How to use findFirstMatchingUnverifiedInvocation method of org.mockito.internal.invocation.InvocationsFinder class

when(myObject.someMethod()).thenReturn(someMockObject);
// run test
verify(myObject, times(n)).someMethod();

See the documentation for more conversion examples.

You do not need mockito to test this method, only a java.io.StringWriter.

Here is how you can write the test for a nominal use:

@Test
void status_written_in_csv_format() {
    // Setup
    WriteCsvToResponse objectUnderTest = new WriteCsvToResponse ();
    StringWriter stringWriter = new StringWriter();
    PrintWriter printWriter = new PrintWriter(stringWriter);

    // Given
    Status status = ...

    // When
    objectUnderTest.writeStatus(printWriter, status);

    // Then
    String actualCsv = stringWriter.toString();
    assertThat(actualCsv.split("\n"))
       .as("Produced CSV")
       .containsExactly(
         "id,storeId,status",
         "42,142,OK");
}

This example assume some things about your Status object, but you have the general idea. For assertions, I use AssertJ, but you can do the same with JUnit5 built-in assertions.

Hope this helps !

Mockito comes with a helper class to save you some reflection boiler plate code:

import org.mockito.internal.util.reflection.Whitebox;

//...

@Mock
private Person mockedPerson;
private Test underTest;

// ...

@Test
public void testMethod() {
    Whitebox.setInternalState(underTest, "person", mockedPerson);
    // ...
}

Update: Unfortunately the mockito team decided to remove the class in Mockito 2. So you are back to writing your own reflection boilerplate code, use another library (e.g. Apache Commons Lang), or simply pilfer the Whitebox class (it is MIT licensed).

Update 2: JUnit 5 comes with its own ReflectionSupport and AnnotationSupport classes that might be useful and save you from pulling in yet another library.

You should use PowerMock if you want Foo to return the same instance of Bar when you call doSomething method on any instance of Foo. Here's an example:

@RunWith(PowerMockRunner.class)
@PrepareForTest(Foo.class)
public class FooMockAllInstanceTest {

    @Test
    public void testMockInstanceofObjectCreation() throws Exception {
        Bar mockBar = PowerMockito.mock(Bar.class);
        when(mockBar.sayHello()).thenReturn("Hi John!");
        PowerMockito.whenNew(Bar.class)
                .withNoArguments()
                .thenReturn(mockBar);

        Foo myFooOne = new Foo();
        assertEquals(mockBar,  myFooOne.doSomething("Jane"));

        Foo myFooTwo = new Foo();
        assertEquals(mockBar,  myFooTwo.doSomething("Sarah"));

        Baz bazOne = new Baz();
        assertEquals(mockBar, bazOne.doSomething("Sam"));

        Baz bazTwo = new Baz();
        assertEquals(mockBar, bazTwo.doSomething("Nina"));
    }
}

This example will return the same Bar object even when Baz is called. Here's the Baz class,

public class Baz {

    public Bar doSomething(String name) {
        Foo foo = new Foo();
        return foo.doSomething(name);
    }
}

Update 2

There's another slightly better way to test with PowerMock. Here it's,

@Test
public void testStubbingMethod() throws Exception {
    Bar mockBar = PowerMockito.mock(Bar.class);
    when(mockBar.sayHello()).thenReturn("Hi John!");

    PowerMockito.stub(PowerMockito.method(Foo.class, "doSomething",
            String.class)).toReturn(mockBar);

    Foo myFooOne = new Foo();
    assertEquals(mockBar, myFooOne.doSomething("Jane"));

    Foo myFooTwo = new Foo();
    assertEquals(mockBar, myFooTwo.doSomething("Sarah"));

    Baz bazOne = new Baz();
    assertEquals(mockBar, bazOne.doSomething("Sam"));

    Baz bazTwo = new Baz();
    assertEquals(mockBar, bazTwo.doSomething("Nina"));
}

I stumbled upon this too and the answer is buried quite deep in the docs, so here it is.

Mockito optionally offers advanced mocking features like mocking final classes. For this to work, they use a different mechanism ("mock maker") that is considered experimental and therefore turned off by default. But:

As a convenience, the Mockito team provides an artifact where this mock maker is preconfigured. Instead of using the mockito-core artifact, include the mockito-inline artifact in your project.

