How to do partial mock of newDirectoryStream from Files.java using PowerMock with Mockito?

Question

I would like to partially mock Files.java newDirectoryStream static method.

Example below work perfectly fine when I use mockStatic. Unfortunately this approach mock all static methods. I would like to mock only one specific so wanted to use partial mocking through calling spy method.

import java.io.IOException;
import java.nio.file.DirectoryStream;
import java.nio.file.Files;
import java.nio.file.Path;

public class FileAccess {

    public static void newDirectoryStreamWrapper(Path path) throws IOException {
        DirectoryStream<Path> paths = Files.newDirectoryStream(path);
        if (paths == null) {
            return;
        }

        for (Path p : paths) {
                // some work
        }
    }
}

import org.junit.Assert;
import org.junit.Rule;
import org.junit.Test;
import org.junit.rules.TemporaryFolder;
import org.junit.runner.RunWith;
import org.mockito.Mockito;
import org.mockito.invocation.InvocationOnMock;
import org.mockito.stubbing.Answer;
import org.powermock.api.mockito.PowerMockito;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;

import java.io.File;
import java.io.IOException;
import java.nio.file.DirectoryStream;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.Map;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertTrue;

@RunWith(PowerMockRunner.class)
@PrepareForTest({Files.class, FileAccess.class})
public class Mocking {

    private boolean called = false;

    @Rule
    public TemporaryFolder folder = new TemporaryFolder();

    @Test
    public void test() throws Exception {

        //test will pass with mockStatic instead of spy
        PowerMockito.spy(Files.class);

        Mockito
            .when(Files.newDirectoryStream(Mockito.any(Path.class)))
            .thenAnswer(new Answer<Object>() {
                @Override
                public Object answer(InvocationOnMock invocationOnMock) throws Throwable {
                    called = true;
                    return invocationOnMock.callRealMethod();
                }
            });

        Path path = folder.newFile().toPath();
        FileAccess.newDirectoryStreamWrapper(path.getParent());

        assertTrue(called);
    }
}

Exception thrown while executing code above:

java.lang.NullPointerException
    at java.nio.file.Files.provider(Files.java:97)
    at java.nio.file.Files.newDirectoryStream(Files.java:457)
    at Mocking.test(Mocking.java:41)
    at org.junit.internal.runners.TestMethod.invoke(TestMethod.java:68)
    at org.junit.internal.runners.MethodRoadie$2.run(MethodRoadie.java:89)
    at org.junit.internal.runners.MethodRoadie.runBeforesThenTestThenAfters(MethodRoadie.java:97)
    at org.junit.rules.ExternalResource$1.evaluate(ExternalResource.java:48)
    at org.junit.internal.runners.MethodRoadie.runTest(MethodRoadie.java:87)
    at org.junit.internal.runners.MethodRoadie.run(MethodRoadie.java:50)
    at org.junit.internal.runners.ClassRoadie.runUnprotected(ClassRoadie.java:34)
    at org.junit.internal.runners.ClassRoadie.runProtected(ClassRoadie.java:44)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
    at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
    at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
    at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)

Answer 1

mockStatic does not mock all methods, it only enables mocking for all methods. So as long as you do not actually mock a methods, the real methods still get called even after mockStatic(). Only methods that are explicitly mocked after mockStatic will respond with the mocked behavior.

So your approach with mockStatic is correct.