Use final modifier where appropriate

Change-Id: I80b73b653e97904605dc62484a7448f3bfbf7226

98 files changed, 149 insertions, 150 deletions

diff --git a/server/src/main/java/com/vaadin/data/Binder.java b/server/src/main/java/com/vaadin/data/Binder.java
index 7912a55d72..2763c7f788 100644
--- a/server/src/main/java/com/vaadin/data/Binder.java
+++ b/server/src/main/java/com/vaadin/data/Binder.java
@@ -794,7 +794,7 @@ public class Binder<BEAN> implements Serializable {
      */
     private static class ValidatorAsConverter<T> implements Converter<T, T> {
 
-        private Validator<? super T> validator;
+        private final Validator<? super T> validator;
 
         /**
          * Creates a new converter wrapping the given validator.
diff --git a/server/src/main/java/com/vaadin/data/validator/AbstractValidator.java b/server/src/main/java/com/vaadin/data/validator/AbstractValidator.java
index 793797ee3f..b6467b07b9 100644
--- a/server/src/main/java/com/vaadin/data/validator/AbstractValidator.java
+++ b/server/src/main/java/com/vaadin/data/validator/AbstractValidator.java
@@ -31,7 +31,7 @@ import com.vaadin.server.SerializableFunction;
  */
 public abstract class AbstractValidator<T> implements Validator<T> {
 
-    private SerializableFunction<T, String> messageProvider;
+    private final SerializableFunction<T, String> messageProvider;
 
     /**
      * Constructs a validator with the given error message. The substring "{0}"
diff --git a/server/src/main/java/com/vaadin/data/validator/RangeValidator.java b/server/src/main/java/com/vaadin/data/validator/RangeValidator.java
index b8ad387308..2f966f81ee 100644
--- a/server/src/main/java/com/vaadin/data/validator/RangeValidator.java
+++ b/server/src/main/java/com/vaadin/data/validator/RangeValidator.java
@@ -35,7 +35,7 @@ public class RangeValidator<T> extends AbstractValidator<T> {
     private T maxValue = null;
     private boolean minValueIncluded = true;
     private boolean maxValueIncluded = true;
-    private Comparator<? super T> comparator;
+    private final Comparator<? super T> comparator;
 
     /**
      * Creates a new range validator of the given type. Passing null to either
diff --git a/server/src/main/java/com/vaadin/data/validator/StringLengthValidator.java b/server/src/main/java/com/vaadin/data/validator/StringLengthValidator.java
index b3e1da18eb..f207e618ee 100644
--- a/server/src/main/java/com/vaadin/data/validator/StringLengthValidator.java
+++ b/server/src/main/java/com/vaadin/data/validator/StringLengthValidator.java
@@ -28,7 +28,7 @@ import com.vaadin.data.util.converter.ValueContext;
 @SuppressWarnings("serial")
 public class StringLengthValidator extends AbstractValidator<String> {
 
-    private RangeValidator<Integer> validator;
+    private final RangeValidator<Integer> validator;
 
     /**
      * Creates a new StringLengthValidator with a given error message and
diff --git a/server/src/main/java/com/vaadin/event/FieldEvents.java b/server/src/main/java/com/vaadin/event/FieldEvents.java
index bb14cc7339..b0e539961b 100644
--- a/server/src/main/java/com/vaadin/event/FieldEvents.java
+++ b/server/src/main/java/com/vaadin/event/FieldEvents.java
@@ -194,7 +194,7 @@ public interface FieldEvents {
     public static abstract class FocusAndBlurServerRpcImpl
             implements FocusAndBlurServerRpc {
 
-        private Component component;
+        private final Component component;
 
         public FocusAndBlurServerRpcImpl(Component component) {
             this.component = component;
diff --git a/server/src/main/java/com/vaadin/event/MouseEvents.java b/server/src/main/java/com/vaadin/event/MouseEvents.java
index d76759343a..e391ade401 100644
--- a/server/src/main/java/com/vaadin/event/MouseEvents.java
+++ b/server/src/main/java/com/vaadin/event/MouseEvents.java
@@ -60,7 +60,7 @@ public interface MouseEvents {
          */
         @Deprecated
         public static final MouseButton BUTTON_RIGHT = MouseButton.RIGHT;
-        private MouseEventDetails details;
+        private final MouseEventDetails details;
 
         public ClickEvent(Component source,
                 MouseEventDetails mouseEventDetails) {
diff --git a/server/src/main/java/com/vaadin/event/TransferableImpl.java b/server/src/main/java/com/vaadin/event/TransferableImpl.java
index 4da28176fc..7ea9ef7b23 100644
--- a/server/src/main/java/com/vaadin/event/TransferableImpl.java
+++ b/server/src/main/java/com/vaadin/event/TransferableImpl.java
@@ -28,7 +28,7 @@ import com.vaadin.ui.Component;
  */
 public class TransferableImpl implements Transferable {
     private Map<String, Object> rawVariables = new HashMap<>();
-    private Component sourceComponent;
+    private final Component sourceComponent;
 
     public TransferableImpl(Component sourceComponent,
             Map<String, Object> rawVariables) {
diff --git a/server/src/main/java/com/vaadin/event/dd/DragAndDropEvent.java b/server/src/main/java/com/vaadin/event/dd/DragAndDropEvent.java
index 062f8aff29..93491b4274 100644
--- a/server/src/main/java/com/vaadin/event/dd/DragAndDropEvent.java
+++ b/server/src/main/java/com/vaadin/event/dd/DragAndDropEvent.java
@@ -34,8 +34,8 @@ import com.vaadin.event.dd.acceptcriteria.AcceptCriterion;
  *
  */
 public class DragAndDropEvent implements Serializable {
-    private Transferable transferable;
-    private TargetDetails dropTargetDetails;
+    private final Transferable transferable;
+    private final TargetDetails dropTargetDetails;
 
     public DragAndDropEvent(Transferable transferable,
             TargetDetails dropTargetDetails) {
diff --git a/server/src/main/java/com/vaadin/event/dd/acceptcriteria/AcceptAll.java b/server/src/main/java/com/vaadin/event/dd/acceptcriteria/AcceptAll.java
index 9694f6525b..b42d2b4684 100644
--- a/server/src/main/java/com/vaadin/event/dd/acceptcriteria/AcceptAll.java
+++ b/server/src/main/java/com/vaadin/event/dd/acceptcriteria/AcceptAll.java
@@ -32,7 +32,7 @@ import com.vaadin.event.dd.DragAndDropEvent;
 public final class AcceptAll extends ClientSideCriterion {
 
     private static final long serialVersionUID = 7406683402153141461L;
-    private static AcceptCriterion singleton = new AcceptAll();
+    private static final AcceptCriterion singleton = new AcceptAll();
 
     private AcceptAll() {
     }
diff --git a/server/src/main/java/com/vaadin/event/dd/acceptcriteria/ContainsDataFlavor.java b/server/src/main/java/com/vaadin/event/dd/acceptcriteria/ContainsDataFlavor.java
index 42d666a4ce..c4fc645fec 100644
--- a/server/src/main/java/com/vaadin/event/dd/acceptcriteria/ContainsDataFlavor.java
+++ b/server/src/main/java/com/vaadin/event/dd/acceptcriteria/ContainsDataFlavor.java
@@ -32,7 +32,7 @@ import com.vaadin.server.PaintTarget;
  */
 public class ContainsDataFlavor extends ClientSideCriterion {
 
-    private String dataFlavorId;
+    private final String dataFlavorId;
 
     /**
      * Constructs a new instance of {@link ContainsDataFlavor}.
diff --git a/server/src/main/java/com/vaadin/event/dd/acceptcriteria/Not.java b/server/src/main/java/com/vaadin/event/dd/acceptcriteria/Not.java
index 057e032d46..f194be7634 100644
--- a/server/src/main/java/com/vaadin/event/dd/acceptcriteria/Not.java
+++ b/server/src/main/java/com/vaadin/event/dd/acceptcriteria/Not.java
@@ -31,7 +31,7 @@ import com.vaadin.server.PaintTarget;
 public class Not extends ClientSideCriterion {
 
     private static final long serialVersionUID = 1131422338558613244L;
-    private AcceptCriterion acceptCriterion;
+    private final AcceptCriterion acceptCriterion;
 
     public Not(ClientSideCriterion acceptCriterion) {
         this.acceptCriterion = acceptCriterion;
diff --git a/server/src/main/java/com/vaadin/event/dd/acceptcriteria/Or.java b/server/src/main/java/com/vaadin/event/dd/acceptcriteria/Or.java
index 20bf507bbe..81ef37631f 100644
--- a/server/src/main/java/com/vaadin/event/dd/acceptcriteria/Or.java
+++ b/server/src/main/java/com/vaadin/event/dd/acceptcriteria/Or.java
@@ -33,7 +33,7 @@ import com.vaadin.server.PaintTarget;
  */
 public class Or extends ClientSideCriterion {
     private static final long serialVersionUID = 1L;
-    private AcceptCriterion criteria[];
+    private final AcceptCriterion criteria[];
 
     /**
      * @param criteria
diff --git a/server/src/main/java/com/vaadin/event/dd/acceptcriteria/SourceIs.java b/server/src/main/java/com/vaadin/event/dd/acceptcriteria/SourceIs.java
index eaccceaca2..49ea24e5cb 100644
--- a/server/src/main/java/com/vaadin/event/dd/acceptcriteria/SourceIs.java
+++ b/server/src/main/java/com/vaadin/event/dd/acceptcriteria/SourceIs.java
@@ -36,7 +36,7 @@ import com.vaadin.ui.Component;
 @SuppressWarnings("serial")
 public class SourceIs extends ClientSideCriterion {
 
