How can I mock a private static method with PowerMockito?

Question

This is the same question as found here. Unfortunately, the accepted answer isn't working for me. I have a static utility class with private methods that I need to test. I find that when I mock methods like this:

PowerMockito.spy(StaticUtil.class);
PowerMockito.when(StaticUtil.class, "getSomethingMethod", someObjectArray, someStringArray, aBoolean, someList).thenReturn(anotherList);

I'm getting a null pointer exception because the getSomethingMethod() is actually being called. When I debug, I see that it isn't being called when I run the method I'm trying to test, but it is running when I am setting up the mock. Based on this site, it looks like that is what is supposed to happen when you create the mock in this format.

So then I try to set up the mock this way:

PowerMockito.spy(StaticUtil.class);        
PowerMockito.doReturn(anotherList).when(StaticUtil.getSomethingMethod( someObjectArray, someStringArray, aBoolean, someList);

However, I am get an error from Eclipse that says I need to change the visibility of getSomethingMethod() to public. Isn't one of the big benefits of using PowerMockito that you can mock private methods? I need to mock this private static method (without actually calling the method during setup).

Answer 1

when you set the expectations on mock object, you have to use argument matchers like Matchers.any() or Matchers.anyString() but not actual arguments.

For more details, see my answer to J-Unit Test: Make static void method in final class throw exception

There is a potential issue with durron597's answer: the syntax in 'testDoReturnClassStringMethod' is not mocked properly. In that method, he tried to mock StaticUtil class but didn't call the test method wrapper. See the example

@Test
public void testDoReturnClassStringMethod() throws Exception {
    System.out.println("Beginning Test doReturn().when(Class klass, String method name");
    PowerMockito.spy(StaticUtil.class);
    PowerMockito.doReturn(anotherList).when(StaticUtil.class, "getSomethingMethod", someObjectArray, someStringArray, aBoolean, someList);
    StaticUtil.Wrapper();
    System.out.println("End Test doReturn");
}

The result for this is

Beginning Test doReturn().when(Class klass, String method name
I happen!
End Test doReturn

'I happen!' is printed. mock is not configured properly.

The correct way of mocking is:

@Test
public void testDoReturnWithProperMock() throws Exception {
    System.out.println("Beginning Test doReturn().when(Class klass, String method name");
    PowerMockito.spy(StaticUtil.class);
    PowerMockito.doReturn(anotherList).when(StaticUtil.class, "getSomethingMethod", Matchers.anyObject(), Matchers.anyObject(), Matchers.anyBoolean(), Matchers.anyList());
    StaticUtil.Wrapper();
    System.out.println("End Test doReturn");

}

For which the result is:

Beginning Test doReturn().when(Class klass, String method name
End Test doReturn

Answer 2

You have to use the exact syntax that they specify in the answer you linked. That syntax is doReturn(returnValue).when(Class, String, arguments);. Neither of the examples you've provided here use that example.

---

Here's some extended explanation. I've thrown together a sample test framework to demonstrate this:

Trying to run tests on this class:

package org.test.stackoverflow;

import java.util.Collections;
import java.util.List;

public class StaticUtil {
  public static void Wrapper() {
    getSomethingMethod(null, null, false, Collections.<String>emptyList());
  }

  private static List<String> getSomethingMethod(Object[] obj,
      String[] str, boolean flag, List<String> aList){ 
    System.out.println("I happen!");
    return aList;
  }
}

If the method itself gets invoked, we'll see I happen!. If it doesn't, we won't.

Then, I use this test class:

package org.test.stackoverflow;

import java.util.List;

import org.junit.runner.RunWith;
import org.junit.*;
import org.powermock.api.mockito.PowerMockito;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;

@RunWith(PowerMockRunner.class)
@PrepareForTest(org.test.stackoverflow.StaticUtil.class)
public class StaticUtilTest {
  Object[] someObjectArray;
  String[] someStringArray;
  boolean aBoolean;
  List<String> someList;
  List<String> anotherList;

  @Test
  public void testWhenClassStringMethod() throws Exception {
    System.out.println("Beginning Test when(Class klass, String method name).doReturn(result)");
    PowerMockito.spy(StaticUtil.class);
    PowerMockito.when(StaticUtil.class, "getSomethingMethod", someObjectArray, someStringArray, aBoolean, someList).thenReturn(anotherList);
    System.out.println("End Test when");
  }

  @Test
  public void testDoReturnActualMethod() throws Exception {
    PowerMockito.spy(StaticUtil.class);
    // This doesn't compile as you've correctly stated
//    PowerMockito.doReturn(anotherList).when(StaticUtil.getSomethingMethod(someObjectArray, someStringArray, aBoolean, someList);
  }

  @Test
  public void testDoReturnClassStringMethod() throws Exception {
    System.out.println("Beginning Test doReturn().when(Class klass, String method name");
    PowerMockito.spy(StaticUtil.class);
    PowerMockito.doReturn(anotherList).when(StaticUtil.class, "getSomethingMethod", someObjectArray, someStringArray, aBoolean, someList);
    System.out.println("End Test doReturn");
  }
}

So, if it prints I happen, then we've used the wrong syntax. When I run this program, we get:

Beginning Test when(Class klass, String method name).doReturn(result)
I happen!
End Test when
Beginning Test doReturn().when(Class klass, String method name)
End Test doReturn

Therefore, you must use the syntax in the third test.

Note: this example uses static, empty arguments; obviously you should configure your example to use Argument Matchers as normal as appropriate for your application.