@Spy for partial mocking in JUnit - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the @Spy annotation in JUnit testing?

The @Spy annotation is used to create a partial mock of an object. It allows you to call real methods unless they are stubbed, combining real behavior with mock control.

beginner

How does @Spy differ from @Mock in JUnit?

@Mock creates a full mock where all methods are stubbed and do nothing unless specified. @Spy wraps a real object and calls real methods by default, allowing selective stubbing.

intermediate

Show a simple example of using @Spy in a JUnit test.

@Spy List<String> spyList = new ArrayList<>(); @Test void testSpy() { spyList.add("one"); verify(spyList).add("one"); assertEquals(1, spyList.size()); } This test uses @Spy to partially mock an ArrayList, calling real methods but verifying interactions.

intermediate

What happens if you stub a method on a @Spy object?

When you stub a method on a @Spy object, the stubbed method returns the specified value instead of calling the real method. Other methods still call real implementations.

beginner

Why might you choose @Spy over @Mock in testing?

Use @Spy when you want to test real behavior but still control or verify some method calls. It helps when you want to partially mock complex objects without rewriting all behavior.

What does the @Spy annotation do in JUnit?

A Creates a full mock where all methods do nothing by default

B Creates a partial mock that calls real methods unless stubbed

C Creates a new instance of the class without mocking

D Automatically verifies all method calls

The @Spy annotation creates a partial mock that calls real methods unless you stub them.

Which statement is true about @Spy and @Mock?

A @Spy calls real methods by default; @Mock does not

B @Mock calls real methods by default; @Spy does not

C Both call real methods by default

D Neither can be used with JUnit

@Spy calls real methods unless stubbed; @Mock creates a full mock with no real method calls.

If you stub a method on a @Spy object, what happens when that method is called?

A The stubbed value is returned instead of calling the real method

B The real method is always called ignoring the stub

C An exception is thrown

D The method call is ignored

Stubbing a method on a @Spy causes that method to return the stubbed value instead of calling the real method.

Which of these is a good use case for @Spy?

A When you want to disable all method calls

B When you want to mock an interface with no implementation

C When you want to create a new object without mocking

D When you want to test real behavior but control some methods

@Spy is useful when you want to test real behavior but selectively control or verify some methods.

In the example: @Spy List<String> spyList = new ArrayList<>(); spyList.add("one"); What will spyList.size() return?

A Throws an exception

B 0

C 1

D Depends on stubbing

Since @Spy calls real methods by default, adding "one" increases the list size to 1.

Explain how @Spy works for partial mocking in JUnit and when you would use it.

Think about mixing real method calls with mocked behavior.

Describe the difference between @Spy and @Mock annotations in JUnit testing.

Focus on default method call behavior.