-    private Component[] components;
+    private final Component[] components;
 
     public SourceIs(Component... component) {
         components = component;
diff --git a/server/src/main/java/com/vaadin/event/dd/acceptcriteria/SourceIsTarget.java b/server/src/main/java/com/vaadin/event/dd/acceptcriteria/SourceIsTarget.java
index 95fbaab85e..8cb16e4313 100644
--- a/server/src/main/java/com/vaadin/event/dd/acceptcriteria/SourceIsTarget.java
+++ b/server/src/main/java/com/vaadin/event/dd/acceptcriteria/SourceIsTarget.java
@@ -38,7 +38,7 @@ import com.vaadin.ui.Component;
 public class SourceIsTarget extends ClientSideCriterion {
 
     private static final long serialVersionUID = -451399314705532584L;
-    private static SourceIsTarget instance = new SourceIsTarget();
+    private static final SourceIsTarget instance = new SourceIsTarget();
 
     private SourceIsTarget() {
     }
diff --git a/server/src/main/java/com/vaadin/server/AbstractClientConnector.java b/server/src/main/java/com/vaadin/server/AbstractClientConnector.java
index f3e79d6579..01222a07dd 100644
--- a/server/src/main/java/com/vaadin/server/AbstractClientConnector.java
+++ b/server/src/main/java/com/vaadin/server/AbstractClientConnector.java
@@ -61,13 +61,13 @@ public abstract class AbstractClientConnector
      * A map from client to server RPC interface class name to the RPC call
      * manager that handles incoming RPC calls for that interface.
      */
-    private Map<String, ServerRpcManager<?>> rpcManagerMap = new HashMap<>();
+    private final Map<String, ServerRpcManager<?>> rpcManagerMap = new HashMap<>();
 
     /**
      * A map from server to client RPC interface class to the RPC proxy that
      * sends ourgoing RPC calls for that interface.
      */
-    private Map<Class<?>, ClientRpc> rpcProxyMap = new HashMap<>();
+    private final Map<Class<?>, ClientRpc> rpcProxyMap = new HashMap<>();
 
     /**
      * Shared state object to be communicated from the server to the client when
@@ -84,7 +84,7 @@ public abstract class AbstractClientConnector
 
     private String connectorId;
 
-    private ArrayList<Extension> extensions = new ArrayList<>();
+    private final ArrayList<Extension> extensions = new ArrayList<>();
 
     /**
      * The EventRouter used for the event model.
@@ -378,7 +378,7 @@ public abstract class AbstractClientConnector
     private class RpcInvocationHandler
             implements InvocationHandler, Serializable {
 
-        private String rpcInterfaceName;
+        private final String rpcInterfaceName;
 
         public RpcInvocationHandler(Class<?> rpcInterface) {
             rpcInterfaceName = rpcInterface.getName().replaceAll("\\$", ".");
diff --git a/server/src/main/java/com/vaadin/server/AbstractErrorMessage.java b/server/src/main/java/com/vaadin/server/AbstractErrorMessage.java
index 491462b53d..6ffbf6063e 100644
--- a/server/src/main/java/com/vaadin/server/AbstractErrorMessage.java
+++ b/server/src/main/java/com/vaadin/server/AbstractErrorMessage.java
@@ -63,7 +63,7 @@ public abstract class AbstractErrorMessage implements ErrorMessage {
      */
     private ErrorLevel level = ErrorLevel.ERROR;
 
-    private List<ErrorMessage> causes = new ArrayList<>();
+    private final List<ErrorMessage> causes = new ArrayList<>();
 
     protected AbstractErrorMessage(String message) {
         this.message = message;
diff --git a/server/src/main/java/com/vaadin/server/ClientConnector.java b/server/src/main/java/com/vaadin/server/ClientConnector.java
index 80453d4733..b54a85029b 100644
--- a/server/src/main/java/com/vaadin/server/ClientConnector.java
+++ b/server/src/main/java/com/vaadin/server/ClientConnector.java
@@ -114,7 +114,7 @@ public interface ClientConnector extends Connector {
     public static class ConnectorErrorEvent
             extends com.vaadin.server.ErrorEvent {
 
-        private Connector connector;
+        private final Connector connector;
 
         public ConnectorErrorEvent(Connector connector, Throwable t) {
             super(t);
diff --git a/server/src/main/java/com/vaadin/server/ClientMethodInvocation.java b/server/src/main/java/com/vaadin/server/ClientMethodInvocation.java
index 28d5d2778d..acf5f3568f 100644
--- a/server/src/main/java/com/vaadin/server/ClientMethodInvocation.java
+++ b/server/src/main/java/com/vaadin/server/ClientMethodInvocation.java
@@ -39,7 +39,7 @@ public class ClientMethodInvocation
     private final String interfaceName;
     private final String methodName;
     private transient Object[] parameters;
-    private Type[] parameterTypes;
+    private final Type[] parameterTypes;
 
     // used for sorting calls between different connectors in the same UI
     private final long sequenceNumber;
diff --git a/server/src/main/java/com/vaadin/server/ComponentSizeValidator.java b/server/src/main/java/com/vaadin/server/ComponentSizeValidator.java
index da8231eec9..a9068495dc 100644
--- a/server/src/main/java/com/vaadin/server/ComponentSizeValidator.java
+++ b/server/src/main/java/com/vaadin/server/ComponentSizeValidator.java
@@ -631,9 +631,9 @@ public class ComponentSizeValidator implements Serializable {
 
     }
 
-    private static Map<Object, FileLocation> creationLocations = new HashMap<>();
-    private static Map<Object, FileLocation> widthLocations = new HashMap<>();
-    private static Map<Object, FileLocation> heightLocations = new HashMap<>();
+    private static final Map<Object, FileLocation> creationLocations = new HashMap<>();
+    private static final Map<Object, FileLocation> widthLocations = new HashMap<>();
+    private static final Map<Object, FileLocation> heightLocations = new HashMap<>();
 
     public static class FileLocation implements Serializable {
         public String method;
diff --git a/server/src/main/java/com/vaadin/server/FontAwesome.java b/server/src/main/java/com/vaadin/server/FontAwesome.java
index 2bb4a9abc8..2ce35328f6 100644
--- a/server/src/main/java/com/vaadin/server/FontAwesome.java
+++ b/server/src/main/java/com/vaadin/server/FontAwesome.java
@@ -731,7 +731,7 @@ public enum FontAwesome implements FontIcon {
     ;
 
     public static final String FONT_FAMILY = "FontAwesome";
-    private int codepoint;
+    private final int codepoint;
 
     FontAwesome(int codepoint) {
         this.codepoint = codepoint;
diff --git a/server/src/main/java/com/vaadin/server/JavaScriptCallbackHelper.java b/server/src/main/java/com/vaadin/server/JavaScriptCallbackHelper.java
index 9b066a4ff8..7e228bb7ca 100644
--- a/server/src/main/java/com/vaadin/server/JavaScriptCallbackHelper.java
+++ b/server/src/main/java/com/vaadin/server/JavaScriptCallbackHelper.java
@@ -47,9 +47,9 @@ public class JavaScriptCallbackHelper implements Serializable {
 
     private static final Method CALL_METHOD = ReflectTools.findMethod(
             JavaScriptCallbackRpc.class, "call", String.class, JsonArray.class);
-    private AbstractClientConnector connector;
+    private final AbstractClientConnector connector;
 
-    private Map<String, JavaScriptFunction> callbacks = new HashMap<>();
+    private final Map<String, JavaScriptFunction> callbacks = new HashMap<>();
     private JavaScriptCallbackRpc javascriptCallbackRpc;
 
     public JavaScriptCallbackHelper(AbstractClientConnector connector) {
diff --git a/server/src/main/java/com/vaadin/server/JsonCodec.java b/server/src/main/java/com/vaadin/server/JsonCodec.java
index 588443d55b..a5725e3d45 100644
--- a/server/src/main/java/com/vaadin/server/JsonCodec.java
+++ b/server/src/main/java/com/vaadin/server/JsonCodec.java
@@ -204,17 +204,17 @@ public class JsonCodec implements Serializable {
      * happens to process Vaadin requests, so it must be protected from
      * corruption caused by concurrent access.
      */
-    private static ConcurrentMap<Class<?>, Collection<BeanProperty>> typePropertyCache = new ConcurrentHashMap<>();
+    private static final ConcurrentMap<Class<?>, Collection<BeanProperty>> typePropertyCache = new ConcurrentHashMap<>();
 
-    private static Map<Class<?>, String> typeToTransportType = new HashMap<>();
+    private static final Map<Class<?>, String> typeToTransportType = new HashMap<>();
 
     /**
      * Note! This does not contain primitives.
      * <p>
      */
-    private static Map<String, Class<?>> transportTypeToType = new HashMap<>();
+    private static final Map<String, Class<?>> transportTypeToType = new HashMap<>();
 
-    private static Map<Class<?>, JSONSerializer<?>> customSerializers = new HashMap<>();
+    private static final Map<Class<?>, JSONSerializer<?>> customSerializers = new HashMap<>();
     static {
         customSerializers.put(Date.class, new DateSerializer());
     }
diff --git a/server/src/main/java/com/vaadin/server/LegacyApplication.java b/server/src/main/java/com/vaadin/server/LegacyApplication.java
index b417c0f9ee..599d9bb385 100644
--- a/server/src/main/java/com/vaadin/server/LegacyApplication.java
+++ b/server/src/main/java/com/vaadin/server/LegacyApplication.java
@@ -42,7 +42,7 @@ public abstract class LegacyApplication implements ErrorHandler {
     private LegacyWindow mainWindow;
     private String theme = "reindeer";
 
-    private Map<String, LegacyWindow> legacyUINames = new HashMap<>();
+    private final Map<String, LegacyWindow> legacyUINames = new HashMap<>();
 
     private boolean isRunning = true;
 
diff --git a/server/src/main/java/com/vaadin/server/LegacyCommunicationManager.java b/server/src/main/java/com/vaadin/server/LegacyCommunicationManager.java
index 7c438ca999..3ef17b618a 100644
--- a/server/src/main/java/com/vaadin/server/LegacyCommunicationManager.java
+++ b/server/src/main/java/com/vaadin/server/LegacyCommunicationManager.java
@@ -69,7 +69,7 @@ public class LegacyCommunicationManager implements Serializable {
     private String requestThemeName;
 
     // TODO Refactor (#11413)
-    private Map<String, Class<?>> publishedFileContexts = new HashMap<>();
+    private final Map<String, Class<?>> publishedFileContexts = new HashMap<>();
 
     /**
      * TODO New constructor - document me!
diff --git a/server/src/main/java/com/vaadin/server/LocaleService.java b/server/src/main/java/com/vaadin/server/LocaleService.java
index 98bbc8275a..c2b28d1cab 100644
--- a/server/src/main/java/com/vaadin/server/LocaleService.java
+++ b/server/src/main/java/com/vaadin/server/LocaleService.java
@@ -41,9 +41,9 @@ import com.vaadin.ui.UI;
  */
 public class LocaleService implements Serializable {
 
