Wednesday, 12 October 2011

Using PowerMock to Mock Static Methods

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...

2 comments:

Diana said...

Hi Roger,

What about code coverage? When I add my class to @PrepareForTest, the coverage is ignored....

I'm running JUnits for code coverage purpose, can you help me with this?

Thanks in advance!

Roger Hughes said...

Diana,
I'm not sure what the answer to your problem is, but I'd hazard a guess that changing from the default JUnit test runner to the PowerMockRunner using:

@RunWith(PowerMockRunner.class)

will be the cause of the problems. As they say on the Powermock website "PowerMock uses a custom classloader and byte code manipulation" - which means that when you then add:

@PrepareForTest(UsesResourceBundle.class)

it'll tell PowerMock to starting doing some jiggery-pokery with your code that will confuse whatever code coverage method you're using.