Intercept test failures due to ClassCastExceptions that GwtMockito ca…

…n't do

anything about so that we can at least provide an error messages to users
hinting how they can work around the problem.

Refs #4, #22, #43, #47.

gwtmockito/src/main/java/com/google/gwtmockito/GwtMockitoTestRunner.java

@@ -1,12 +1,12 @@
 /*
  * Copyright 2013 Google Inc.
- * 
+ *
  * 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
@@ -56,6 +56,10 @@
 import com.google.gwt.user.client.ui.Widget;
 import com.google.gwtmockito.impl.StubGenerator;
 
+import org.junit.runner.Description;
+import org.junit.runner.Result;
+import org.junit.runner.notification.Failure;
+import org.junit.runner.notification.RunListener;
 import org.junit.runner.notification.RunNotifier;
 import org.junit.runners.BlockJUnit4ClassRunner;
 import org.junit.runners.ParentRunner;
@@ -295,16 +299,72 @@ protected List<String> getAdditionalClasspaths() {
    * the context classloader.
    */
   @Override
-  public void run(RunNotifier notifier) {
+  public void run(final RunNotifier notifier) {
     // When running the test, we want to be sure to use our custom classloader as the context
     // classloader. This is important because Mockito will create mocks using the context
     // classloader. Things that can go wrong if this isn't set include not being able to mock
     // package-private classes, since the mock implementation would be created by a different
     // classloader than the class being mocked.
     ClassLoader originalClassLoader = Thread.currentThread().getContextClassLoader();
     Thread.currentThread().setContextClassLoader(gwtMockitoClassLoader);
+    RunNotifier wrapperNotifier = new RunNotifier();
+    wrapperNotifier.addListener(new RunListener() {
+      @Override
+      public void testAssumptionFailure(Failure failure) {
+        notifier.fireTestAssumptionFailed(failure);
+      }
+      @Override
+      public void testFailure(Failure failure) throws Exception {
+        if (failure.getException() instanceof ClassCastException
+            && failure.getMessage().contains("EnhancerByMockitoWithCGLIB")) {
+          notifier.fireTestFailure(new Failure(
+              failure.getDescription(),
+              new FailedCastException(
+                  "The test failed with a ClassCastException. This often indicates that you are "
+                      + "using a library that invokes JavaScriptObject.cast() to convert an object "
+                      + "from one type to another in a way that can't be mimicked in pure Java. "
+                      + "There are a few ways to deal with this:\n"
+                      + "  1) Inject the class that calls cast() into the class being tested so "
+                      + "that you can replace it with a mock.\n"
+                      + "  2) Annotate you test class with @WithClassesToStub and pass in the "
+                      + "class that is causing problems.\n"
+                      + "  3) If the class is part of GWT and not a third-party library, try "
+                      + "reporting it on the issue tracker at "
+                      + "https://github.com/google/gwtmockito/issues and it might be possible to "
+                      + " insert a workaround.\n",
+                  failure.getException())));
+        } else {
+          notifier.fireTestFailure(failure);
+        }
+      }
+
+      @Override
+      public void testFinished(Description description) throws Exception {
+        notifier.fireTestFinished(description);
+      }
+
+      @Override
+      public void testIgnored(Description description) throws Exception {
+        notifier.fireTestIgnored(description);
+      }
+
+      @Override
+      public void testRunFinished(Result result) throws Exception {
+        notifier.fireTestRunFinished(result);
+      }
+
+      @Override
+      public void testRunStarted(Description description) throws Exception {
+        notifier.fireTestRunStarted(description);
+      }
+
+      @Override
+      public void testStarted(Description description) throws Exception {
+        notifier.fireTestStarted(description);
+      }
+    });
     try {
-      super.run(notifier);
+      super.run(wrapperNotifier);
     } finally {
       Thread.currentThread().setContextClassLoader(originalClassLoader);
     }
@@ -445,4 +505,10 @@ private String newMockForClassSnippet(CtClass paramClass) {
     @Override
     public void start(ClassPool pool) {}
   }
+
+  private static class FailedCastException extends Exception {
+    FailedCastException(String message, Throwable cause) {
+      super(message, cause);
+    }
+  }
 }