-    private UI ui;
+    private final UI ui;
 
-    private LocaleServiceState state;
+    private final LocaleServiceState state;
 
     /**
      * Creates a LocaleService bound to the given UI
diff --git a/server/src/main/java/com/vaadin/server/Page.java b/server/src/main/java/com/vaadin/server/Page.java
index d4eb2432f0..d067ffc7dc 100644
--- a/server/src/main/java/com/vaadin/server/Page.java
+++ b/server/src/main/java/com/vaadin/server/Page.java
@@ -333,7 +333,7 @@ public class Page implements Serializable {
 
     private static class InjectedStyleString implements InjectedStyle {
 
-        private String css;
+        private final String css;
 
         public InjectedStyleString(String css) {
             this.css = css;
diff --git a/server/src/main/java/com/vaadin/server/RestrictedRenderResponse.java b/server/src/main/java/com/vaadin/server/RestrictedRenderResponse.java
index 587bf0ae69..d62acfa8c0 100644
--- a/server/src/main/java/com/vaadin/server/RestrictedRenderResponse.java
+++ b/server/src/main/java/com/vaadin/server/RestrictedRenderResponse.java
@@ -40,7 +40,7 @@ import org.w3c.dom.Element;
  */
 class RestrictedRenderResponse implements RenderResponse, Serializable {
 
-    private RenderResponse response;
+    private final RenderResponse response;
 
     RestrictedRenderResponse(RenderResponse response) {
         this.response = response;
diff --git a/server/src/main/java/com/vaadin/server/SizeWithUnit.java b/server/src/main/java/com/vaadin/server/SizeWithUnit.java
index 17624b4712..b53e4533f4 100644
--- a/server/src/main/java/com/vaadin/server/SizeWithUnit.java
+++ b/server/src/main/java/com/vaadin/server/SizeWithUnit.java
@@ -30,8 +30,8 @@ import com.vaadin.shared.util.SharedUtil;
  * @author Vaadin Ltd
  */
 public class SizeWithUnit implements Serializable {
-    private float size;
-    private Unit unit;
+    private final float size;
+    private final Unit unit;
     private static final Pattern sizePattern = Pattern
             .compile(SharedUtil.SIZE_PATTERN);
 
diff --git a/server/src/main/java/com/vaadin/server/Sizeable.java b/server/src/main/java/com/vaadin/server/Sizeable.java
index d731908e31..f3fa8ed0cc 100644
--- a/server/src/main/java/com/vaadin/server/Sizeable.java
+++ b/server/src/main/java/com/vaadin/server/Sizeable.java
@@ -132,7 +132,7 @@ public interface Sizeable extends Serializable {
          */
         PERCENTAGE("%");
 
-        private String symbol;
+        private final String symbol;
 
         private Unit(String symbol) {
             this.symbol = symbol;
diff --git a/server/src/main/java/com/vaadin/server/VaadinPortletResponse.java b/server/src/main/java/com/vaadin/server/VaadinPortletResponse.java
index 4a739760f4..f7a5abadff 100644
--- a/server/src/main/java/com/vaadin/server/VaadinPortletResponse.java
+++ b/server/src/main/java/com/vaadin/server/VaadinPortletResponse.java
@@ -47,7 +47,7 @@ public class VaadinPortletResponse implements VaadinResponse {
     }
 
     private final PortletResponse response;
-    private VaadinPortletService vaadinService;
+    private final VaadinPortletService vaadinService;
 
     /**
      * Wraps a portlet response and an associated vaadin service
diff --git a/server/src/main/java/com/vaadin/server/VaadinServletResponse.java b/server/src/main/java/com/vaadin/server/VaadinServletResponse.java
index 07c3964f5d..7513dd4d97 100644
--- a/server/src/main/java/com/vaadin/server/VaadinServletResponse.java
+++ b/server/src/main/java/com/vaadin/server/VaadinServletResponse.java
@@ -31,7 +31,7 @@ import javax.servlet.http.HttpServletResponseWrapper;
 public class VaadinServletResponse extends HttpServletResponseWrapper
         implements VaadinResponse {
 
-    private VaadinServletService vaadinService;
+    private final VaadinServletService vaadinService;
 
     /**
      * Wraps a http servlet response and an associated vaadin service
diff --git a/server/src/main/java/com/vaadin/server/VaadinSession.java b/server/src/main/java/com/vaadin/server/VaadinSession.java
index d2cf4e740a..ebda81b9c7 100644
--- a/server/src/main/java/com/vaadin/server/VaadinSession.java
+++ b/server/src/main/java/com/vaadin/server/VaadinSession.java
@@ -85,7 +85,7 @@ public class VaadinSession implements HttpSessionBindingListener, Serializable {
         private final Map<Class<?>, CurrentInstance> instances = CurrentInstance
                 .getInstances(true);
         private final VaadinSession session;
-        private Runnable runnable;
+        private final Runnable runnable;
 
         /**
          * Creates an instance for the given runnable
diff --git a/server/src/main/java/com/vaadin/server/communication/AtmospherePushConnection.java b/server/src/main/java/com/vaadin/server/communication/AtmospherePushConnection.java
index 60ab1d0651..1189ff6023 100644
--- a/server/src/main/java/com/vaadin/server/communication/AtmospherePushConnection.java
+++ b/server/src/main/java/com/vaadin/server/communication/AtmospherePushConnection.java
@@ -131,7 +131,7 @@ public class AtmospherePushConnection implements PushConnection {
         CONNECTED;
     }
 
-    private UI ui;
+    private final UI ui;
     private transient State state = State.DISCONNECTED;
     private transient AtmosphereResource resource;
     private transient FragmentedMessage incomingMessage;
diff --git a/server/src/main/java/com/vaadin/server/communication/JSR356WebsocketInitializer.java b/server/src/main/java/com/vaadin/server/communication/JSR356WebsocketInitializer.java
index 95b796a2c7..2119de6f37 100644
--- a/server/src/main/java/com/vaadin/server/communication/JSR356WebsocketInitializer.java
+++ b/server/src/main/java/com/vaadin/server/communication/JSR356WebsocketInitializer.java
@@ -65,8 +65,8 @@ public class JSR356WebsocketInitializer implements ServletContextListener {
      */
     public static class FakeServletConfig implements ServletConfig {
 
-        private ServletRegistration servletRegistration;
-        private ServletContext servletContext;
+        private final ServletRegistration servletRegistration;
+        private final ServletContext servletContext;
 
         public FakeServletConfig(ServletRegistration servletRegistration,
                 ServletContext servletContext) {
diff --git a/server/src/main/java/com/vaadin/server/communication/PushHandler.java b/server/src/main/java/com/vaadin/server/communication/PushHandler.java
index 5423226e6a..75a5c9d94f 100644
--- a/server/src/main/java/com/vaadin/server/communication/PushHandler.java
+++ b/server/src/main/java/com/vaadin/server/communication/PushHandler.java
@@ -156,7 +156,7 @@ public class PushHandler {
         }
     };
 
-    private VaadinServletService service;
+    private final VaadinServletService service;
 
     public PushHandler(VaadinServletService service) {
         this.service = service;
diff --git a/server/src/main/java/com/vaadin/server/communication/UidlRequestHandler.java b/server/src/main/java/com/vaadin/server/communication/UidlRequestHandler.java
index 0f5ce37fa9..6afb6434a4 100644
--- a/server/src/main/java/com/vaadin/server/communication/UidlRequestHandler.java
+++ b/server/src/main/java/com/vaadin/server/communication/UidlRequestHandler.java
@@ -51,7 +51,7 @@ public class UidlRequestHandler extends SynchronizedRequestHandler
 
     public static final String UIDL_PATH = "UIDL/";
 
-    private ServerRpcHandler rpcHandler;
+    private final ServerRpcHandler rpcHandler;
 
     public UidlRequestHandler() {
         rpcHandler = createRpcHandler();
diff --git a/server/src/main/java/com/vaadin/server/data/BackEndDataSource.java b/server/src/main/java/com/vaadin/server/data/BackEndDataSource.java
index 2eedeb6839..3ca31de15a 100644
--- a/server/src/main/java/com/vaadin/server/data/BackEndDataSource.java
+++ b/server/src/main/java/com/vaadin/server/data/BackEndDataSource.java
@@ -30,8 +30,8 @@ import com.vaadin.server.SerializableFunction;
  */
 public class BackEndDataSource<T> extends AbstractDataSource<T> {
 
-    private SerializableFunction<Query, Stream<T>> request;
-    private SerializableFunction<Query, Integer> sizeCallback;
+    private final SerializableFunction<Query, Stream<T>> request;
+    private final SerializableFunction<Query, Integer> sizeCallback;
 
     /**
      * Constructs a new DataSource to request data from an arbitrary back end
diff --git a/server/src/main/java/com/vaadin/server/data/DataCommunicator.java b/server/src/main/java/com/vaadin/server/data/DataCommunicator.java
index a0ce859892..24bb464722 100644
--- a/server/src/main/java/com/vaadin/server/data/DataCommunicator.java
+++ b/server/src/main/java/com/vaadin/server/data/DataCommunicator.java
@@ -177,11 +177,11 @@ public class DataCommunicator<T> extends AbstractExtension {
         }
     }
 
-    private Collection<DataGenerator<T>> generators = new LinkedHashSet<>();
-    private ActiveDataHandler handler = new ActiveDataHandler();
+    private final Collection<DataGenerator<T>> generators = new LinkedHashSet<>();
+    private final ActiveDataHandler handler = new ActiveDataHandler();
 
     private DataSource<T> dataSource = DataSource.create();
-    private DataKeyMapper<T> keyMapper;
+    private final DataKeyMapper<T> keyMapper;
 
     private boolean reset = false;
     private final Set<T> updatedData = new HashSet<>();
@@ -189,8 +189,8 @@ public class DataCommunicator<T> extends AbstractExtension {
 
     private Comparator<T> inMemorySorting;
     private SerializablePredicate<T> inMemoryFilter;
-    private List<SortOrder<String>> backEndSorting = new ArrayList<>();
-    private DataCommunicatorClientRpc rpc;
+    private final List<SortOrder<String>> backEndSorting = new ArrayList<>();
+    private final DataCommunicatorClientRpc rpc;
 
     public DataCommunicator() {
         addDataGenerator(handler);
diff --git a/server/src/main/java/com/vaadin/server/widgetsetutils/ClassPathExplorer.java b/server/src/main/java/com/vaadin/server/widgetsetutils/ClassPathExplorer.java
index b5e377fffc..7e7539b865 100644
--- a/server/src/main/java/com/vaadin/server/widgetsetutils/ClassPathExplorer.java
+++ b/server/src/main/java/com/vaadin/server/widgetsetutils/ClassPathExplorer.java
@@ -98,14 +98,14 @@ public class ClassPathExplorer {
      * entries that could include widgets/widgetsets are listed (primarily
      * directories, Vaadin JARs and add-on JARs).
      */
-    private static List<String> rawClasspathEntries = getRawClasspathEntries();
+    private static final List<String> rawClasspathEntries = getRawClasspathEntries();
 
