How to solve null pointer exception in spring boot test?

I have two simple Consumer components and one Consumer service. Then I want to write unit test for this service. But always got null pointer exception.

I have tried: Autowire construction, Autowire setter, Put mock into test method body, put mock into @Before method, Add/Remove @PrepareForTest({MyService.class}), etc. None of them is working.

Here is my code:

@Component
public class MyComponent1 implements Consumer<Employee> { }

@Component
public class MyComponent2 implements Consumer<Employee> { }

@Service
public class MyService implements Consumer<List<Employee>> {
    @Autowired
    private MyComponent1 myComponent1;
    @Autowired
    private MyComponent2 myComponent2;

    @Override
    public void accept(List<Employee> employeeList) {
        employeeList.forEach(myComponent1.andThen(myComponent2));
    }
}

@SpringBootTest
@ActiveProfiles("test")
@RunWith(SpringRunner.class)
//@PrepareForTest({MyService.class})
public class MyServiceTest {

    @MockBean
    private MyComponent1 mockedComponent1;

    @MockBean
    private MyComponent2 mockedComponent2;

    @Autowired
    private MyService myService;

    @Before
    public void init() {
        PowerMockito.doNothing().when(mockedComponent1).accept(any(Employee.class));
        PowerMockito.doNothing().when(mockedComponent2).accept(any(Employee.class));
    }
    @Test
    public void myTest() {
        Employee employee1 = new Employee("abc", 43);
        List<Employee> employeeList = new LinkedList<>();
        employeeList.add(employee1);

        myService.accept(employeeList);

        verify(mockedComponent1, times(1)).accept(any(Employee.class));
        verify(mockedComponent2, times(1)).accept(any(Employee.class));
    }
}

Error stack

java.lang.NullPointerException
    at java.util.Objects.requireNonNull(Objects.java:203)
    at java.lang.Iterable.forEach(Iterable.java:73)
    at com.wanghongliang.service.MyService.accept(MyService.java:31)
    at com.wanghongliang.service.MyServiceTest.myTest(MyServiceTest.java:54)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    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.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.springframework.test.context.junit4.statements.RunBeforeTestExecutionCallbacks.evaluate(RunBeforeTestExecutionCallbacks.java:74)
    at org.springframework.test.context.junit4.statements.RunAfterTestExecutionCallbacks.evaluate(RunAfterTestExecutionCallbacks.java:84)
    at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
    at org.springframework.test.context.junit4.statements.RunBeforeTestMethodCallbacks.evaluate(RunBeforeTestMethodCallbacks.java:75)
    at org.springframework.test.context.junit4.statements.RunAfterTestMethodCallbacks.evaluate(RunAfterTestMethodCallbacks.java:86)
    at org.springframework.test.context.junit4.statements.SpringRepeat.evaluate(SpringRepeat.java:84)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:251)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:97)
    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$2.evaluate(ParentRunner.java:268)
    at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
    at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:70)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:190)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
    at com.intellij.rt.execution.junit.IdeaTestRunner$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

The problem is that in your method under test you call:

myComponent1.andThen(myComponent2)

As myComponent1 is a mock and you haven’t stubbed andThen method call, the expression evaluates to null, which you pass to forEach.

Fix 1:

create a new mock of Consumer.class. Let’s call it combinedConsumer
stub myComponent1.andThen to return combinedConsumer

Fix 2:

Instruct Mockito to call real method for andThen

Mockito.when(consumer1.andThen(consumer2)).thenCallRealMethod();

More general remarks:

You don’t need a @SpringBootTest for a single service test. I would opt for MockitoExtension / Runner. Your test will be less fragile and faster, as currently entire spring app context needs to be created.
doNothing is the default action for a void method (in your case Consumer.accept). You don’t need to stub it.
Mockito is enough for this test (nothing PopwerMockito-specific is used)