Powermock - mocking org.apache.log4j.Logger? How can I?

Question

I'm wondering what is the easiest and most concise way to mock org.apache.log4j with powermock (and mockito).

I have tried a few approaches (which I won't illustrate here) but have not yet found a way to achieve what I want. I have created a simple class to test below and I want to call the run method and verify that the log.info message has been called. How do I do this? I'm sure it's quite easy when you know how!

(I'm using @Rule as I want to run under spring test but that should make no difference).

Thanks a millon for the correct code.

import org.apache.log4j.Logger;
import org.junit.Rule;
import org.junit.Test;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.rule.PowerMockRule;

public class MockLog4JTest
{
    @Rule
    public PowerMockRule rule = new PowerMockRule();

    private static class ClassUnderTest
    {
        private static Logger logger = Logger.getLogger(ClassUnderTest.class);

        public void run() {
            logger.info("Hello.");
        }

    }

    @Test
    public void testLog()
    {
        ClassUnderTest classUnderTest = new ClassUnderTest();
        classUnderTest.run();
    }

}

Have you tried mocking the static `Logger` class? This page shows you how: https://code.google.com/p/powermock/wiki/MockStatic — Chris Mantle, Sep 25 '13 at 13:25
I have tried that but I didn't manage to get it working. The above is so simple an an example, I am hoping that somebody will post an answer with the few lines of code to get this working, rather than suggestions (as I have tried quite a few things) - but thanks for that Chris all the same. — fergal_dd, Sep 25 '13 at 14:01
Hm, I'd second Chris' approach to mocking `Logger` in that its static `getLogger` method will return a mocked `Logger` instance. Injecting the Logger instance into the ClassUnderTest is not an option, right? — Fildor, Sep 25 '13 at 14:46

score 10 · Accepted Answer · answered Sep 25 '13 at 15:27

Chris and Fildor are correct to try to mock the Logger.getLogger(). Unfortunately the way Log4J works makes that technically tricky.

Here's the code I came up with (tested) based on your sample above.

ClassUnderTest.java

import org.apache.log4j.Logger;

public class ClassUnderTest {
    private static Logger logger = Logger.getLogger(ClassUnderTest.class);

    public void run() {
        logger.info("Hello.");
    }
}

MockLog4JTest.java

import static org.mockito.Matchers.eq;
import static org.mockito.Mockito.verify;
import static org.powermock.api.mockito.PowerMockito.mock;

import org.apache.log4j.Logger;
import org.junit.BeforeClass;
import org.junit.Rule;
import org.junit.Test;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.rule.PowerMockRule;
import org.powermock.reflect.Whitebox;

@PrepareForTest(ClassUnderTest.class)
public class MockLog4JTest {
    @Rule
    public PowerMockRule rule = new PowerMockRule();

    @BeforeClass
    public static void oneTimeSetup() {
        System.setProperty("log4j.defaultInitOverride", Boolean.toString(true));
        System.setProperty("log4j.ignoreTCL", Boolean.toString(true));
    }

    @Test
    public void testLog()
    {
        ClassUnderTest classUnderTest = new ClassUnderTest();
        Logger mockLogger = mock(Logger.class);

        Whitebox.setInternalState(ClassUnderTest.class, "logger", mockLogger);

        classUnderTest.run();

        verify(mockLogger).info(eq("Hello."));
    }
}

I chose to go with using Whitebox to outright set the static field on the class under test to my mockLogger instance. After that, verification was pretty straightforward.

The above code works perfectly. I tried it out. Thanks. It does answer the question. — fergal_dd, Sep 25 '13 at 16:17
For some reason under spring testing I couldn't get this working until I changed the line with WhiteBox. see http://stackoverflow.com/questions/3301635/change-private-static-final-field-using-java-reflection — fergal_dd, Sep 25 '13 at 16:47
I'm glad that worked for you; could you please "accept" the answer? What change did you have to make to the Whitebox line? I haven't used Spring testing with Powermock so I'm curious about the integration issues that come up. — Matt Lachman, Sep 25 '13 at 19:02
I've "answered" that - might be useful for others. I accepted your answer. Cheers for code. All working perfectly under spring now. — fergal_dd, Sep 26 '13 at 08:40

score 1 · Answer 2 · edited May 23 '17 at 12:17

Matt Lachman's answer worked perfectly for me - until I tried it using Spring. In Spring, I got an runtime exception when attempting to change the logger to the mockLogger. To get it working in Spring I had to do the following:

change the line

Whitebox.setInternalState(ClassUnderTest.class, "logger", mockLogger);

to

EncapsulationBreaker.setFinalStatic(ClassUnderTest.class.getDeclaredField("logger"),  mockLogger);

and the EncapsulationBreaker looks like this:

  public class EncapsulationBreaker
 {
    public static void setFinalStatic(Field field, Object newValue) throws Exception {
    field.setAccessible(true);

    Field modifiersField = Field.class.getDeclaredField("modifiers");
    modifiersField.setAccessible(true);
    modifiersField.setInt(field, field.getModifiers() & ~Modifier.FINAL);

    field.set(null, newValue);
 }

}

To read more about setting private static final members see Change private static final field using Java reflection

Also note, I'm only doing this for testing purposes.

Powermock - mocking org.apache.log4j.Logger? How can I?

2 Answers2

ClassUnderTest.java

MockLog4JTest.java