     /**
      * Map from identifiers (either a package name preceded by the path and a
      * slash, or a URL for a JAR file) to the corresponding URLs. This is
      * constructed from the class path.
      */
-    private static Map<String, URL> classpathLocations = getClasspathLocations(
+    private static final Map<String, URL> classpathLocations = getClasspathLocations(
             rawClasspathEntries);
 
     private static boolean debug = false;
diff --git a/server/src/main/java/com/vaadin/ui/AbsoluteLayout.java b/server/src/main/java/com/vaadin/ui/AbsoluteLayout.java
index e968568a09..49d020d32f 100644
--- a/server/src/main/java/com/vaadin/ui/AbsoluteLayout.java
+++ b/server/src/main/java/com/vaadin/ui/AbsoluteLayout.java
@@ -55,12 +55,11 @@ public class AbsoluteLayout extends AbstractLayout
     private static final String ATTR_Z_INDEX = ":z-index";
 
     private final AbsoluteLayoutServerRpc rpc = (MouseEventDetails mouseDetails,
-            Connector clickedConnector) -> {
-        fireEvent(LayoutClickEvent.createEvent(AbsoluteLayout.this,
-                mouseDetails, clickedConnector));
-    };
+            Connector clickedConnector) -> fireEvent(
+                    LayoutClickEvent.createEvent(AbsoluteLayout.this,
+                            mouseDetails, clickedConnector));
     // Maps each component to a position
-    private LinkedHashMap<Component, ComponentPosition> componentToCoordinates = new LinkedHashMap<>();
+    private final LinkedHashMap<Component, ComponentPosition> componentToCoordinates = new LinkedHashMap<>();
 
     /**
      * Creates an AbsoluteLayout with full size.
diff --git a/server/src/main/java/com/vaadin/ui/AbstractColorPicker.java b/server/src/main/java/com/vaadin/ui/AbstractColorPicker.java
index 7f79cecf55..7a40798815 100644
--- a/server/src/main/java/com/vaadin/ui/AbstractColorPicker.java
+++ b/server/src/main/java/com/vaadin/ui/AbstractColorPicker.java
@@ -75,7 +75,7 @@ public abstract class AbstractColorPicker extends AbstractField<Color> {
         /** A simple popup with only the swatches (palette) tab. */
         POPUP_SIMPLE("simple");
 
-        private String style;
+        private final String style;
 
         PopupStyle(String styleName) {
             style = styleName;
diff --git a/server/src/main/java/com/vaadin/ui/AbstractJavaScriptComponent.java b/server/src/main/java/com/vaadin/ui/AbstractJavaScriptComponent.java
index 580754be01..8e70e9cb03 100644
--- a/server/src/main/java/com/vaadin/ui/AbstractJavaScriptComponent.java
+++ b/server/src/main/java/com/vaadin/ui/AbstractJavaScriptComponent.java
@@ -149,7 +149,7 @@ import elemental.json.JsonValue;
  * @since 7.0.0
  */
 public abstract class AbstractJavaScriptComponent extends AbstractComponent {
-    private JavaScriptCallbackHelper callbackHelper = new JavaScriptCallbackHelper(
+    private final JavaScriptCallbackHelper callbackHelper = new JavaScriptCallbackHelper(
             this);
 
     @Override
diff --git a/server/src/main/java/com/vaadin/ui/AbstractOrderedLayout.java b/server/src/main/java/com/vaadin/ui/AbstractOrderedLayout.java
index 6c995611fb..6858a18999 100644
--- a/server/src/main/java/com/vaadin/ui/AbstractOrderedLayout.java
+++ b/server/src/main/java/com/vaadin/ui/AbstractOrderedLayout.java
@@ -45,10 +45,10 @@ public abstract class AbstractOrderedLayout extends AbstractLayout
         LayoutClickNotifier, Layout.MarginHandler {
 
     private final AbstractOrderedLayoutServerRpc rpc = (
-            MouseEventDetails mouseDetails, Connector clickedConnector) -> {
-        fireEvent(LayoutClickEvent.createEvent(AbstractOrderedLayout.this,
-                mouseDetails, clickedConnector));
-    };
+            MouseEventDetails mouseDetails,
+            Connector clickedConnector) -> fireEvent(
+                    LayoutClickEvent.createEvent(AbstractOrderedLayout.this,
+                            mouseDetails, clickedConnector));
 
     public static final Alignment ALIGNMENT_DEFAULT = Alignment.TOP_LEFT;
 
diff --git a/server/src/main/java/com/vaadin/ui/AbstractSplitPanel.java b/server/src/main/java/com/vaadin/ui/AbstractSplitPanel.java
index 73bedc8470..e7a66cebe5 100644
--- a/server/src/main/java/com/vaadin/ui/AbstractSplitPanel.java
+++ b/server/src/main/java/com/vaadin/ui/AbstractSplitPanel.java
@@ -54,7 +54,7 @@ public abstract class AbstractSplitPanel extends AbstractComponentContainer {
     private Unit posMinUnit;
     private Unit posMaxUnit;
 
-    private AbstractSplitPanelRpc rpc = new AbstractSplitPanelRpc() {
+    private final AbstractSplitPanelRpc rpc = new AbstractSplitPanelRpc() {
 
         @Override
         public void splitterClick(MouseEventDetails mouseDetails) {
diff --git a/server/src/main/java/com/vaadin/ui/ConnectorTracker.java b/server/src/main/java/com/vaadin/ui/ConnectorTracker.java
index fd2c0029d1..2ba6f5e895 100644
--- a/server/src/main/java/com/vaadin/ui/ConnectorTracker.java
+++ b/server/src/main/java/com/vaadin/ui/ConnectorTracker.java
@@ -68,8 +68,8 @@ import elemental.json.JsonObject;
 public class ConnectorTracker implements Serializable {
 
     private final HashMap<String, ClientConnector> connectorIdToConnector = new HashMap<>();
-    private Set<ClientConnector> dirtyConnectors = new HashSet<>();
-    private Set<ClientConnector> uninitializedConnectors = new HashSet<>();
+    private final Set<ClientConnector> dirtyConnectors = new HashSet<>();
+    private final Set<ClientConnector> uninitializedConnectors = new HashSet<>();
 
     /**
      * Connectors that have been unregistered and should be cleaned up the next
@@ -80,7 +80,7 @@ public class ConnectorTracker implements Serializable {
 
     private boolean writingResponse = false;
 
-    private UI uI;
+    private final UI uI;
     private transient Map<ClientConnector, JsonObject> diffStates = new HashMap<>();
 
     /** Maps connectorIds to a map of named StreamVariables */
@@ -96,7 +96,7 @@ public class ConnectorTracker implements Serializable {
      * @see #getCurrentSyncId()
      * @see #cleanConcurrentlyRemovedConnectorIds(long)
      */
-    private TreeMap<Integer, Set<String>> syncIdToUnregisteredConnectorIds = new TreeMap<>();
+    private final TreeMap<Integer, Set<String>> syncIdToUnregisteredConnectorIds = new TreeMap<>();
 
     /**
      * Gets a logger for this class
diff --git a/server/src/main/java/com/vaadin/ui/DragAndDropWrapper.java b/server/src/main/java/com/vaadin/ui/DragAndDropWrapper.java
index 7f2e558220..4b9a7afbfb 100644
--- a/server/src/main/java/com/vaadin/ui/DragAndDropWrapper.java
+++ b/server/src/main/java/com/vaadin/ui/DragAndDropWrapper.java
@@ -351,7 +351,7 @@ public class DragAndDropWrapper extends CustomComponent
 
     final class ProxyReceiver implements StreamVariable {
 
-        private String id;
+        private final String id;
         private Html5File file;
 
         public ProxyReceiver(String id, Html5File file) {
@@ -425,7 +425,7 @@ public class DragAndDropWrapper extends CustomComponent
         class ReceivingEventWrapper implements StreamingErrorEvent,
                 StreamingEndEvent, StreamingStartEvent, StreamingProgressEvent {
 
-            private StreamingEvent wrappedEvent;
+            private final StreamingEvent wrappedEvent;
 
             ReceivingEventWrapper(StreamingEvent e) {
                 wrappedEvent = e;
diff --git a/server/src/main/java/com/vaadin/ui/Grid.java b/server/src/main/java/com/vaadin/ui/Grid.java
index b738b71323..63c8fe3e6f 100644
--- a/server/src/main/java/com/vaadin/ui/Grid.java
+++ b/server/src/main/java/com/vaadin/ui/Grid.java
@@ -650,8 +650,8 @@ public class Grid<T> extends AbstractSingleSelect<T> implements HasComponents {
      */
     public static class DetailsManager<T> extends AbstractGridExtension<T> {
 
-        private Set<T> visibleDetails = new HashSet<>();
-        private Map<T, Component> components = new HashMap<>();
+        private final Set<T> visibleDetails = new HashSet<>();
+        private final Map<T, Component> components = new HashMap<>();
         private DetailsGenerator<T> generator;
 
         /**
@@ -1821,17 +1821,17 @@ public class Grid<T> extends AbstractSingleSelect<T> implements HasComponents {
         }
     };
 
-    private Set<Column<T, ?>> columnSet = new LinkedHashSet<>();
-    private Map<String, Column<T, ?>> columnKeys = new HashMap<>();
+    private final Set<Column<T, ?>> columnSet = new LinkedHashSet<>();
+    private final Map<String, Column<T, ?>> columnKeys = new HashMap<>();
 
