Mock Static Methods with PowerMock

In a recent blog, I tried to highlight the benefits of using dependency injection and expressing the idea that one of the main benefits of this technique is that it allows you to test your code more easily by providing a high degree of isolation between classes, and coming to the conclusion that lots of good tests equals good code.

But, what happens when you don’t have dependency injection and you’re using a third party library that contains classes of a certain vintage that contains static methods? One way is to isolate those classes by writing a wrapper or adaptor around them and using this to provide isolation during testing; however, there’s also another way: using PowerMock. PowerMock is a mocking framework that extends other mocking frameworks to provide much needed additional functionality. To para-phase an old advert: “it refreshes the parts that other mocking frameworks fail to reach”.

This blog takes a look at PowerMock’s ability to mock static methods, providing an example of mocking the JDK’s ResourceBundle class, which as many of you know uses ResourceBundle.getBundle(…) to, well… load resource bundles.

I, like many other bloggers and writers, usually present some highly contrived scenario to highlight the problem. Today is different, I’ve simply got a class that uses a ResourceBundle called: UsesResourceBundle:

public class UsesResourceBundle {

  private static Logger logger = LoggerFactory.getLogger(UsesResourceBundle.class);

  private ResourceBundle bundle;

  public String getResourceString(String key) {

    if (isNull(bundle)) {
      // Lazy load of the resource bundle
      Locale locale = getLocale();

      if (isNotNull(locale)) {
        this.bundle = ResourceBundle.getBundle("SomeBundleName", locale);
      } else {
        handleError();
      }
    }

    return bundle.getString(key);
  }

  private boolean isNull(Object obj) {
    return obj == null;
  }

  private Locale getLocale() {

    return Locale.ENGLISH;
  }

  private boolean isNotNull(Object obj) {
    return obj != null;
  }

  private void handleError() {
    String msg = "Failed to retrieve the locale for this page";
    logger.error(msg);
    throw new RuntimeException(msg);
  }
}

You can see that there’s one method: getResourceString(…), which given a key will retrieve a resource string from a bundle. In order to make this work a little more efficiently, I’ve lazily loaded my resource bundle, and once loaded, I call bundle.getString(key) to retrieve my resource. To test this I’ve written a PowerMock JUnit test:

import static org.easymock.EasyMock.expect;
import static org.junit.Assert.assertEquals;
import static org.powermock.api.easymock.PowerMock.mockStatic;
import static org.powermock.api.easymock.PowerMock.replayAll;
import static org.powermock.api.easymock.PowerMock.verifyAll;

import java.util.Locale;
import java.util.MissingResourceException;
import java.util.ResourceBundle;

import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.powermock.api.easymock.annotation.Mock;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;

@RunWith(PowerMockRunner.class)
@PrepareForTest(UsesResourceBundle.class)
public class UsesResourceBundleTest {

  @Mock
  private ResourceBundle bundle;

  private UsesResourceBundle instance;

  @Before
  public void setUp() {
    instance = new UsesResourceBundle();
  }

  @Test
  public final void testGetResourceStringAndSucceed() {

    mockStatic(ResourceBundle.class);
    expect(ResourceBundle.getBundle("SomeBundleName", Locale.ENGLISH)).andReturn(bundle);

    final String key = "DUMMY";
    final String message = "This is a Message";
    expect(bundle.getString(key)).andReturn(message);

    replayAll();
    String result = instance.getResourceString(key);
    verifyAll();
    assertEquals(message, result);
  }

  @Test(expected = MissingResourceException.class)
  public final void testGetResourceStringWithStringMissing() {

    mockStatic(ResourceBundle.class);
    expect(ResourceBundle.getBundle("SomeBundleName", Locale.ENGLISH)).andReturn(bundle);

    final String key = "DUMMY";
    Exception e = new MissingResourceException(key, key, key);
    expect(bundle.getString(key)).andThrow(e);

    replayAll();
    instance.getResourceString(key);
  }

  @Test(expected = MissingResourceException.class)
  public final void testGetResourceStringWithBundleMissing() {

    mockStatic(ResourceBundle.class);
    final String key = "DUMMY";
    Exception e = new MissingResourceException(key, key, key);
    expect(ResourceBundle.getBundle("SomeBundleName", Locale.ENGLISH)).andThrow(e);

    replayAll();
    instance.getResourceString(key);
  }

}

In the code above I’ve taken the unusual step of including the import statements. This is to highlight that we’re using PowerMock’s versions of the import statics and not EasyMock’s. If you accidentally import EasyMock’s statics, then the whole thing just won’t work.

There are four easy steps in setting up a test that mocks a static call:

1. Use the PowerMock JUnit runner:

@RunWith(PowerMockRunner.class)

2. Declare the test class that we’re mocking:

@PrepareForTest(UsesResourceBundle.class)

3. Tell PowerMock the name of the class that contains static methods:

mockStatic(ResourceBundle.class);

4. Setup the expectations, telling PowerMock to expect a call to a static method:

expect(ResourceBundle.getBundle("SomeBundleName", Locale.ENGLISH)).andReturn(bundle);

The rest is plain sailing, you set up expectations for other standard method calls and the tell PowerMock/EasyMock to run the test, verifying the results:

final String key = "DUMMY";
final String message = "This is a Message";
expect(bundle.getString(key)).andReturn(message);

replayAll();
String result = instance.getResourceString(key);
verifyAll();

PowerMock can do lots more, such as mocking constructors and private method calls. More on that later perhaps…

Reference: Using PowerMock to Mock Static Methods from our JCG partner Roger at Captain Debug’s Blog.

Related Articles :

Do you want to know how to develop your skillset to become a Java Rockstar?

Subscribe to our newsletter to start Rocking right now!

To get you started we give you two of our best selling eBooks for FREE!

JPA Mini Book

Learn how to leverage the power of JPA in order to create robust and flexible Java applications. With this Mini Book, you will get introduced to JPA and smoothly transition to more advanced concepts.

JVM Troubleshooting Guide

The Java virtual machine is really the foundation of any Java EE platform. Learn how to master it with this advanced guide!

Given email address is already subscribed, thank you!
Oops. Something went wrong. Please try again later.
Please provide a valid email address.
Thank you, your sign-up request was successful! Please check your e-mail inbox.
Please complete the CAPTCHA.
Please fill in the required fields.

Leave a Reply


six − = 2



Java Code Geeks and all content copyright © 2010-2014, Exelixis Media Ltd | Terms of Use | Privacy Policy | Contact
All trademarks and registered trademarks appearing on Java Code Geeks are the property of their respective owners.
Java is a trademark or registered trademark of Oracle Corporation in the United States and other countries.
Java Code Geeks is not connected to Oracle Corporation and is not sponsored by Oracle Corporation.
Do you want to know how to develop your skillset and become a ...
Java Rockstar?

Subscribe to our newsletter to start Rocking right now!

To get you started we give you two of our best selling eBooks for FREE!

Get ready to Rock!
You can download the complementary eBooks using the links below:
Close