In Java 8 it is even simpler than all of the above:

when(mockObject.myMethod(anyString()))
    .thenAnswer(invocation -> 
        invocation.getArgumentAt(0, String.class));

Two unrelated surprises are causing this problem together:

• Mockito.any(Class) doesn't actually return an object of that class. It returns null and stashes a "disregard the parameter and accept anything" matcher on a secret internal matcher stack called ArgumentMatcherStorage. That argument value will actually be null, but in most cases you won't see it.

• The statement when(foo.bar()).thenReturn(baz) actually calls foo.bar(), always. Typically this has no side effects—especially if you're stubbing its first chain of actions—so you don't notice it.

During the stub, Java calls your real answer, and tries to call setState on your matcher-based (null) argument. Based on Java evaluation order, this makes sense: Mockito calls your answer as if it were the system under test calling your answer, because there's no way for Mockito to know that the call to mobileMethod immediately precedes a call to when. It hasn't gotten there yet.

The answer is to use the "doVerb" methods, such as doAnswer, doReturn, and doThrow, which I like to call "Yoda syntax". Because these contain when(object).method() instead of when(object.method()), Mockito has a chance to deactivate your previously-set expectations, and your original answer is never triggered. It would look like this:

MyService myService = Mockito.mock(MyService.class, new StandardServiceAnswer());
Mockito.doAnswer(new MobileServiceAnswer())
    .when(myService).mobileMethod(
          Mockito.any(MobileCommand.class), Mockito.any(Context.class));

It's worth noting that the exception is the only reason that your override didn't work. Under normal circumstances "when-thenVerb" is absolutely fine for overriding, and will backtrack over the previous action so as not to throw off consecutive actions like .thenReturn(...).thenThrow(...). It's also worth noting that when(mobileMethod(command, context)) would have changed command and context during the stub without throwing an exception, which can introduce subtle testing gaps.

Some developers go so far as to prefer the "doVerb-when" syntax over the "when-thenVerb" syntax at all times, because it has that nice behavior of never calling the other mock. You're welcome to come to the same conclusion—"doVerb" does everything "when-thenVerb" does, but is safer to use when overriding behavior in mocks and spies. I prefer "when" syntax myself—it's a little nicer to read, and it does type-check return values—as long as you remember that sometimes "doVerb" is the only way to get where you need to go.

This comes down to the implementation of the spy. According to the docs, the Spy is created as a copy of the real instance:

Mockito does not delegate calls to the passed real instance, instead it actually creates a copy of it. So if you keep the real instance and interact with it, don't expect the spied to be aware of those interaction and their effect on real instance state. The corollary is that when an unstubbed method is called on the spy but not on the real instance, you won't see any effects on the real instance.

It seems to be a shallow copy. As a result, as far as my debugging shows, the CacheLoader is shared between the copy and the original object, but its reference to its enclosing object is the original object, not the spy. Therefore the real retrieveValue is called instead of the mocked one.

I'm not sure offhand what the best way to resolve this would be. One way for this specific example would be to invert the CacheLoader dependency (i.e. pass it into Subject instead of Subject defining it internally), and mock that instead of Subject.

From the last stack trace you've sent I assume, that you need to remove the logback-core and logback-classic dependencies from cassandra-unit in pom.xml.

Your dependencies will look like the following:

<dependency>
    <groupId>org.cassandraunit</groupId>
    <artifactId>cassandra-unit</artifactId>
    <version>3.3.0.2</version>
    <scope>test</scope>
    <exclusions>
        <exclusion>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-log4j12</artifactId>
        </exclusion>
        <exclusion>
            <groupId>ch.qos.logback</groupId>
            <artifactId>logback-core</artifactId>
        </exclusion>
        <exclusion>
            <groupId>ch.qos.logback</groupId>
            <artifactId>logback-classic</artifactId>
        </exclusion>
        <exclusion>
            <groupId>com.google.guava</groupId>
            <artifactId>guava</artifactId>
        </exclusion>
        <exclusion>
            <groupId>org.hibernate</groupId>
            <artifactId>hibernate-validator</artifactId>
        </exclusion>
        <exclusion>
            <groupId>org.apache.httpcomponents</groupId>
            <artifactId>httpclient</artifactId>
        </exclusion>
        <exclusion>
            <groupId>commons-codec</groupId>
            <artifactId>commons-codec</artifactId>
        </exclusion>
    </exclusions>
</dependency>

Let me know if you still have the issue