-    private List<SortOrder<Column<T, ?>>> sortOrder = new ArrayList<>();
-    private DetailsManager<T> detailsManager;
-    private Set<Component> extensionComponents = new HashSet<>();
+    private final List<SortOrder<Column<T, ?>>> sortOrder = new ArrayList<>();
+    private final DetailsManager<T> detailsManager;
+    private final Set<Component> extensionComponents = new HashSet<>();
     private StyleGenerator<T> styleGenerator = item -> null;
     private DescriptionGenerator<T> descriptionGenerator;
 
-    private Header header = new HeaderImpl();
-    private Footer footer = new FooterImpl();
+    private final Header header = new HeaderImpl();
+    private final Footer footer = new FooterImpl();
 
     private int counter = 0;
 
diff --git a/server/src/main/java/com/vaadin/ui/Html5File.java b/server/src/main/java/com/vaadin/ui/Html5File.java
index b3fcdb826f..b4434e75c9 100644
--- a/server/src/main/java/com/vaadin/ui/Html5File.java
+++ b/server/src/main/java/com/vaadin/ui/Html5File.java
@@ -27,10 +27,10 @@ import com.vaadin.server.StreamVariable;
  */
 public class Html5File implements Serializable {
 
-    private String name;
-    private long size;
+    private final String name;
+    private final long size;
     private StreamVariable streamVariable;
-    private String type;
+    private final String type;
 
     Html5File(String name, long size, String mimeType) {
         this.name = name;
diff --git a/server/src/main/java/com/vaadin/ui/LoadingIndicatorConfiguration.java b/server/src/main/java/com/vaadin/ui/LoadingIndicatorConfiguration.java
index 32b9356d76..bd56036aee 100644
--- a/server/src/main/java/com/vaadin/ui/LoadingIndicatorConfiguration.java
+++ b/server/src/main/java/com/vaadin/ui/LoadingIndicatorConfiguration.java
@@ -83,7 +83,7 @@ public interface LoadingIndicatorConfiguration extends Serializable {
 
 class LoadingIndicatorConfigurationImpl
         implements LoadingIndicatorConfiguration {
-    private UI ui;
+    private final UI ui;
 
     public LoadingIndicatorConfigurationImpl(UI ui) {
         this.ui = ui;
diff --git a/server/src/main/java/com/vaadin/ui/LoginForm.java b/server/src/main/java/com/vaadin/ui/LoginForm.java
index 74432d001a..9acd0735b1 100644
--- a/server/src/main/java/com/vaadin/ui/LoginForm.java
+++ b/server/src/main/java/com/vaadin/ui/LoginForm.java
@@ -62,7 +62,7 @@ public class LoginForm extends AbstractSingleComponentContainer {
      */
     public static class LoginEvent extends Component.Event {
 
-        private Map<String, String> params;
+        private final Map<String, String> params;
 
         /**
          * Creates a login event using the given source and the given
diff --git a/server/src/main/java/com/vaadin/ui/Notification.java b/server/src/main/java/com/vaadin/ui/Notification.java
index 536598448b..7ee78fb807 100644
--- a/server/src/main/java/com/vaadin/ui/Notification.java
+++ b/server/src/main/java/com/vaadin/ui/Notification.java
@@ -70,7 +70,7 @@ public class Notification implements Serializable {
          */
         ASSISTIVE_NOTIFICATION("assistive");
 
-        private String style;
+        private final String style;
 
         Type(String style) {
             this.style = style;
diff --git a/server/src/main/java/com/vaadin/ui/NotificationConfiguration.java b/server/src/main/java/com/vaadin/ui/NotificationConfiguration.java
index f7591af13c..0062bb969f 100644
--- a/server/src/main/java/com/vaadin/ui/NotificationConfiguration.java
+++ b/server/src/main/java/com/vaadin/ui/NotificationConfiguration.java
@@ -118,7 +118,7 @@ public interface NotificationConfiguration extends Serializable {
 
 class NotificationConfigurationImpl implements NotificationConfiguration {
 
-    private UI ui;
+    private final UI ui;
 
     public NotificationConfigurationImpl(UI ui) {
         this.ui = ui;
diff --git a/server/src/main/java/com/vaadin/ui/PushConfiguration.java b/server/src/main/java/com/vaadin/ui/PushConfiguration.java
index cad96a5952..bb722ec2d2 100644
--- a/server/src/main/java/com/vaadin/ui/PushConfiguration.java
+++ b/server/src/main/java/com/vaadin/ui/PushConfiguration.java
@@ -165,7 +165,7 @@ public interface PushConfiguration extends Serializable {
 }
 
 class PushConfigurationImpl implements PushConfiguration {
-    private UI ui;
+    private final UI ui;
 
     public PushConfigurationImpl(UI ui) {
         this.ui = ui;
diff --git a/server/src/main/java/com/vaadin/ui/ReconnectDialogConfiguration.java b/server/src/main/java/com/vaadin/ui/ReconnectDialogConfiguration.java
index d4df9fdf28..8a36de3510 100644
--- a/server/src/main/java/com/vaadin/ui/ReconnectDialogConfiguration.java
+++ b/server/src/main/java/com/vaadin/ui/ReconnectDialogConfiguration.java
@@ -132,7 +132,7 @@ public interface ReconnectDialogConfiguration extends Serializable {
 }
 
 class ReconnectDialogConfigurationImpl implements ReconnectDialogConfiguration {
-    private UI ui;
+    private final UI ui;
 
     public ReconnectDialogConfigurationImpl(UI ui) {
         this.ui = ui;
diff --git a/server/src/main/java/com/vaadin/ui/TabSheet.java b/server/src/main/java/com/vaadin/ui/TabSheet.java
index 5623166357..7ad40c2771 100644
--- a/server/src/main/java/com/vaadin/ui/TabSheet.java
+++ b/server/src/main/java/com/vaadin/ui/TabSheet.java
@@ -1101,7 +1101,7 @@ public class TabSheet extends AbstractComponentContainer
      */
     public class TabSheetTabImpl implements Tab {
 
-        private TabState tabState;
+        private final TabState tabState;
 
         private Focusable defaultFocus;
 
diff --git a/server/src/main/java/com/vaadin/ui/TooltipConfiguration.java b/server/src/main/java/com/vaadin/ui/TooltipConfiguration.java
index b5af10961b..1bf9fd7b81 100644
--- a/server/src/main/java/com/vaadin/ui/TooltipConfiguration.java
+++ b/server/src/main/java/com/vaadin/ui/TooltipConfiguration.java
@@ -123,7 +123,7 @@ public interface TooltipConfiguration extends Serializable {
 }
 
 class TooltipConfigurationImpl implements TooltipConfiguration {
-    private UI ui;
+    private final UI ui;
 
     public TooltipConfigurationImpl(UI ui) {
         this.ui = ui;
diff --git a/server/src/main/java/com/vaadin/ui/components/colorpicker/ColorPickerHistory.java b/server/src/main/java/com/vaadin/ui/components/colorpicker/ColorPickerHistory.java
index a32378aeaf..ae1c304fd2 100644
--- a/server/src/main/java/com/vaadin/ui/components/colorpicker/ColorPickerHistory.java
+++ b/server/src/main/java/com/vaadin/ui/components/colorpicker/ColorPickerHistory.java
@@ -40,7 +40,7 @@ public class ColorPickerHistory extends CustomField<Color> {
     private static final int COLUMNS = 15;
 
     /** Temporary color history for when the component is detached. */
-    private ArrayBlockingQueue<Color> tempHistory = new ArrayBlockingQueue<>(
+    private final ArrayBlockingQueue<Color> tempHistory = new ArrayBlockingQueue<>(
             ROWS * COLUMNS);
 
     @Override
diff --git a/server/src/main/java/com/vaadin/ui/components/colorpicker/ColorPickerSelect.java b/server/src/main/java/com/vaadin/ui/components/colorpicker/ColorPickerSelect.java
index 20d68da0fa..ec200082ab 100644
--- a/server/src/main/java/com/vaadin/ui/components/colorpicker/ColorPickerSelect.java
+++ b/server/src/main/java/com/vaadin/ui/components/colorpicker/ColorPickerSelect.java
@@ -38,7 +38,7 @@ public class ColorPickerSelect extends CustomField<Color> {
         ALL("All colors"), RED("Red colors"), GREEN("Green colors"), BLUE(
                 "Blue colors");
 
-        private String caption;
+        private final String caption;
 
         ColorRange(String caption) {
             this.caption = caption;
diff --git a/server/src/main/java/com/vaadin/ui/components/grid/StaticSection.java b/server/src/main/java/com/vaadin/ui/components/grid/StaticSection.java
index eb02be2bbc..678ab2a935 100644
--- a/server/src/main/java/com/vaadin/ui/components/grid/StaticSection.java
+++ b/server/src/main/java/com/vaadin/ui/components/grid/StaticSection.java
@@ -150,8 +150,8 @@ public abstract class StaticSection<ROW extends StaticSection.StaticRow<?>>
      */
     abstract static class StaticCell implements Serializable {
 
-        private CellState cellState = new CellState();
-        private StaticRow<?> row;
+        private final CellState cellState = new CellState();
+        private final StaticRow<?> row;
 
         protected StaticCell(StaticRow<?> row) {
             this.row = row;
diff --git a/server/src/main/java/com/vaadin/ui/declarative/DesignAttributeHandler.java b/server/src/main/java/com/vaadin/ui/declarative/DesignAttributeHandler.java
index 00fb32392c..5b0968bae6 100644
--- a/server/src/main/java/com/vaadin/ui/declarative/DesignAttributeHandler.java
+++ b/server/src/main/java/com/vaadin/ui/declarative/DesignAttributeHandler.java
@@ -430,7 +430,7 @@ public class DesignAttributeHandler implements Serializable {
      * @author Vaadin Ltd
      */
     private static class AttributeCacheEntry implements Serializable {
-        private Map<String, Method[]> accessMethods = new ConcurrentHashMap<>();
+        private final Map<String, Method[]> accessMethods = new ConcurrentHashMap<>();
 
         private void addAttribute(String attribute, Method getter,
                 Method setter) {
diff --git a/server/src/main/java/com/vaadin/ui/declarative/DesignContext.java b/server/src/main/java/com/vaadin/ui/declarative/DesignContext.java
index c8e794c46a..761c7edd04 100644
--- a/server/src/main/java/com/vaadin/ui/declarative/DesignContext.java
+++ b/server/src/main/java/com/vaadin/ui/declarative/DesignContext.java
@@ -686,9 +686,9 @@ public class DesignContext implements Serializable {
      * @author Vaadin Ltd
      */
     public class ComponentCreatedEvent implements Serializable {
-        private String localId;
-        private Component component;
-        private DesignContext context;
+        private final String localId;
+        private final Component component;
+        private final DesignContext context;
 
