From 7cab7fd80f85b9a7846b1d771aa3dce592ae5d42 Mon Sep 17 00:00:00 2001
From: Leif Åstrand <leif@vaadin.com>
Date: Thu, 13 Mar 2014 13:23:21 +0200
Subject: Improve error message when reusing UI instance (#13457)

Change-Id: I49ec1e837a1a2a04dfadef5fd5fb5b6fd10ffcbc
---
 server/src/com/vaadin/ui/UI.java | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

(limited to 'server/src/com/vaadin/ui/UI.java')

diff --git a/server/src/com/vaadin/ui/UI.java b/server/src/com/vaadin/ui/UI.java
index 746fa194ac..0412cb1882 100644
--- a/server/src/com/vaadin/ui/UI.java
+++ b/server/src/com/vaadin/ui/UI.java
@@ -601,7 +601,18 @@ public abstract class UI extends AbstractSingleComponentContainer implements
      */
     public void doInit(VaadinRequest request, int uiId) {
         if (this.uiId != -1) {
-            throw new IllegalStateException("UI id has already been defined");
+            String message = "This UI instance is already initialized (as UI id "
+                    + this.uiId
+                    + ") and can therefore not be initialized again (as UI id "
+                    + uiId + "). ";
+
+            if (getSession() != null
+                    && !getSession().equals(VaadinSession.getCurrent())) {
+                message += "Furthermore, it is already attached to another VaadinSession. ";
+            }
+            message += "Please make sure you are not accidentally reusing an old UI instance.";
+
+            throw new IllegalStateException(message);
         }
         this.uiId = uiId;
 
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