Mockito测试-上下文onject为空异常

Question

I don't know why but I get null pointer exception when running this code. This is a really basic Mockito test which I copied from Android's official documentation.

Don't mind the logic. I made it super simple to show that even this basic example doesn't work

    package com.matala3.me.matala3;

import static org.hamcrest.MatcherAssert.assertThat;
import static org.hamcrest.CoreMatchers.*;
import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.Mock;
import org.mockito.Mockito;
import org.mockito.runners.MockitoJUnitRunner;
import android.content.Context;

@RunWith(MockitoJUnitRunner.class)
public class Mock_tests {

    private static final String FAKE_CONNECED_STRING = "Connected as";

    @Mock
    Context mMockContext;

    @Before
    public void before(){
        Mockito.when(mMockContext.getString(R.string.connected_as))
                .thenReturn("Connected as");
    }

    @Test
    public void checkHeaderString() {

        assertThat("Connected as", is(FAKE_CONNECED_STRING));
    }
}

I'm getting:

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.res.Resources.getString(int)' on a null object reference
at android.content.Context.getString(Context.java:476)
at com.matala3.me.matala3.ExampleInstrumentedTest.before(ExampleInstrumentedTest.java:26)
at java.lang.reflect.Method.invoke(Native Method)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:24)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.mockito.internal.runners.JUnit45AndHigherRunnerImpl.run(JUnit45AndHigherRunnerImpl.java:37)
at org.mockito.runners.MockitoJUnitRunner.run(MockitoJUnitRunner.java:62)
at org.junit.runners.Suite.runChild(Suite.java:128)
at org.junit.runners.Suite.runChild(Suite.java:27)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at org.junit.runner.JUnitCore.run(JUnitCore.java:115)
at android.support.test.internal.runner.TestExecutor.execute(TestExecutor.java:58)
at android.support.test.runner.AndroidJUnitRunner.onStart(AndroidJUnitRunner.java:375)
at android.app.Instrumentation$InstrumentationThread.run(Instrumentation.java:1932)

Answer 1

This statement :

Mockito.when(mMockContext.getString(R.string.connected_as))
       .thenReturn("Connected as");

provokes this exception :

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.res.Resources.getString(int)' on a null object reference

because mMockContext is null .
It means that the mocking initialization was not performed here :

@Mock
Context mMockContext;

I tested on my machine with Eclipse and Maven and I have the expected behavior : the mock is correctly initialized.

I could not explain the cause but I suspect a side effect with some classes or the way which you run the test (IDE maybe ?).
This first call :

android.app.Instrumentation$InstrumentationThread.run(Instrumentation.java:1932)

refers to an instrumented test, that is, with device/emulator aware while you didn't write a instrumented test as you mock device features.

So as advice : you could check you don't have configuration or dependencies that enable the instrumentation and try to execute it from command line to ensure that it is not a IDE issue.

Answer 2

If you are using Mockito 2.x, the org.mockito.runners.MockitoJUnitRunner is deprecated and org.mockito.junit.MockitoJUnitRunner should be used.

If problem still exists you could initialize the @Mocks by adding this the @Before method:

@Before
public void before(){
    MockitoAnnotations.initMocks(this);
    Mockito.when(mMockContext.getString(R.string.connected_as))
            .thenReturn("Connected as");
}

Update :

I looked a bit at the Context.getString(int) javadocs and it is a final method. There is not out-of-the-box support for mocking that kind of methods in Mockito.

You would need to use a special plugin , or Powermock which is also explained on that link.