PowerMock for Integration Testing

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A while ago I blogged about how to use the PowerMock Rule to bootstrap PowerMock without using a JUnit runner. A problem that you'll likely run into sooner or later when using this approach in an integration test is the need to ignore certain classes or packages from being loaded by the PowerMock classloader using the @PowerMockIgnore annotation. This is actually not limited to the Rule but happens when using the standard JUnit runner as well but it's much more likely to occur in an integration test. Frequent candidates for this are various XML, log, and persistence frameworks . The reason is that some frameworks tries to instantiate classes using reflection and does this from the thread context classloader (PowerMock's classloader) but then tries to assign the created object to a field not loaded by the same classloader. So by using @PowerMockIgnore you can can instruct PowerMock to defer the loading of a certain package to the parent classloader. What you need to ignore is case specific but usually it's e.g. the XML framework or some packages that interact with it. E.g. @PowerMockIgnore({"org.xml.*", "javax.xml.*"}).

Example

Let's say we have a (very stupid) Spring bean like this:

 
@Component
public class MyBean {
 
    @Autowired
    private CompanyRepository companyRepository;
 
    public Message generateMessage() throws SAXException {
        final String[] allEmployees = companyRepository.getAllEmployees();
        final String message = StringUtils.join(allEmployees, ", ");
 
        final long id = IdGenerator.generateNewId();
        return new Message(id, message);
    }
}
 

and for demonstration we'd like to mock the static method call to IdGenerator.generateNewId(). Let's write a simple test bootstrapped by Spring using the classloader version of PowerMockRule:

 
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration("classpath:/example-context.xml")
@PrepareForTest(IdGenerator.class)
public class SpringExampleTest {
 
    @Rule
    public PowerMockRule rule = new PowerMockRule();
 
    @Autowired
    private MyBean myBean;
 
    @Test
    public void mockStaticMethod() throws Exception {
        // Given
        final long expectedId = 2L;
        mockStatic(IdGenerator.class);
        when(IdGenerator.generateNewId()).thenReturn(expectedId);
 
        // When
        final Message message = myBean.generateMessage();
 
        // Then
        assertEquals(expectedId, message.getId());
        assertEquals("John Doe, Paul Anderson, Jane Doe", message.getContent());
    }
 

If you run this test you'll get the following error:

 
java.lang.RuntimeException: java.lang.ClassCastException: com.sun.org.apache.xerces.internal.jaxp.DocumentBuilderFactoryImpl cannot be cast to javax.xml.parsers.DocumentBuilderFactory
	at powermock.examples.spring.CompanyRepository.getAllEmployees(CompanyRepository.java:42)
	at powermock.examples.spring.MyBean.generateMessage(MyBean.java:31)
	at org.powermock.examples.spring.mockito.SpringExampleTest.mockStaticMethod(SpringExampleTest.java:58)
...
 

The reason is that companyRepository.getAllEmployees() implementation called from MyBean bean above parses the employee list using Java's XML parsing (javax.xml.parsers.DocumentBuilderFactory). Thus we must use the PowerMockIgnore annotation to ignore the framework:

 
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration("classpath:/example-context.xml")
@PrepareForTest(IdGenerator.class)
@PowerMockIgnore({"org.w3c.*", "javax.xml.*"})
public class SpringExampleTest {
 
    @Rule
    public PowerMockRule rule = new PowerMockRule();
 
    ....
    // Same code as in the previous example
 

Re-run the test and it'll now pass.

Introducing PowerMock Java Agent

To make it easier to use PowerMock for integration testing version 1.4.9 introduces yet another way to bootstrap the framework, this time using a java agent. Instead of using classloaders to bootstrap and perform byte-code manipulation we now use Java's instrumentation API. This means that you won't run into classloading issues as those presented in the previous example. The test looks exactly the same though:

 
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration("classpath:/example-context.xml")
@PrepareForTest(IdGenerator.class)
public class SpringExampleTest {
 
    @Rule
    public PowerMockRule rule = new PowerMockRule();
 
    @Autowired
    private MyBean myBean;
 
    @Test
    public void mockStaticMethod() throws Exception {
        // Given
        final long expectedId = 2L;
        mockStatic(IdGenerator.class);
        when(IdGenerator.generateNewId()).thenReturn(expectedId);
 
        // When
        final Message message = myBean.generateMessage();
 
        // Then
        assertEquals(expectedId, message.getId());
        assertEquals("John Doe, Paul Anderson, Jane Doe", message.getContent());
    }
 

Even though the test looks exactly the same the internal implementation has a lot of differences. To have a look at the real code behind the to examples have a look in subversion here for the first example and here for the java agent example.

Round-up

Using the Java agent can help you out when you run into hard-to-resolve classloading issues with PowerMock. The implementation is still experimental and we are aware of some bugs and limitations. To know more and find help getting started with the agent please have a look at the documentation.

Johan Haleby
Consultant at Jayway