         /**
          * Creates a new instance of ComponentCreatedEvent
diff --git a/server/src/main/java/com/vaadin/ui/declarative/converters/DesignEnumConverter.java b/server/src/main/java/com/vaadin/ui/declarative/converters/DesignEnumConverter.java
index 93ccee3f37..a32c68998f 100644
--- a/server/src/main/java/com/vaadin/ui/declarative/converters/DesignEnumConverter.java
+++ b/server/src/main/java/com/vaadin/ui/declarative/converters/DesignEnumConverter.java
@@ -33,7 +33,7 @@ import com.vaadin.ui.declarative.DesignAttributeHandler;
 public class DesignEnumConverter<T extends Enum>
         implements Converter<String, T> {
 
-    private Class<T> type;
+    private final Class<T> type;
 
     /**
      * Creates a converter for the given enum type.
diff --git a/server/src/main/java/com/vaadin/ui/declarative/converters/DesignResourceConverter.java b/server/src/main/java/com/vaadin/ui/declarative/converters/DesignResourceConverter.java
index ef6e10ef5c..c03d3750d1 100644
--- a/server/src/main/java/com/vaadin/ui/declarative/converters/DesignResourceConverter.java
+++ b/server/src/main/java/com/vaadin/ui/declarative/converters/DesignResourceConverter.java
@@ -170,7 +170,7 @@ public class DesignResourceConverter implements Converter<String, Resource> {
             return ((ExternalResource) value).getURL();
         }
 
-        private static Map<Class<? extends Resource>, ResourceConverterByProtocol> typeToConverter = new HashMap<>();
+        private static final Map<Class<? extends Resource>, ResourceConverterByProtocol> typeToConverter = new HashMap<>();
         static {
             typeToConverter.put(ExternalResource.class, HTTP);
             // ^ any of non-specialized would actually work
diff --git a/server/src/main/java/com/vaadin/ui/renderers/ClickableRenderer.java b/server/src/main/java/com/vaadin/ui/renderers/ClickableRenderer.java
index 3073517b65..8686c391c5 100644
--- a/server/src/main/java/com/vaadin/ui/renderers/ClickableRenderer.java
+++ b/server/src/main/java/com/vaadin/ui/renderers/ClickableRenderer.java
@@ -70,8 +70,8 @@ public abstract class ClickableRenderer<T, V> extends AbstractRenderer<T, V> {
      */
     public static class RendererClickEvent<T> extends ClickEvent {
 
-        private T item;
-        private Column column;
+        private final T item;
+        private final Column column;
 
         protected RendererClickEvent(Grid<T> source, T item, Column column,
                 MouseEventDetails mouseEventDetails) {
diff --git a/server/src/main/java/com/vaadin/util/CurrentInstance.java b/server/src/main/java/com/vaadin/util/CurrentInstance.java
index f8d6af2a4c..43b158a4ff 100644
--- a/server/src/main/java/com/vaadin/util/CurrentInstance.java
+++ b/server/src/main/java/com/vaadin/util/CurrentInstance.java
@@ -64,7 +64,7 @@ public class CurrentInstance implements Serializable {
     private final WeakReference<Object> instance;
     private final boolean inheritable;
 
-    private static InheritableThreadLocal<Map<Class<?>, CurrentInstance>> instances = new InheritableThreadLocal<Map<Class<?>, CurrentInstance>>() {
+    private static final InheritableThreadLocal<Map<Class<?>, CurrentInstance>> instances = new InheritableThreadLocal<Map<Class<?>, CurrentInstance>>() {
         @Override
         protected Map<Class<?>, CurrentInstance> childValue(
                 Map<Class<?>, CurrentInstance> parentValue) {
diff --git a/server/src/main/java/com/vaadin/util/FileTypeResolver.java b/server/src/main/java/com/vaadin/util/FileTypeResolver.java
index 4df2785ce8..4cbac69df5 100644
--- a/server/src/main/java/com/vaadin/util/FileTypeResolver.java
+++ b/server/src/main/java/com/vaadin/util/FileTypeResolver.java
@@ -53,7 +53,7 @@ public class FileTypeResolver implements Serializable {
     /**
      * Initial file extension to mime-type mapping.
      */
-    static private String initialExtToMIMEMap = "application/cu-seeme                            csm cu,"
+    private static final String initialExtToMIMEMap = "application/cu-seeme                            csm cu,"
             + "application/dsptype                             tsp,"
             + "application/futuresplash                        spl,"
             + "application/mac-binhex40                        hqx,"
diff --git a/server/src/main/java/com/vaadin/util/SerializerHelper.java b/server/src/main/java/com/vaadin/util/SerializerHelper.java
index 5b6b06c0c1..41ef150d51 100644
--- a/server/src/main/java/com/vaadin/util/SerializerHelper.java
+++ b/server/src/main/java/com/vaadin/util/SerializerHelper.java
@@ -105,7 +105,7 @@ public class SerializerHelper {
      * List of primitive classes. Google App Engine has problems
      * serializing/deserializing these (#3064).
      */
-    private static Class<?>[] primitiveClasses = new Class<?>[] { byte.class,
+    private static final Class<?>[] primitiveClasses = new Class<?>[] { byte.class,
             short.class, int.class, long.class, float.class, double.class,
             boolean.class, char.class };
 
diff --git a/server/src/test/java/com/vaadin/data/BinderMultiSelectTest.java b/server/src/test/java/com/vaadin/data/BinderMultiSelectTest.java
index 3b35f86144..f59c89e1d8 100644
--- a/server/src/test/java/com/vaadin/data/BinderMultiSelectTest.java
+++ b/server/src/test/java/com/vaadin/data/BinderMultiSelectTest.java
@@ -54,7 +54,7 @@ public class BinderMultiSelectTest
         }
     }
 
-    private Binder<AtomicReference<String>> converterBinder = new Binder<>();
+    private final Binder<AtomicReference<String>> converterBinder = new Binder<>();
 
     private CheckBoxGroup<TestEnum> select;
 
diff --git a/server/src/test/java/com/vaadin/data/Jsr303Test.java b/server/src/test/java/com/vaadin/data/Jsr303Test.java
index da30f56743..016018a6b3 100644
--- a/server/src/test/java/com/vaadin/data/Jsr303Test.java
+++ b/server/src/test/java/com/vaadin/data/Jsr303Test.java
@@ -78,7 +78,7 @@ public class Jsr303Test {
 
     public static class Jsr303UnitTest implements UnitTest {
 
-        private TextField nameField = new TextField();
+        private final TextField nameField = new TextField();
 
         @Override
         public void execute() {
diff --git a/server/src/test/java/com/vaadin/data/validator/StringLengthValidatorTest.java b/server/src/test/java/com/vaadin/data/validator/StringLengthValidatorTest.java
index dd2b1be50b..7fb1c2ff76 100644
--- a/server/src/test/java/com/vaadin/data/validator/StringLengthValidatorTest.java
+++ b/server/src/test/java/com/vaadin/data/validator/StringLengthValidatorTest.java
@@ -7,7 +7,7 @@ import org.junit.Test;
 
 public class StringLengthValidatorTest extends ValidatorTestBase {
 
-    private static String LONG_STRING = Stream.generate(() -> "x").limit(1000)
+    private static final String LONG_STRING = Stream.generate(() -> "x").limit(1000)
             .collect(Collectors.joining());
 
     @Test
diff --git a/server/src/test/java/com/vaadin/server/AbstractDeploymentConfigurationTest.java b/server/src/test/java/com/vaadin/server/AbstractDeploymentConfigurationTest.java
index c274bf35c3..2d4e7fad0a 100644
--- a/server/src/test/java/com/vaadin/server/AbstractDeploymentConfigurationTest.java
+++ b/server/src/test/java/com/vaadin/server/AbstractDeploymentConfigurationTest.java
@@ -95,7 +95,7 @@ public class AbstractDeploymentConfigurationTest {
     private static class DeploymentConfigImpl
             extends AbstractDeploymentConfiguration {
 
-        private Properties properties;
+        private final Properties properties;
 
         DeploymentConfigImpl(Properties props) {
             properties = props;
diff --git a/server/src/test/java/com/vaadin/server/DownloadStreamTest.java b/server/src/test/java/com/vaadin/server/DownloadStreamTest.java
index f302163ef7..81f45f6173 100644
--- a/server/src/test/java/com/vaadin/server/DownloadStreamTest.java
+++ b/server/src/test/java/com/vaadin/server/DownloadStreamTest.java
@@ -13,7 +13,7 @@ import org.junit.Before;
 import org.junit.Test;
 
 public class DownloadStreamTest {
-    private String filename = "日本語.png";
+    private final String filename = "日本語.png";
     private DownloadStream stream;
 
     @Before
diff --git a/server/src/test/java/com/vaadin/server/MockVaadinSession.java b/server/src/test/java/com/vaadin/server/MockVaadinSession.java
index 04d516ecb4..07854560b3 100644
--- a/server/src/test/java/com/vaadin/server/MockVaadinSession.java
+++ b/server/src/test/java/com/vaadin/server/MockVaadinSession.java
@@ -66,5 +66,5 @@ public class MockVaadinSession extends VaadinSession {
 
     private int closeCount;
 
-    private ReentrantLock lock = new ReentrantLock();
+    private final ReentrantLock lock = new ReentrantLock();
 }
diff --git a/server/src/test/java/com/vaadin/server/communication/FileUploadHandlerTest.java b/server/src/test/java/com/vaadin/server/communication/FileUploadHandlerTest.java
index 3726ed7cea..60b2b2a02d 100644
--- a/server/src/test/java/com/vaadin/server/communication/FileUploadHandlerTest.java
+++ b/server/src/test/java/com/vaadin/server/communication/FileUploadHandlerTest.java
@@ -58,7 +58,7 @@ public class FileUploadHandlerTest {
     @Mock
     private OutputStream responseOutput;
 
