Mocking SWT Image and Display with PowerMockito
Posted on 09 February 2014 by Ben Limmer
I’ve been working in SWT quite a bit during my career at ReadyTalk. In many cases, I’ll need to mock out two common SWT classes, Image and Display. Unfortunately, mocking out these classes (or static calls to these classes) is a bit difficult. This is where PowerMockito comes in handy.
A co-worker of mine, David, refers to PowerMock as the “circular saw in your kitchen”. 99 times out of 100 you won’t need to use it, but it does have its uses.
Setup
Anytime you’re utilizing PowerMockito, you’re going to need a class level annotation telling JUnit that we’re going to be doing some heavy lifting with PowerMock. Your test class should look something like this
import org.junit.runner.RunWith;
import org.powermock.modules.junit4.PowerMockRunner;
@RunWith(PowerMockRunner.class)
public class SWTTestClass {
...
}If you don’t specify the PowerMockRunner, you’ll notice that all your other calls to mock out objects will not work. You get errors like
java.lang.IllegalArgumentException: Cannot subclass final class class org.eclipse.swt.graphics.Image if you don't mark your test class with this annotation.
Mocking SWT Image
Now that you have your class marked with the @PowerMockRunner annotation, you can mock the final class Image.
First, you need to tell PowerMock that we’re going to mock the Image class. Use the @PrepareForTest class-level annotation for Image.
import org.eclipse.swt.graphics.Image;
import org.junit.runner.RunWith;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;
@RunWith(PowerMockRunner.class)
@PrepareForTest(Image.class)
public class SWTTestClass {
...
}Now you can mock calls in your tests. For instance, if you wanted to mock the Image.getBounds() call, you use the when pattern to return an SWT Rectangle.
import org.eclipse.swt.graphics.Image;
import org.eclipse.swt.graphics.Rectangle;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.powermock.api.mockito.PowerMockito;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;
import static junit.framework.Assert.assertEquals;
import static org.mockito.Mockito.when;
@RunWith(PowerMockRunner.class)
@PrepareForTest(Image.class)
public class SWTTestClass {
@Test
public void imageMockReturnsDefinedRectangle() {
Image image = PowerMockito.mock(Image.class);
Rectangle rectangle = new Rectangle(0, 0, 10, 10);
when(image.getBounds()).thenReturn(rectangle);
assertEquals(rectangle, image.getBounds());
}
}It’s as easy as that. Rinse and repeat for mocking other calls to Image.
Mocking Static Calls to SWT Display
Mocking the SWT Display object with Mockito is pretty straightforward. However, sometimes you’ll want to mock out the return values from static calls to Display, like Display.getDefault() or Display.getCurrent(). Here’s how you do it.
Similar to what we did with Image, we need to prepare the Display with the @PrepareForTest class-level annotation.
import org.eclipse.swt.widgets.Display;
import org.junit.runner.RunWith;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;
@RunWith(PowerMockRunner.class)
@PrepareForTest(Display.class)
public class SWTTestClass {
...
}Now, we can mock out the static calls to Display to return another mock object. For instance, you might want to make sure some method returns the current Display.
import org.eclipse.swt.widgets.Display;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.powermock.api.mockito.PowerMockito;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;
import static junit.framework.Assert.assertEquals;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.when;
@RunWith(PowerMockRunner.class)
@PrepareForTest(Display.class)
public class SWTTestClass {
@Test
public void getCurrentDisplayReturnsMockDisplay() {
Display mockDisplay = mock(Display.class);
PowerMockito.mockStatic(Display.class);
when(Display.getCurrent()).thenReturn(mockDisplay);
assertEquals(mockDisplay, Display.getCurrent());
}
}It’s as simple as that. Happy testing!