Creating a battery of good unit test cases is an important part of ensuring the quality of your application over its lifecycle. To aid developers with their testing efforts, GWT provides integration with the popular JUnit unit testing framework and Emma code coverage tool. GWT allows JUnit test cases to run in either development mode or production mode.

Note: To run through the steps to add JUnit tests to a sample GWT app, see the tutorial Unit Testing GWT Applications with JUnit.

  1. Architecting Your App for Testing
  2. Creating & Running a Test Case
  3. Asynchronous Testing
  4. Combining TestCase classes into a TestSuite
  5. Setting up and tearing down JUnit test cases that use GWT code
  6. Running tests in Eclipse

Architecting Your App for Testing

The bulk of this page is dedicated to explaining how to unit test your GWT code via the GWTTestCase class, which at the end of the day must pay performance penalties for running in a browser. But that’s not always what you want to do.

It will be well worth your effort to architect your app so that the bulk of your code has no idea that it will live in a browser. Code that you isolate this way can be tested in plain old JUnit test cases running in a JRE, and so execute much faster. The same good habits of separation of concerns, dependency injection and the like will benefit your GWT app just as they would any other, perhaps even more than usual.

For some tips along these lines take a look at the Best Practices For Architecting Your GWT App talk (video or slides) given at Google I/O in May of 2009. And keep an eye on this site for more more articles in the same vein.

Creating a Test Case

This section will describe how to create and run a set of unit test cases for your GWT project. In order to use this facility, you must have the JUnit library installed on your system.

The GWTTestCase Class

GWT includes a special GWTTestCase base class that provides JUnit integration. Running a compiled GWTTestCase subclass under JUnit launches the HtmlUnit browser which serves to emulate your application behavior during test execution.

GWTTestCase is derived from JUnit’s TestCase. The typical way to setup a JUnit test case class is to have it extend TestCase, and then run the it with the JUnit TestRunner. TestCase uses reflection to discover the test methods defined in your derived class. It is convention to begin the name of all test methods with the prefix test.

Using webAppCreator

The webAppCreator that GWT includes can generate a starter test case for you, plus ant targets and eclipse launch configs for testing in both development mode and production mode.

For example, to create a starter application along with test cases in the directory fooApp, where module name is com.example.foo.Foo:

~/Foo> webAppCreator -out fooApp
        -junit /opt/eclipse/plugins/org.junit_3.8.1/junit.jar
Created directory fooApp/src
Created directory fooApp/war
Created directory fooApp/war/WEB-INF
Created directory fooApp/war/WEB-INF/lib
Created directory fooApp/src/com/example/foo
Created directory fooApp/src/com/example/foo/client
Created directory fooApp/src/com/example/foo/server
Created directory fooApp/test/com/example/foo/client
Created file fooApp/src/com/example/foo/Foo.gwt.xml
Created file fooApp/war/Foo.html
Created file fooApp/war/Foo.css
Created file fooApp/war/WEB-INF/web.xml
Created file fooApp/src/com/example/foo/client/Foo.java
Created file fooApp/src/com/example/foo/client/GreetingService.java
Created file fooApp/src/com/example/foo/client/GreetingServiceAsync.java
Created file fooApp/src/com/example/foo/server/GreetingServiceImpl.java
Created file fooApp/build.xml
Created file fooApp/README.txt
Created file fooApp/test/com/example/foo/client/FooTest.java
Created file fooApp/.project
Created file fooApp/.classpath
Created file fooApp/Foo.launch
Created file fooApp/FooTest-dev.launch
Created file fooApp/FooTest-prod.launch
Created file fooApp/war/WEB-INF/lib/gwt-servlet.jar

Follow the instructions in the generated fooApp/README.txt file. You have two ways to run your tests: using ant or using Eclipse. There are ant targets ant test.dev and ant test.web for running your tests in development and production mode, respectively. Similarly, you can follow the instructions in the README.txt file to import your project in Eclipse or your favorite IDE, and use the launch configs FooTest-dev and FooTest-prod to run your tests in development and production mode using eclipse. As you keep adding your testing logic to the skeleton FooTest.java, you can continue using the above techniques to run your tests.

