Advanced Features

Assertions with Timeouts

Sometimes it is necessary to have a timeout when verifying that a particular condition has been satisfied or the application has reached a particular state. For example, some UI tests may involve asynchronous operations or time-consuming tasks.

The following code listing shows how to perform assertions using timeouts. In our example, we are going to assume that a JButton is enabled after calling a web service asynchronously. In our example, if the JButton is not enabled after 10 seconds, the test will fail. The exception thrown will include the description of the Condition.

// import static org.assertj.swing.timing.Pause.pause;
// import static org.assertj.swing.timing.Timeout.timeout;
// import static org.assertj.swing.edt.GuiActionRunner.execute;

final JButton okButton = //obtain a reference to the "OK" button
pause(new Condition("OK button to be enabled") {

  public boolean test() {
    return execute(okButton::isEnabled);

}, timeout(10000));

Component Formatters

A component formatter is an implementation of the interface ComponentFormatter that creates a String representation of a GUI component. AssertJ Swing provides default component formatters for all the Swing components in the JDK. Unlike the toString() method in Swing components, the provided component formatters display only the information that can help developers solve problems in functional tests, excluding any information related to the appearance of GUI components (e.g. colors, layouts, sizes, etc.). Here are some examples:

org.assertj.swing.test.TestFrame[name='frame', title='FormattingTest', enabled=true, showing=true]
javax.swing.JButton[name='button', text='A button', enabled=false]
javax.swing.JList[name='list', selectedValues=['One', 2], contents=['One', 2, 'Three', 4], selectionMode=MULTIPLE_INTERVAL_SELECTION, enabled=true]
javax.swing.JOptionPane[message='A message', messageType=ERROR_MESSAGE, optionType=DEFAULT_OPTION, enabled=true, showing=false]
javax.swing.JTabbedPane[name='tabbedPane', selectedTabIndex=1, selectedTabTitle='Second', tabCount=2, tabTitles=['First', 'Second'], enabled=true]:

Custom Component Formatters

There might be cases that you might want to create your own custom formatter to override an existing one or to add support for custom GUI components.

Implement the ComponentFormatter interface

The interface ComponentFormatter provides two methods:

  • Class<? extends Component> targetType() returns the type of component this formatter supports. For example, by returning JButton.class a formatter indicates that it supports instances of JButton and its subclasses.
  • String format(Component c) returns the String representation of the given GUI component.

Configure an IntrospectionComponentFormatter

The easiest way to create a component formatter is to configure an instance of IntrospectionComponentFormatter, which, as the name suggests, uses introspection to display property values of a GUI component. The following code listing shows how to configure a IntrospectionComponentFormatter to support JLabel (and subclasses) and the properties to show:

IntrospectionComponentFormatter formatter =
  new IntrospectionComponentFormatter(JLabel.class, "name", "text", "enabled");

Register your custom formatter

After creating a custom formatter, we need to register it with AssertJ Swing. It is very simple, we only need to call the static method register in org.assertj.swing.format.Formatting.

Formatting.register(new MyFormatter());
AssertJ Swing uses the formatter that supports the type that is the closest to the type of the given GUI component. For example, if we have formatter1, a formatter registered to format instances of JButton, and formatter2, a formatter registered to format instances of MyOwnButton (a subclass of JButton), AssertJ Swing will use formatter2 to format instances of MyOwnButton.

Extending AssertJ Swing

It is very likely that, in your application, you are using custom Swing components (e.g. Flamingo, JIDE, or your own). Since AssertJ Swing currently supports only the standard Swing components (the ones that come in the JDK), you might want to create your own AssertJ Swing fixtures to test your application.

The following are some suggestions or recommendations you can follow when creating your own AssertJ Swing fixture:

Take a look at the code and Javadocs of existing AssertJ Swing fixtures

By reading AssertJ Swing's code you can learn to use the BasicRobot to simulate a user moving the mouse, clicking a mouse button or pressing keyboard keys. We have separated the structure of a component fixture in several layers (from bottom to top):

  1. BasicRobot. Simulates a user interacting with a mouse and keyboard. It uses the AWT Robot to generate native input events.
  2. Component driver. This layer does all the heavy lifting. All interaction with a GUI component is done in this layer. It knows how to simulate events and check the state of a specific GUI component. For example, JComboBoxDriver knows how to simulate a user using a JComboBox (selecting a particular element) and how to verify the state of it (which element should be selected).
  3. Component fixture. This layer sits on top of the driver. It provides a fluent interface to that makes the API easier to write and read. Users of AssertJ Swing write their GUI tests using fixtures, not drivers. Fixtures can be considered the user interface of the AssertJ Swing library.

Extend an existing AssertJ Swing fixture

If the component you want to test is a subclass of a JDK Swing component (e.g. you have a custom button that extends JButton), you can extend an existing concrete AssertJ Swing fixture (JButtonFixture in our example).

