Implementation for issue junit-team#144

Added additional public method, setMissingExceptionMessage, to allow a
user to specify a custom message that would be thrown in the event a
test did not throw the expected exception. If one is not provided the
class falls back to the previous behavior.

.gitignore

@@ -9,4 +9,5 @@ MaxCore.max
 *.iml
 *.iws
 out
-java.hprof.txt
+java.hprof.txt
+.gitattributes

src/main/java/org/junit/rules/ExpectedException.java

@@ -96,6 +96,8 @@ public static ExpectedException none() {
     private boolean handleAssumptionViolatedExceptions = false;
 
     private boolean handleAssertionErrors = false;
+
+    private String missingExceptionMessage;
 
     private ExpectedException() {
     }
@@ -109,6 +111,17 @@ public ExpectedException handleAssumptionViolatedExceptions() {
         handleAssumptionViolatedExceptions = true;
         return this;
     }
+
+    /**
+     * Specifies the detail message for an exception to be thrown if the test does 
+     * not throw the expected exception.
+     * @param providedMessage exception detail message
+     * @return self
+     */
+    public ExpectedException setMissingExceptionMessage(String providedMessage) {
+    	missingExceptionMessage = providedMessage;
+    	return this;
+    }
 
     public Statement apply(Statement base,
             org.junit.runner.Description description) {
@@ -180,8 +193,16 @@ public void evaluate() throws Throwable {
     }
 
     private void failDueToMissingException() throws AssertionError {
-        String expectation = StringDescription.toString(fMatcherBuilder.build());
-        fail("Expected test to throw " + expectation);
+    	String failureMessage;
+
+    	if ( isMissingExceptionMessageEmpty() ) {
+    		String expectation = StringDescription.toString(fMatcherBuilder.build());
+    		failureMessage = "Expected test to throw " + expectation;
+    	} else {
+    		failureMessage = missingExceptionMessage;
+    	}
+
+        fail(failureMessage);
     }
 
     private void optionallyHandleException(Throwable e, boolean handleException)
@@ -200,4 +221,8 @@ private void handleException(Throwable e) throws Throwable {
             throw e;
         }
     }
+
+    private boolean isMissingExceptionMessageEmpty() {
+    	return missingExceptionMessage == null || missingExceptionMessage.isEmpty();
+    }
 }

src/test/java/org/junit/tests/experimental/rules/ExpectedExceptionTest.java

@@ -88,7 +88,8 @@ public static Collection<Object[]> testsWithEventMatcher() {
                                 containsString("exception with cause is <java.lang.NullPointerException: expected cause>"),
                                 containsString("cause was <java.lang.NullPointerException: an unexpected cause>"),
                                 containsString("Stacktrace was: java.lang.IllegalArgumentException: Ack!"),
-                                containsString("Caused by: java.lang.NullPointerException: an unexpected cause")))}
+                                containsString("Caused by: java.lang.NullPointerException: an unexpected cause")))},
+                { CustomMessageWithoutExpectedException.class, hasSingleFailureWithMessage(ARBITRARY_MESSAGE) }
         });
     }
 
@@ -368,4 +369,16 @@ public void throwWithCause() {
             throw new IllegalArgumentException("Ack!", new NullPointerException("an unexpected cause"));
         }
     }
+
+    public static class CustomMessageWithoutExpectedException {
+
+        @Rule
+        public ExpectedException thrown = ExpectedException.none();
+
+        @Test
+        public void throwWithCause() {
+            thrown.expect(IllegalArgumentException.class);
+            thrown.setMissingExceptionMessage(ARBITRARY_MESSAGE);
+        }
+    }
 }