Creating a Test Case by Hand

If you prefer not to use webAppCreator, you may create a test case suite by hand by following the instructions below:

  1. Define a class that extends GWTTestCase. Make sure your test class is on the module source path (e.g. in the client subpackage of your module.) You can add new source paths by editing the module XML file and adding a <source> element.
  2. If you do not have a GWT module yet, create a module that causes the source for your test case to be included. If you are adding a test case to an existing GWT app, you can just use the existing module.
  3. Implement the method GWTTestCase.getModuleName() to return the fully-qualified name of the module. This is the glue that tells the JUnit test case which module to instantiate.
  4. Compile your test case class to bytecode. You can use the Java compiler directly using javac or a Java IDE such as Eclipse.
  5. Run your test case. Use the class junit.textui.TestRunner as your main class and pass the full name of your test class as the command line argument, e.g. com.example.foo.client.FooTest. When running the test case, make sure your classpath includes:
    • Your project’s src directory
    • Your project’s bin directory
    • The gwt-user.jar library
    • The gwt-dev.jar library
    • The junit.jar library

Client side Example

First of all, you will need a valid GWT module to host your test case class. Usually, you do not need to create a new module XML file - you can just use the one you have already created to develop your GWT module. But if you did not already have a module, you might create one like this:

  <!-- Module com.example.foo.Foo -->

  <!-- Standard inherit.                                           -->
  <inherits name='com.google.gwt.user.User'/>

  <!-- implicitly includes com.example.foo.client package          -->


  <!-- It's okay for your module to declare an entry point.        -->
  <!-- This gets ignored when running under JUnit.                 -->
  <entry-point class='com.example.foo.FooModule'/>

  <!-- You can also test remote services during a JUnit run.       -->
  <servlet path='/foo' class='com.example.foo.server.FooServiceImpl'/>

Tip: You do not need to create a separate module for every test case, and in fact will pay a startup penalty for every module you do use. In the example above, any test cases in com.example.foo.client (or any subpackage) can share the com.example.foo.Foo module.

Suppose you had created a widget under the foo package, UpperCasingLabel, which ensures that the text it shows is all upper case. Here is how you might test it.

package com.example.foo.client;
import com.google.gwt.junit.client.GWTTestCase;

public class UpperCasingLabelTest extends GWTTestCase {

   * Specifies a module to use when running this test case. The returned
   * module must include the source for this class.
   * @see com.google.gwt.junit.client.GWTTestCase#getModuleName()
  public String getModuleName() {
    return "com.example.foo.Foo";
  public void testUpperCasingLabel() {
    UpperCasingLabel upperCasingLabel = new UpperCasingLabel();
    assertEquals("FOO", upperCasingLabel.getText());

    assertEquals("BAR", upperCasingLabel.getText());

    assertEquals("BAZ", upperCasingLabel.getText());

Now, there are several ways to run your tests. Just look at the sample ant scripts or launch configs generated by webAppCreator, as in the previous subsection.

Passing Arguments to the Test Infrastructure

The main class in the test infrastructure is JUnitShell. To control aspects of how your tests execute, you must pass arguments to this class. Arguments cannot be passed directly through the command-line because normal command-line arguments go directly to the JUnit runner. Instead, define the system property gwt.args to pass arguments to JUnitShell.

For example, to run tests in (legacy) development mode (that is, run the tests in Java in the JVM), declare -Dgwt.args="-devMode" as a JVM argument when invoking JUnit. To get a full list of supported options, declare -Dgwt.args="-help" (instead of running the test, help is printed to the console).