If the custom GUI component does not extend any JDK Swig component, or if you prefer to create a AssertJ Swing fixture from scratch, please read the following

  • Extend ComponentFixture. This class provides all the necessary wiring of a GUI component to test and a Robot. It also provides some very basic functionality and convenience methods.
  • If want to simulate a user right-clicking on your component and showing a JPopupMenu, extend JPopupMenuInvokerFixture.
  • Implement any of the following interfaces if needed:
    • CommonComponentFixture
    • FocusableComponentFixture
    • KeyboardInputSimulationFixture
    • MouseInputSimulationFixture
    • StateVerificationFixture
  • You can also extend GenericComponentFixture, which extends ComponentFixture and implements the interfaces mentioned in the previous point.
To avoid unexpected side effects in your tests, you must access Swing components in the event dispatch thread.

Create an extension for ContainerFixture

By default, implementations of ContainerFixture provide shortcut methods to access the standard Swing components (the ones that come in the JDK) in a Container. For example, the following code listing shows shortcuts methods to access a JLabel and a JTree from a JFrame being managed by a FrameFixture:

FrameFixture frame = new FrameFixture(new MyFrame());

If you have created a fixture for your custom GUI component, it is not possible to add a shortcut to Container due to the lack of extension methods in Java. For example, let's assume you have created a custom GUI component called MyCalendar and a fixture to use this custom component in your GUI tests called MyCalendarFixture. It is not possible to add the shortcut method calendar to ContainerFixture and have all its implementations look like this:


To overcome this limitation, AssertJ Swing provides ComponentFixtureExtension. The following code listing shows an extension to add a shortcut to a MyCalendarFixture. This extension looks up a MyCalendar that has a matching name and is showing on the screen:

public class MyCalendarFixtureExtension extends ComponentFixtureExtension {

  public static MyCalendarFixtureExtension calendarWithName(String name) {
    return new MyCalendarFixtureExtension(name);

  private final String name;

  private MyCalendarFixtureExtension(String name) { = name;

  public MyCalendarFixture createFixture(Robot robot, Container root) {
    MyCalendar calendar = robot.finder().findByName(root, name, MyCalendar.class, true);
    return new MyCalendarFixture(robot, calendar);

The only method that needs to be implemented is createFixture(Robot, Container), which is responsible for creating our custom fixture. The static method calendarWithName(String) is just a convenience factory method, which we will use to connect our extension to any implementation of ContainerFixture:

// import static org.assertj.swing.sample.MyCalendarFixtureExtension.calendarWithName;

FrameFixture frame = new FrameFixture(new MyFrame());

Handling System.exit

A common topic in the mailing list is how to handle System.exit. The typical example is simulating a user selecting the menus File, Exit from a GUI test. By doing this, the application will quit and any GUI test trying to use the application will fail.

To prevent this failure and to test that in fact System.exit is being handled correctly by the application, AssertJ Swing provides a NoExitSecurityManager that will prevent the application to end. To use it, simply add a call to NoExitSecurityManagerInstaller as follows:

private static NoExitSecurityManagerInstaller noExitSecurityManagerInstaller;

public static void setUpOnce() {
  noExitSecurityManagerInstaller = NoExitSecurityManagerInstaller.install();

public static void tearDown() {

By default, NoExitSecurityManager will just prevent the application from quitting if System.exit is called. If you want to do something when System.exit is called, do the following:

  • Implement a ExitHook
  • Pass an instance of your ExitHook to NoExitSecurityManagerInstaller.install(ExitHook)

Support for Platform-Specific Features

Sometimes, when testing our applications, we need to deal with platform-specific issues. AssertJ Swing provides the utility class Platform to overcome this type of issues.

Currently Platform provides one method, controlOrCommandKey() that returns the code of either the Command key (if MacOS) or the code of the Control key (if other than MacOS, for example Windows). A good example of the utility of this method is simulation of a user copying some text to the clipboard. In windows we need to simulate a user pressing the keys Ctrl and C, while in MacOS we need to simulate a user pressing the keys Cmd and C. With AssertJ Swing we can simply write:

// import static org.assertj.swing.util.Platform.controlOrCommandKey;
// import static java.awt.event.KeyEvent.VK_C;

int controlOrCommand = controlOrCommandKey();


Testing containers without a frame

AssertJ Swing provides the utility class Containers to help test a Container (e.g. a JPanel) without having a Frame (or JFrame). The following code listing shows how to test our own JPanel of type MyCoolJPanel:

// import static org.assertj.swing.fixture.Containers.showInFrame;

private FrameFixture frameFixture;

public void setUp() {
  MyCoolJPanel panel = createPanel() // create panel in EDT.
  frameFixture = showInFrame(panel);

AssertJ Swing will show the panel in a JFrame, wrapped in a FrameFixture.