-    private int uiId = 123;
+    private final int uiId = 123;
     private final String connectorId = "connectorId";
     private final String variableName = "name";
     private final String expectedSecurityKey = "key";
diff --git a/server/src/test/java/com/vaadin/server/data/DataCommunicatorTest.java b/server/src/test/java/com/vaadin/server/data/DataCommunicatorTest.java
index b82d571f9b..c9a30fa517 100644
--- a/server/src/test/java/com/vaadin/server/data/DataCommunicatorTest.java
+++ b/server/src/test/java/com/vaadin/server/data/DataCommunicatorTest.java
@@ -85,7 +85,7 @@ public class DataCommunicatorTest {
         }
     }
 
-    private MockVaadinSession session = new MockVaadinSession(
+    private final MockVaadinSession session = new MockVaadinSession(
             Mockito.mock(VaadinService.class));
 
     @Test
diff --git a/server/src/test/java/com/vaadin/server/data/datasource/StrBean.java b/server/src/test/java/com/vaadin/server/data/datasource/StrBean.java
index 24f478ad47..44650d63bb 100644
--- a/server/src/test/java/com/vaadin/server/data/datasource/StrBean.java
+++ b/server/src/test/java/com/vaadin/server/data/datasource/StrBean.java
@@ -7,7 +7,7 @@ import java.util.Random;
 
 class StrBean implements Serializable {
 
-    private static String[] values = new String[] { "Foo", "Bar", "Baz" };
+    private static final String[] values = new String[] { "Foo", "Bar", "Baz" };
 
     private String value;
     private final int id;
diff --git a/server/src/test/java/com/vaadin/tests/data/bean/AnotherTestEnum.java b/server/src/test/java/com/vaadin/tests/data/bean/AnotherTestEnum.java
index fc8f22a947..934a2ce696 100644
--- a/server/src/test/java/com/vaadin/tests/data/bean/AnotherTestEnum.java
+++ b/server/src/test/java/com/vaadin/tests/data/bean/AnotherTestEnum.java
@@ -3,7 +3,7 @@ package com.vaadin.tests.data.bean;
 public enum AnotherTestEnum {
     ONE("ONE"), TWO("TWO");
 
-    private String id;
+    private final String id;
 
     private AnotherTestEnum(String id) {
         this.id = id;
diff --git a/server/src/test/java/com/vaadin/tests/data/bean/BeanToValidate.java b/server/src/test/java/com/vaadin/tests/data/bean/BeanToValidate.java
index 9f0b244360..23270cab5e 100644
--- a/server/src/test/java/com/vaadin/tests/data/bean/BeanToValidate.java
+++ b/server/src/test/java/com/vaadin/tests/data/bean/BeanToValidate.java
@@ -42,7 +42,7 @@ public class BeanToValidate {
     @Valid
     private Address address;
 
-    private String readOnlyProperty = "READONLY DATA";
+    private final String readOnlyProperty = "READONLY DATA";
 
     private String writeOnlyProperty;
 
diff --git a/server/src/test/java/com/vaadin/tests/data/bean/Country.java b/server/src/test/java/com/vaadin/tests/data/bean/Country.java
index 159333212f..c7795d5f6d 100644
--- a/server/src/test/java/com/vaadin/tests/data/bean/Country.java
+++ b/server/src/test/java/com/vaadin/tests/data/bean/Country.java
@@ -5,7 +5,7 @@ public enum Country {
     FINLAND("Finland"), SWEDEN("Sweden"), USA("USA"), RUSSIA(
             "Russia"), NETHERLANDS("Netherlands"), SOUTH_AFRICA("South Africa");
 
-    private String name;
+    private final String name;
 
     private Country(String name) {
         this.name = name;
diff --git a/server/src/test/java/com/vaadin/tests/data/bean/Sex.java b/server/src/test/java/com/vaadin/tests/data/bean/Sex.java
index a4e3f20a11..489588c640 100644
--- a/server/src/test/java/com/vaadin/tests/data/bean/Sex.java
+++ b/server/src/test/java/com/vaadin/tests/data/bean/Sex.java
@@ -3,7 +3,7 @@ package com.vaadin.tests.data.bean;
 public enum Sex {
     MALE("Male"), FEMALE("Female"), UNKNOWN("Unknown");
 
-    private String stringRepresentation;
+    private final String stringRepresentation;
 
     private Sex(String stringRepresentation) {
         this.stringRepresentation = stringRepresentation;
diff --git a/server/src/test/java/com/vaadin/tests/data/bean/TestEnum.java b/server/src/test/java/com/vaadin/tests/data/bean/TestEnum.java
index bf6f721052..f79298b6ad 100644
--- a/server/src/test/java/com/vaadin/tests/data/bean/TestEnum.java
+++ b/server/src/test/java/com/vaadin/tests/data/bean/TestEnum.java
@@ -3,7 +3,7 @@ package com.vaadin.tests.data.bean;
 public enum TestEnum {
     ONE("1"), TWO("2");
 
-    private String id;
+    private final String id;
 
     private TestEnum(String id) {
         this.id = id;
diff --git a/server/src/test/java/com/vaadin/tests/data/converter/StringToBooleanConverterTest.java b/server/src/test/java/com/vaadin/tests/data/converter/StringToBooleanConverterTest.java
index 4de05b9f09..d3d539ae8f 100644
--- a/server/src/test/java/com/vaadin/tests/data/converter/StringToBooleanConverterTest.java
+++ b/server/src/test/java/com/vaadin/tests/data/converter/StringToBooleanConverterTest.java
@@ -21,7 +21,7 @@ public class StringToBooleanConverterTest extends AbstractStringConverterTest {
             getErrorMessage(), "yes", "no");
     private StringToBooleanConverter emptyTrueConverter = new StringToBooleanConverter(
             getErrorMessage(), "", "ABSENT");
-    private StringToBooleanConverter localeConverter = new StringToBooleanConverter(
+    private final StringToBooleanConverter localeConverter = new StringToBooleanConverter(
             getErrorMessage()) {
         @Override
         public String getFalseString(Locale locale) {
diff --git a/server/src/test/java/com/vaadin/tests/design/DeclarativeTestBase.java b/server/src/test/java/com/vaadin/tests/design/DeclarativeTestBase.java
index 2b73408abc..3e8e4dccd6 100644
--- a/server/src/test/java/com/vaadin/tests/design/DeclarativeTestBase.java
+++ b/server/src/test/java/com/vaadin/tests/design/DeclarativeTestBase.java
@@ -31,7 +31,7 @@ import com.vaadin.ui.Flash;
 public abstract class DeclarativeTestBase<T extends Component>
         extends DeclarativeTestBaseBase<T> {
 
-    private static boolean debug = false;
+    private static final boolean debug = false;
 
     private final Map<Class<?>, EqualsAsserter<?>> comparators = new HashMap<>();
     private static final EqualsAsserter standardEqualsComparator = (EqualsAsserter<Object>) Assert::assertEquals;
diff --git a/server/src/test/java/com/vaadin/tests/design/designroot/ExtendedDesignWithAnnotation.java b/server/src/test/java/com/vaadin/tests/design/designroot/ExtendedDesignWithAnnotation.java
index 28d419b27e..b1f6297d9e 100644
--- a/server/src/test/java/com/vaadin/tests/design/designroot/ExtendedDesignWithAnnotation.java
+++ b/server/src/test/java/com/vaadin/tests/design/designroot/ExtendedDesignWithAnnotation.java
@@ -18,7 +18,7 @@ package com.vaadin.tests.design.designroot;
 import com.vaadin.ui.TextField;
 
 public class ExtendedDesignWithAnnotation extends DesignWithAnnotation {
-    private TextField customField = new TextField();
+    private final TextField customField = new TextField();
 
     public ExtendedDesignWithAnnotation() {
         customField.setPlaceholder("Something");
diff --git a/server/src/test/java/com/vaadin/tests/design/designroot/ExtendedDesignWithEmptyAnnotation.java b/server/src/test/java/com/vaadin/tests/design/designroot/ExtendedDesignWithEmptyAnnotation.java
index ea23f847cf..cfea7a6fda 100644
--- a/server/src/test/java/com/vaadin/tests/design/designroot/ExtendedDesignWithEmptyAnnotation.java
+++ b/server/src/test/java/com/vaadin/tests/design/designroot/ExtendedDesignWithEmptyAnnotation.java
@@ -23,7 +23,7 @@ import com.vaadin.ui.TextField;
 public class ExtendedDesignWithEmptyAnnotation
         extends DesignWithEmptyAnnotation {
 
-    private TextField customField = new TextField();
+    private final TextField customField = new TextField();
 
     public ExtendedDesignWithEmptyAnnotation() {
         super();
diff --git a/server/src/test/java/com/vaadin/tests/design/nested/NestedCustomLayoutsTest.java b/server/src/test/java/com/vaadin/tests/design/nested/NestedCustomLayoutsTest.java
index 6479e1995a..341397ff81 100644
--- a/server/src/test/java/com/vaadin/tests/design/nested/NestedCustomLayoutsTest.java
+++ b/server/src/test/java/com/vaadin/tests/design/nested/NestedCustomLayoutsTest.java
@@ -49,7 +49,7 @@ import com.vaadin.ui.declarative.Design;
  */
 public class NestedCustomLayoutsTest {
 
-    private static String PACKAGE_MAPPING = "com_vaadin_tests_design_nested_customlayouts:com.vaadin.tests.design.nested.customlayouts";
+    private static final String PACKAGE_MAPPING = "com_vaadin_tests_design_nested_customlayouts:com.vaadin.tests.design.nested.customlayouts";
 
     @Test
     public void testNestedLayouts() throws IOException {
diff --git a/server/src/test/java/com/vaadin/tests/server/ClassesSerializableTest.java b/server/src/test/java/com/vaadin/tests/server/ClassesSerializableTest.java
index 865856f79c..e33f8373ce 100644
--- a/server/src/test/java/com/vaadin/tests/server/ClassesSerializableTest.java
+++ b/server/src/test/java/com/vaadin/tests/server/ClassesSerializableTest.java
@@ -38,11 +38,11 @@ public class ClassesSerializableTest {
      * JARs that will be scanned for classes to test, in addition to classpath
      * directories.
      */
-    private static String JAR_PATTERN = ".*vaadin.*\\.jar";
+    private static final String JAR_PATTERN = ".*vaadin.*\\.jar";
 
-    private static String[] BASE_PACKAGES = { "com.vaadin" };
+    private static final String[] BASE_PACKAGES = { "com.vaadin" };
 