Running your test in (legacy) Development Mode

When using the webAppCreator tool, you get the ability to launch your tests in either (legacy) development mode or production mode. By default, tests are run in production mode, thus they’re compiled to JavaScript before being executed.

Otherwise, in (legacy) development mode tests are run as normal Java bytecode in a JVM. While this makes it easier to debug them, note that, although rare, there are some differences between Java and JavaScript that could cause your code to produce different results when deployed.

If you instead decide to run the JUnit TestRunner from the command line, you need to pass arguments to JUnitShell to get your unit tests running in (legacy) development mode


Running your test in Manual Mode

Manual-mode tests allow you to run unit tests manually on any browser. In this mode, the JUnitShell main class runs as usual on a specified GWT module, but instead of running the test immediately, it prints out a URL and waits for a browser to connect. You can manually cut and paste this URL into the browser of your choice, and the unit tests will run in that browser.

For example, if you want to run a test in a single browser, you would use the following arguments:

-runStyle Manual:1

GWT will then show a console message like the following:

Please navigate your browser to this URL:

Point your browser to the specified URL, and the test will run. You may be prompted by the GWT Developer Plugin to accept the connection the first time the test is run.

Manual-mode test targets are not generated by the webAppCreator tool, but you can easily create one by copying the test.prod ant target in the build.xml file to test.manual and adding -runStyle Manual:1 to the -Dgwt.args part. Manual mode can also be used for remote browser testing.

Running your test on Remote Systems

Since different browsers can often behave in unexpected ways, it is important for developers to test their applications on all browsers they plan to support. GWT simplifies remote browser testing by enabling you to run tests on remote systems, as explained in the Remote Browser Testing page.

Automating your Test Cases

When developing a large project, a good practice is to integrate the running of your test cases with your regular build process. When you build manually, such as using ant from the command line or using your desktop IDE, this is as simple as just adding the invocation of JUnit into your regular build process. As mentioned before, when you run GWTTestCase tests, an HtmlUnit browser runs the tests. However, all tests might not run successfully on HtmlUnit, as explained earlier. GWT provides remote testing solutions that allow you to use a selenium server to run tests. Also, consider organizing your tests into GWTTestSuite classes to get the best performance from your unit tests.

Server side testing

The tests described above are intended to assist with testing client-side code. The test case wrapper GWTTestCase will launch either a development mode session or a web browser to test the generated JavaScript. On the other hand, server-side code runs as native Java in a JVM without being translated to JavaScript, so it is not necessary to run tests of server-side code using GWTTestCase as the base class for your tests. Instead, use JUnit’s TestCase and other related classes directly when writing tests for your application’s server-side code. That said, you may want both GWTTestCase and TestCase coverage of code that will be used on both the client and the server.

Asynchronous Testing

GWT’s JUnit integration provides special support for testing functionality that cannot be executed in straight-line code. For example, you might want to make an RPC call to a server and then validate the response. However, in a normal JUnit test run, the test stops as soon as the test method returns control to the caller, and GWT does not support multiple threads or blocking. To support this use case, GWTTestCase has extended the TestCase API. The two key methods are GWTTestCase.delayTestFinish(int) and GWTTestCase.finishTest(). Calling delayTestFinish() during a test method’s execution puts that test in asynchronous mode, which means the test will not finish when the test method returns control to the caller. Instead, a delay period begins, which lasts the amount of time specified in the call to delayTestFinish(). During the delay period, the test system will wait for one of three things to happen:

  1. If finishTest() is called before the delay period expires, the test will succeed.
  2. If any exception escapes from an event handler during the delay period, the test will error with the thrown exception.
  3. If the delay period expires and neither of the above has happened, the test will error with a TimeoutException.

The normal use pattern is to setup an event in the test method and call delayTestFinish() with a timeout significantly longer than the event is expected to take. The event handler validates the event and then calls finishTest().


public void testTimer() {
  // Setup an asynchronous event handler.
  Timer timer = new Timer() {
    public void run() {
      // do some validation logic

      // tell the test system the test is now done

  // Set a delay period significantly longer than the
  // event is expected to take.

