Improve message when session is already set (#9989)

Change-Id: If75868863999f61d014402eb98c74a142dc22c4a

1 files changed, 13 insertions, 1 deletions

diff --git a/server/src/com/vaadin/ui/UI.java b/server/src/com/vaadin/ui/UI.java
index 2878d10fa1..d57d38b371 100644
--- a/server/src/com/vaadin/ui/UI.java
+++ b/server/src/com/vaadin/ui/UI.java
@@ -709,7 +709,10 @@ public abstract class UI extends AbstractComponentContainer implements
     public void setSession(VaadinServiceSession session) {
         if ((session == null) == (this.session == null)) {
             throw new IllegalStateException(
-                    "VaadinSession has already been set");
+                    "VaadinServiceSession has already been set. Old session: "
+                            + getSessionDetails(this.session)
+                            + ". New session: " + getSessionDetails(session)
+                            + ".");
         } else {
             if (session == null) {
                 detach();
@@ -722,6 +725,15 @@ public abstract class UI extends AbstractComponentContainer implements
         }
     }
 
+    private static String getSessionDetails(VaadinServiceSession session) {
+        if (session == null) {
+            return null;
+        } else {
+            return session.toString() + " for "
+                    + session.getService().getServiceName();
+        }
+    }
+
     /**
      * Gets the id of the UI, used to identify this UI within its application
      * when processing requests. The UI id should be present in every request to