java - how to inject mock into private method?

Question

the mock object is working fine when i access public methods. but it s not working when i access private method.

My Mock Class:

@component
public class Test{

public List<String> list(){
 // some function}
}

My Main Class:

@component
public class Test2{
private string method(String method){
//here where i have to use mock object
//some function
}
}

My TestCases:

public class JunitTestCases{

@Mock
Test test;

@Autowired
@InjectMocks
Test2 test2
public void Oncall{
Test2 test=new Test2();
Method method=Test2.class.getDeclaredMethod("method",String.class);
method.setAccessible(true);
method.invoke(test, "data");}
}

i am getting the following errors.

java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.TestCases.method(TestClass.java:198)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
at org.springframework.test.context.junit4.statements.RunBeforeTestMethodCallbacks.evaluate(RunBeforeTestMethodCallbacks.java:75)
at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
at org.springframework.test.context.junit4.statements.RunAfterTestMethodCallbacks.evaluate(RunAfterTestMethodCallbacks.java:86)
at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:298)
at org.junit.internal.runners.statements.FailOnTimeout$CallableStatement.call(FailOnTimeout.java:292)
at java.util.concurrent.FutureTask.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.NullPointerException
at com.MainClass.validate(MainClass.java:149)
... 21 more

any suggestion? how can i make it work?

Answer 1

Demo classs For Writing Tests

public class PowerMockDemo {

    public Point callPrivateMethod() {
        return privateMethod(new Point(1, 1));
    }

    private Point privateMethod(Point point) {
        return new Point(point.getX() + 1, point.getY() + 1);
    }
}

Test Class For Demo class

@RunWith(PowerMockRunner.class)
@PrepareForTest(PowerMockDemo.class)
public class PowerMockDemoTest {

    private PowerMockDemo powerMockDemoSpy;

    @Before
    public void setUp() {
        powerMockDemoSpy = PowerMockito.spy(new PowerMockDemo());
    }

    @Test
    public void testMockPrivateMethod() throws Exception {
        Point mockPoint = mock(Point.class);

        PowerMockito.doReturn(mockPoint)
            .when(powerMockDemoSpy, "privateMethod", anyObject());

        Point actualMockPoint = powerMockDemoSpy.callPrivateMethod();

        assertThat(actualMockPoint, is(mockPoint));
    }
}