  // Schedule the event and return control to the test system.

The recommended pattern is to test one asynchronous event per test method. If you need to test multiple events in the same method, here are a couple of techniques:

  • “Chain” the events together. Trigger the first event during the test method’s execution; when that event fires, call delayTestFinish() again with a new timeout and trigger the next event. When the last event fires, call finishTest() as normal.
  • Set a counter containing the number of events to wait for. As each event comes in, decrement the counter. Call finishTest() when the counter reaches 0.

Combining TestCase classes into a TestSuite

The GWTTestSuite mechanism has the overhead of having to start a development mode shell and servlet or compile your code. There is also overhead for each test module within a suite.

Ideally you should group your tests into as few modules as is practical, and should avoid having tests in a particular module run by more than one suite. (Tests are in the same module if they return the same value from getModuleName().)

GWTTestSuite class re-orders the test cases so that all cases that share a module are run back to back.

Creating a suite is simple if you have already defined individual JUnit TestCases or GWTTestCases. Here is an example:

public class MapsTestSuite extends GWTTestSuite {
  public static Test suite() {
    TestSuite suite = new TestSuite("Test for a Maps Application");
    return suite;

The three test cases MapTest, EventTest, and CopyTest can now all run in the same instance of JUnitShell.

java -Xmx256M -cp "./src:./test:./bin:./junit.jar:/gwt/gwt-user.jar:/gwt/gwt-dev.jar:/gwt/gwt-maps.jar" junit.textui.TestRunner com.example.MapsTestSuite

Setting up and tearing down JUnit test cases that use GWT code

When using a test method in a JUnit TestCase, any objects your test creates and leaves a reference to will remain active. This could interfere with future test methods. You can override two new methods to prepare for and/or clean up after each test method.

  • gwtSetUp() runs before each test method in a test case.
  • gwtTearDown() runs after each test method in a test case.

The following example shows how to defensively cleanup the DOM before the next test run using gwtSetUp(). It skips over <iframe> and <script> tags so that the GWT test infrastructure is not accidentally removed.

import com.google.gwt.junit.client.GWTTestCase;
  import com.google.gwt.user.client.DOM;
  import com.google.gwt.user.client.Element;

  private static native String getNodeName(Element elem) /*-{
     return (elem.nodeName || "").toLowerCase();

   * Removes all elements in the body, except scripts and iframes.
  public void gwtSetUp () {
    Element bodyElem = RootPanel.getBodyElement();

    List<Element> toRemove = new ArrayList<Element>();
    for (int i = 0, n = DOM.getChildCount(bodyElem); i < n; ++i) {
      Element elem = DOM.getChild(bodyElem, i);
      String nodeName = getNodeName(elem);
      if (!"script".equals(nodeName) &amp;&amp; !"iframe".equals(nodeName)) {

    for (int i = 0, n = toRemove.size(); i < n; ++i) {
      DOM.removeChild(bodyElem, toRemove.get(i));

Running Tests in Eclipse

The webAppCreator tool provides a simple way to generate example launch configurations that can be used to run both development and production mode tests in Eclipse. You can generate additional launch configurations by copying it and replacing the project name appropriately.

Alternatively, one can also directly generate launch configurations. Create a normal JUnit run configuration by right-clicking on the Test file that extends GWTTestCase and selecting Run as > JUnit Test. Though the first run will fail, a new JUnit run configuration will be generated. Modify the run configuration by adding the project’s src and test directories to the classpath, like so:

  • click the Classpath tab
  • select User Entries
  • click the Advanced button
  • select the Add Folders radio button
  • add your src and test directories

Launch the run config to see the tests running in development mode.

To run tests in production mode, copy the development mode launch configuration and pass VM arguments (by clicking the Arguments tab and adding to the VM arguments textarea) -Dgwt.args="-prod"