?>, CurrentInstance> oldInstances = CurrentInstance
                .getInstances();
        CurrentInstance.setCurrent(session);
        try {
            while ((pendingAccess = session.getPendingAccessQueue()
                    .poll()) != null) {
                if (!pendingAccess.isCancelled()) {
                    pendingAccess.run();

                    try {
                        pendingAccess.get();
                    } catch (Exception exception) {
                        if (exception instanceof ExecutionException) {
                            Throwable cause = exception.getCause();
                            if (cause instanceof Exception) {
                                exception = (Exception) cause;
                            }
                        }
                        pendingAccess.handleError(exception);
                    }
                }
            }
        } finally {
            CurrentInstance.clearAll();
            CurrentInstance.restoreInstances(oldInstances);
        }
    }

    /**
     * Adds a service destroy listener that gets notified when this service is
     * destroyed.
     * <p>
     * The listeners may be invoked in a non-deterministic order. In particular,
     * it is not guaranteed that listeners will be invoked in the order they
     * were added.
     *
     * @since 8.0
     * @param listener
     *            the service destroy listener to add
     *
     * @see #destroy()
     * @see #removeServiceDestroyListener(ServiceDestroyListener)
     * @see ServiceDestroyListener
     * @return a registration object for removing the listener
     */
    public Registration addServiceDestroyListener(
            ServiceDestroyListener listener) {
        serviceDestroyListeners.add(listener);
        return () -> serviceDestroyListeners.remove(listener);
    }

    /**
     * Removes a service destroy listener that was previously added with
     * {@link #addServiceDestroyListener(ServiceDestroyListener)}.
     *
     * @since 7.2
     * @param listener
     *            the service destroy listener to remove
     * @deprecated use the {@link Registration} object returned by
     *             {@link #addServiceDestroyListener(ServiceDestroyListener)} to
     *             remove the listener
     */
    @Deprecated
    public void removeServiceDestroyListener(ServiceDestroyListener listener) {
        serviceDestroyListeners.remove(listener);
    }

    /**
     * Called when the servlet, portlet or similar for this service is being
     * destroyed. After this method has been called, no more requests will be
     * handled by this service.
     *
     * @see #addServiceDestroyListener(ServiceDestroyListener)
     * @see Servlet#destroy()
     * @see Portlet#destroy()
     *
     * @since 7.2
     */
    public void destroy() {
        ServiceDestroyEvent event = new ServiceDestroyEvent(this);
        serviceDestroyListeners
                .forEach(listener -> listener.serviceDestroy(event));
    }

    /**
     * Tries to acquire default class loader and sets it as a class loader for
     * this {@link VaadinService} if found. If current security policy disallows
     * acquiring class loader instance it will log a message and re-throw
     * {@link SecurityException}
     *
     * @throws SecurityException
     *             If current security policy forbids acquiring class loader
     *
     * @since 7.3.5
     */
    protected void setDefaultClassLoader() {
        try {
            setClassLoader(
                    VaadinServiceClassLoaderUtil.findDefaultClassLoader());
        } catch (SecurityException e) {
            getLogger().log(Level.SEVERE,
                    Constants.CANNOT_ACQUIRE_CLASSLOADER_SEVERE, e);
            throw e;
        }
    }

    /**
     * Called when the VaadinSession should be stored.
     * <p>
     * By default stores the VaadinSession in the underlying HTTP session.
     *
     * @since 7.6
     * @param session
     *            the VaadinSession to store
     * @param wrappedSession
     *            the underlying HTTP session
     */
    protected void storeSession(VaadinSession session,
            WrappedSession wrappedSession) {
        assert VaadinSession.hasLock(this, wrappedSession);
        writeToHttpSession(wrappedSession, session);
        session.refreshTransients(wrappedSession, this);
    }

    /**
     * Performs the actual write of the VaadinSession to the underlying HTTP
     * session after sanity checks have been performed.
     * <p>
     * Called by {@link #storeSession(VaadinSession, WrappedSession)}
     *
     * @since 7.6
     * @param wrappedSession
     *            the underlying HTTP session
     * @param session
     *            the VaadinSession to store
     */
    protected void writeToHttpSession(WrappedSession wrappedSession,
            VaadinSession session) {
        wrappedSession.setAttribute(getSessionAttributeName(), session);
    }

    /**
     * Called when the VaadinSession should be loaded from the underlying HTTP
     * session.
     *
     * @since 7.6
     * @param wrappedSession
     *            the underlying HTTP session
     * @return the VaadinSession in the HTTP session or null if not found
     */
    protected VaadinSession loadSession(WrappedSession wrappedSession) {
        assert VaadinSession.hasLock(this, wrappedSession);

        VaadinSession vaadinSession = readFromHttpSession(wrappedSession);
        if (vaadinSession == null) {
            return null;
        }
        vaadinSession.refreshTransients(wrappedSession, this);
        return vaadinSession;
    }

    /**
     * Performs the actual read of the VaadinSession from the underlying HTTP
     * session after sanity checks have been performed.
     * <p>
     * Called by {@link #loadSession(WrappedSession)}.
     *
     * @param wrappedSession
     *            the underlying HTTP session
     * @since 7.6
     * @return the VaadinSession or null if no session was found
     */
    protected VaadinSession readFromHttpSession(WrappedSession wrappedSession) {
        return (VaadinSession) wrappedSession
                .getAttribute(getSessionAttributeName());
    }

    /**
     * Called when the VaadinSession should be removed from the underlying HTTP
     * session.
     *
     * @since 7.6
     * @param wrappedSession
     *            the underlying HTTP session
     */
    public void removeSession(WrappedSession wrappedSession) {
        assert VaadinSession.hasLock(this, wrappedSession);
        removeFromHttpSession(wrappedSession);
    }

    /**
     * Performs the actual removal of the VaadinSession from the underlying HTTP
     * session after sanity checks have been performed.
     *
     * @since 7.6
     * @param wrappedSession
     *            the underlying HTTP session
     */
    protected void removeFromHttpSession(WrappedSession wrappedSession) {
        wrappedSession.removeAttribute(getSessionAttributeName());

    }

    /**
     * Returns the name used for storing the VaadinSession in the underlying
     * HTTP session.
     *
     * @since 7.6
     * @return the attribute name used for storing the VaadinSession
     */
    protected String getSessionAttributeName() {
        return VaadinSession.class.getName() + "." + getServiceName();
    }

    /**
     * Generates a unique id to use for a newly attached connector.
     *
     * @see ConnectorIdGenerator
     * @see #initConnectorIdGenerator(List)
     *
     * @since 8.1
     *
     * @param session
     *            the session to which the connector has been attached, not
     *            <code>null</code>
     * @param connector
     *            the attached connector for which to generate an id, not
     *            <code>null</code>
     * @return a string id that is unique within the session, not
     *         <code>null</code>
     */
    public String generateConnectorId(VaadinSession session,
            ClientConnector connector) {
        assert session.getService() == this;
        String connectorId = connectorIdGenerator.generateConnectorId(
                new ConnectorIdGenerationEvent(session, connector));

        assert connectorId != null;

        return connectorId;
    }

}