3 files changed, 13 insertions, 3 deletions
diff --git a/server/src/com/vaadin/server/AbstractClientConnector.java b/server/src/com/vaadin/server/AbstractClientConnector.java
index 4c22a96782..2fb468bd1c 100644
--- a/server/src/com/vaadin/server/AbstractClientConnector.java
+++ b/server/src/com/vaadin/server/AbstractClientConnector.java
@@ -146,6 +146,8 @@ public abstract class AbstractClientConnector implements ClientConnector {
     }
 
     protected SharedState getState() {
+        assert getSession() == null
+                || getSession().getLock().isHeldByCurrentThread() : VaadinSession.SESSION_LOCK_MESSAGE;
         if (null == sharedState) {
             sharedState = createState();
         }
diff --git a/server/src/com/vaadin/server/VaadinSession.java b/server/src/com/vaadin/server/VaadinSession.java
index a91c011ddf..03e6420008 100644
--- a/server/src/com/vaadin/server/VaadinSession.java
+++ b/server/src/com/vaadin/server/VaadinSession.java
@@ -28,7 +28,6 @@ import java.util.LinkedList;
 import java.util.List;
 import java.util.Locale;
 import java.util.Map;
-import java.util.concurrent.locks.Lock;
 import java.util.concurrent.locks.ReentrantLock;
 import java.util.logging.Logger;
 
@@ -71,6 +70,8 @@ public class VaadinSession implements HttpSessionBindingListener, Serializable {
      */
     public static final String UI_PARAMETER = "UI";
 
+    public static final String SESSION_LOCK_MESSAGE = "You are accessing UI state without proper synchronization!";
+
     private static final Method BOOTSTRAP_FRAGMENT_METHOD = ReflectTools
             .findMethod(BootstrapListener.class, "modifyBootstrapFragment",
                     BootstrapFragmentResponse.class);
@@ -78,7 +79,7 @@ public class VaadinSession implements HttpSessionBindingListener, Serializable {
             .findMethod(BootstrapListener.class, "modifyBootstrapPage",
                     BootstrapPageResponse.class);
 
-    private final Lock lock = new ReentrantLock();
+    private final ReentrantLock lock = new ReentrantLock();
 
     /**
      * An event sent to {@link #start(SessionStartEvent)} when a new Application
@@ -206,6 +207,10 @@ public class VaadinSession implements HttpSessionBindingListener, Serializable {
 
     private final Map<String, Object> attributes = new HashMap<String, Object>();
 
+    public VaadinSession() {
+        // TODO Auto-generated constructor stub
+    }
+
     /**
      * @see javax.servlet.http.HttpSessionBindingListener#valueBound(HttpSessionBindingEvent)
      */
@@ -1304,7 +1309,7 @@ public class VaadinSession implements HttpSessionBindingListener, Serializable {
      * 
      * @return the lock that should be used for synchronization
      */
-    public Lock getLock() {
+    public ReentrantLock getLock() {
         return lock;
     }
 
diff --git a/server/src/com/vaadin/ui/ConnectorTracker.java b/server/src/com/vaadin/ui/ConnectorTracker.java
index d454df98ee..dbada63a47 100644
--- a/server/src/com/vaadin/ui/ConnectorTracker.java
+++ b/server/src/com/vaadin/ui/ConnectorTracker.java
@@ -29,6 +29,7 @@ import com.vaadin.server.AbstractClientConnector;
 import com.vaadin.server.AbstractCommunicationManager;
 import com.vaadin.server.ClientConnector;
 import com.vaadin.server.GlobalResourceHandler;
+import com.vaadin.server.VaadinSession;
 
 /**
  * A class which takes care of book keeping of {@link ClientConnector}s for a
@@ -286,6 +287,8 @@ public class ConnectorTracker implements Serializable {
      *            The connector that should be marked clean.
      */
     public void markDirty(ClientConnector connector) {
+        assert uI.getSession() == null
+                || uI.getSession().getLock().isHeldByCurrentThread() : VaadinSession.SESSION_LOCK_MESSAGE;
         if (isWritingResponse()) {
             throw new IllegalStateException(
                     "A connector should not be marked as dirty while a response is being written.");