-    private static String[] EXCLUDED_PATTERNS = { "com\\.vaadin\\.demo\\..*", //
+    private static final String[] EXCLUDED_PATTERNS = { "com\\.vaadin\\.demo\\..*", //
             "com\\.vaadin\\.external\\.org\\.apache\\.commons\\.fileupload\\..*", //
             "com\\.vaadin\\.launcher\\..*", //
             "com\\.vaadin\\.client\\..*", //
diff --git a/server/src/test/java/com/vaadin/tests/server/CsrfTokenMissingTest.java b/server/src/test/java/com/vaadin/tests/server/CsrfTokenMissingTest.java
index 22d2fb283b..2f4ba832d3 100644
--- a/server/src/test/java/com/vaadin/tests/server/CsrfTokenMissingTest.java
+++ b/server/src/test/java/com/vaadin/tests/server/CsrfTokenMissingTest.java
@@ -184,7 +184,7 @@ public class CsrfTokenMissingTest {
                 rpcRequest.getCsrfToken());
     }
 
-    private static Logger LOGGER = Logger
+    private static final Logger LOGGER = Logger
             .getLogger(CsrfTokenMissingTest.class.getName());
     static {
         LOGGER.setLevel(Level.ALL);
diff --git a/server/src/test/java/com/vaadin/tests/server/clientconnector/AttachDetachListenersTest.java b/server/src/test/java/com/vaadin/tests/server/clientconnector/AttachDetachListenersTest.java
index b870e0ab4f..7a44e861da 100644
--- a/server/src/test/java/com/vaadin/tests/server/clientconnector/AttachDetachListenersTest.java
+++ b/server/src/test/java/com/vaadin/tests/server/clientconnector/AttachDetachListenersTest.java
@@ -151,7 +151,7 @@ public class AttachDetachListenersTest {
     public static class EventEquals<E extends ConnectorEvent>
             implements IArgumentMatcher {
 
-        private E expected;
+        private final E expected;
 
         public EventEquals(E expected) {
             this.expected = expected;
diff --git a/server/src/test/java/com/vaadin/tests/server/component/StateGetDoesNotMarkDirtyTest.java b/server/src/test/java/com/vaadin/tests/server/component/StateGetDoesNotMarkDirtyTest.java
index 57647f1e6a..70b7b0b77c 100644
--- a/server/src/test/java/com/vaadin/tests/server/component/StateGetDoesNotMarkDirtyTest.java
+++ b/server/src/test/java/com/vaadin/tests/server/component/StateGetDoesNotMarkDirtyTest.java
@@ -19,7 +19,7 @@ import com.vaadin.ui.UI;
 
 public class StateGetDoesNotMarkDirtyTest {
 
-    private Set<String> excludedMethods = new HashSet<>();
+    private final Set<String> excludedMethods = new HashSet<>();
 
     @Before
     public void setUp() {
diff --git a/server/src/test/java/com/vaadin/tests/server/component/csslayout/AddComponentsTest.java b/server/src/test/java/com/vaadin/tests/server/component/csslayout/AddComponentsTest.java
index 4da1b1380a..9fe1e4210a 100644
--- a/server/src/test/java/com/vaadin/tests/server/component/csslayout/AddComponentsTest.java
+++ b/server/src/test/java/com/vaadin/tests/server/component/csslayout/AddComponentsTest.java
@@ -16,7 +16,7 @@ import com.vaadin.ui.Layout;
 
 public class AddComponentsTest {
 
-    private Component[] children = new Component[] { new Label("A"),
+    private final Component[] children = new Component[] { new Label("A"),
             new Label("B"), new Label("C"), new Label("D") };
 
     @Test
diff --git a/server/src/test/java/com/vaadin/tests/server/component/customlayout/CustomLayoutTest.java b/server/src/test/java/com/vaadin/tests/server/component/customlayout/CustomLayoutTest.java
index d4880a3d2e..9d60628e12 100644
--- a/server/src/test/java/com/vaadin/tests/server/component/customlayout/CustomLayoutTest.java
+++ b/server/src/test/java/com/vaadin/tests/server/component/customlayout/CustomLayoutTest.java
@@ -142,6 +142,6 @@ public class CustomLayoutTest {
 
         private int readCount;
         private boolean isClosed;
-        private int maxArrayLength;
+        private final int maxArrayLength;
     }
 }
diff --git a/server/src/test/java/com/vaadin/tests/server/component/menubar/MenuBarIdsTest.java b/server/src/test/java/com/vaadin/tests/server/component/menubar/MenuBarIdsTest.java
index f42b9a5683..bd8c2dd443 100644
--- a/server/src/test/java/com/vaadin/tests/server/component/menubar/MenuBarIdsTest.java
+++ b/server/src/test/java/com/vaadin/tests/server/component/menubar/MenuBarIdsTest.java
@@ -23,7 +23,7 @@ public class MenuBarIdsTest implements Command {
     private MenuItem menuFileOpen;
     private MenuItem menuFileSave;
     private MenuItem menuFileExit;
-    private Set<MenuItem> menuItems = new HashSet<>();
+    private final Set<MenuItem> menuItems = new HashSet<>();
 
     private MenuBar menuBar;
 
diff --git a/server/src/test/java/com/vaadin/tests/server/component/ui/CustomUIClassLoaderTest.java b/server/src/test/java/com/vaadin/tests/server/component/ui/CustomUIClassLoaderTest.java
index f7a78b34ec..4e4f5160ba 100644
--- a/server/src/test/java/com/vaadin/tests/server/component/ui/CustomUIClassLoaderTest.java
+++ b/server/src/test/java/com/vaadin/tests/server/component/ui/CustomUIClassLoaderTest.java
@@ -37,7 +37,7 @@ public class CustomUIClassLoaderTest {
      */
     public class LoggingClassLoader extends ClassLoader {
 
-        private List<String> requestedClasses = new ArrayList<>();
+        private final List<String> requestedClasses = new ArrayList<>();
 
         @Override
         protected synchronized Class<?> loadClass(String name, boolean resolve)
diff --git a/server/src/test/java/com/vaadin/tests/server/component/window/AttachDetachWindowTest.java b/server/src/test/java/com/vaadin/tests/server/component/window/AttachDetachWindowTest.java
index 5d0c0cc55d..872683cec3 100644
--- a/server/src/test/java/com/vaadin/tests/server/component/window/AttachDetachWindowTest.java
+++ b/server/src/test/java/com/vaadin/tests/server/component/window/AttachDetachWindowTest.java
@@ -21,7 +21,7 @@ import com.vaadin.ui.Window;
 
 public class AttachDetachWindowTest {
 
-    private VaadinSession testApp = new AlwaysLockedVaadinSession(null);
+    private final VaadinSession testApp = new AlwaysLockedVaadinSession(null);
 
     private interface TestContainer {
         public boolean attachCalled();
@@ -36,7 +36,7 @@ public class AttachDetachWindowTest {
     private class TestWindow extends Window implements TestContainer {
         boolean windowAttachCalled = false;
         boolean windowDetachCalled = false;
-        private TestContent testContent = new TestContent();
+        private final TestContent testContent = new TestContent();
 
         TestWindow() {
             setContent(testContent);
@@ -81,7 +81,7 @@ public class AttachDetachWindowTest {
         boolean contentAttachCalled = false;
         boolean childAttachCalled = false;
 
-        private Label child = new Label() {
+        private final Label child = new Label() {
             @Override
             public void attach() {
                 super.attach();
@@ -115,7 +115,7 @@ public class AttachDetachWindowTest {
     private class TestUI extends UI implements TestContainer {
         boolean rootAttachCalled = false;
         boolean rootDetachCalled = false;
-        private TestContent testContent = new TestContent();
+        private final TestContent testContent = new TestContent();
 
         public TestUI() {
             setContent(testContent);
diff --git a/server/src/test/java/com/vaadin/tests/server/navigator/NavigatorTest.java b/server/src/test/java/com/vaadin/tests/server/navigator/NavigatorTest.java
index be3f402e16..20271defe5 100644
--- a/server/src/test/java/com/vaadin/tests/server/navigator/NavigatorTest.java
+++ b/server/src/test/java/com/vaadin/tests/server/navigator/NavigatorTest.java
@@ -217,7 +217,7 @@ public class NavigatorTest {
             return page;
         }
 
-        private Page page;
+        private final Page page;
     }
 
     private static class TestPage extends Page {
diff --git a/server/src/test/java/com/vaadin/tests/util/MockDeploymentConfiguration.java b/server/src/test/java/com/vaadin/tests/util/MockDeploymentConfiguration.java
index e4665adf52..4f6ea6625d 100644
--- a/server/src/test/java/com/vaadin/tests/util/MockDeploymentConfiguration.java
+++ b/server/src/test/java/com/vaadin/tests/util/MockDeploymentConfiguration.java
@@ -17,10 +17,10 @@ public class MockDeploymentConfiguration
     private int heartbeatInterval = 300;
     private boolean closeIdleSessions = false;
     private PushMode pushMode = PushMode.DISABLED;
-    private Properties initParameters = new Properties();
-    private Map<String, String> applicationOrSystemProperty = new HashMap<>();
+    private final Properties initParameters = new Properties();
+    private final Map<String, String> applicationOrSystemProperty = new HashMap<>();
     private boolean syncIdCheckEnabled = true;
-    private boolean sendUrlsAsParameters = true;
+    private final boolean sendUrlsAsParameters = true;
 
     @Override
     public boolean isProductionMode() {
diff --git a/server/src/test/java/com/vaadin/ui/declarative/DesignTest.java b/server/src/test/java/com/vaadin/ui/declarative/DesignTest.java
index 2b39cc7c51..769a02dedc 100644
--- a/server/src/test/java/com/vaadin/ui/declarative/DesignTest.java
+++ b/server/src/test/java/com/vaadin/ui/declarative/DesignTest.java
@@ -43,9 +43,9 @@ public class DesignTest {
     private static final Charset CP1251_CHARSET = Charset.forName("cp1251");
     private static final Charset UTF8_CHARSET = StandardCharsets.UTF_8;
 
-    private static String NON_ASCII_STRING = "\u043C";
+    private static final String NON_ASCII_STRING = "\u043C";
 
-    private static Charset DEFAULT_CHARSET = Charset.defaultCharset();
+    private static final Charset DEFAULT_CHARSET = Charset.defaultCharset();
 
     @AfterClass
     public static void restoreCharset()