How to use multiple ArgumentCaptor parameter

Question

When I use more than 1 ArgumentCaptor parameter I receive a Nullpointer. May be anyone has an idea what goes wrong?

@RunWith(PowerMockRunner.class)
@PrepareForTest(FileHelper.class)
public class MyTest {

...

@Before
public void setUp() throws Exception {
PowerMockito.mockStatic(FileHelper.class);
}


@Test
public void save() throws Exception {
ArgumentCaptor<String> argName = ArgumentCaptor.forClass(String.class);
ArgumentCaptor<List> argList = ArgumentCaptor.forClass(List.class);
PowerMockito.doNothing().when(FileHelper.class, "saveTextFile", argName.capture(),
    argList.capture());

...
}

The saveTextFile methods in the FileHelper class:

public static void saveTextFile(String filename, List<String> data) {
...
}

If I run the code I get this exception:

java.lang.NullPointerException at java.lang.Class.isAssignableFrom(Native Method) at org.powermock.reflect.internal.WhiteboxImpl.checkIfParameterTypesAreSame(WhiteboxImpl.java:2432) at org.powermock.reflect.internal.WhiteboxImpl.getMethods(WhiteboxImpl.java:1934) at org.powermock.reflect.internal.WhiteboxImpl.getBestMethodCandidate(WhiteboxImpl.java:1025) at org.powermock.reflect.internal.WhiteboxImpl.findMethodOrThrowException(WhiteboxImpl.java:948) at org.powermock.reflect.internal.WhiteboxImpl.doInvokeMethod(WhiteboxImpl.java:882) at org.powermock.reflect.internal.WhiteboxImpl.invokeMethod(WhiteboxImpl.java:859) at org.powermock.reflect.Whitebox.invokeMethod(Whitebox.java:466) at org.powermock.api.mockito.internal.expectation.PowerMockitoStubberImpl.when(PowerMockitoStubberImpl.java:106)

If I call a method that has only one argument, i.e. the list argument everything works fine.

Answer 1

I think you probably want to use "verify" instead of when.

You are setting an expectation to "do nothing" on your mock.

Wouldn't it be better to call the method being tested and then verify what you expected to happen did happen?

For example, consider these two classes...

HelperClass.java

public class HelperClass {

    public static void nastyStaticCall(String filename, List<String> data) {
       System.out.println("REAL METHOD CALLED!!");
    }
}

MyClass.java

import java.util.Arrays;

public class MyClass {

    public void testMethod() {
        HelperClass.nastyStaticCall("FILENAME", Arrays.asList("Data"));
    }
}

If I wanted to test "MyClass", I'd do something like this...

import static org.fest.assertions.Assertions.assertThat;
import static org.powermock.api.mockito.PowerMockito.mockStatic;
import static org.powermock.api.mockito.PowerMockito.verifyStatic;

import java.util.List;

import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.ArgumentCaptor;
import org.mockito.Captor;
import org.mockito.MockitoAnnotations;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;

@RunWith(PowerMockRunner.class)
@PrepareForTest(HelperClass.class)
public class MyClassTest {

    private MyClass classUnderTest;

    @Captor
    private ArgumentCaptor<String> fileNameCaptor;

    @Captor
    private ArgumentCaptor<List<String>> dataCaptor;

    @Before
    public void setup() {
        MockitoAnnotations.initMocks(this);
        mockStatic(HelperClass.class);
        classUnderTest = new MyClass();
    }

    @Test
    public void shouldMakeNastyStaticCall() {
        classUnderTest.testMethod();

        verifyStatic();
        HelperClass.nastyStaticCall(fileNameCaptor.capture(), dataCaptor.capture());

        assertThat(fileNameCaptor.getValue()).isEqualTo("FILENAME");
        assertThat(dataCaptor.getValue()).containsOnly("Data");
    }
}

As you can see, all this test method does is call the actual code and then verify that the helper class was called with the correct arguments.

If you actually expected a static method call to return a value that would be used by the rest of the method you are testing, then you would need to use a "when", but as this returns "void", you don;t need to do anything.

In general, personally I avoid using argument captors in "when" clauses and leave these for the "verify" stages.

Hope this helps.