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1. Introduction

In the past, we’ve talked extensively about JMockit and Mockito.

In this tutorial, we’ll give an introduction to another mocking tool – EasyMock.

2. Maven Dependencies

Before we dive in, let’s add the following dependency to our pom.xml:

<dependency>
    <groupId>org.easymock</groupId>
    <artifactId>easymock</artifactId>
    <version>3.5.1</version>
    <scope>test</scope>
</dependency>

The latest version can always be found here.

3. Core Concepts

When generating a mock, we can simulate the target object, specify its behavior, and finally verify whether it’s used as expected.

Working with EasyMock’s mocks involves four steps:

  1. creating a mock of the target class
  2. recording its expected behavior, including the action, result, exceptions, etc.
  3. using mocks in tests
  4. verifying if it’s behaving as expected

After our recording finishes, we switch it to “replay” mode, so that the mock behaves as recorded when collaborating with any object that will be using it.

Eventually, we verify if everything goes as expected.

The four steps mentioned above relate to methods in org.easymock.EasyMock:

  1. mock(…): generates a mock of the target class, be it a concrete class or an interface. Once created, a mock is in “recording” mode, meaning that EasyMock will record any action the Mock Object takes, and replay them in the “replay” mode
  2. expect(…): with this method, we can set expectations, including calls, results, and exceptions, for associated recording actions
  3. replay(…): switches a given mock to “replay” mode. Then, any action triggering previously recorded method calls will replay “recorded results”
  4. verify(…): verifies that all expectations were met and that no unexpected call was performed on a mock

In the next section, we’ll show how these steps work in action, using real-world examples.

4. A Practical Example of Mocking

Before we continue, let’s take a look at the example context: say we have a reader of the Baeldung blog, who likes to browse articles on the website, and then he/she tries to write articles.

Let’s start by creating the following model:

public class BaeldungReader {

    private ArticleReader articleReader;
    private IArticleWriter articleWriter;

    // constructors

    public BaeldungArticle readNext(){
        return articleReader.next();
    }

    public List<BaeldungArticle> readTopic(String topic){
        return articleReader.ofTopic(topic);
    }

    public String write(String title, String content){
        return articleWriter.write(title, content);
    }
}

In this model, we have two private members: the articleReader(a concrete class) and the articleWriter (an interface).

Next, we’ll mock them to verify BaeldungReader‘s behavior.

5. Mock with Java Code

Let’s begin with mocking an ArticleReader.

5.1. Typical Mocking

We expect the articleReader.next() method to be called when a reader skips an article:

@Test
public void whenReadNext_thenNextArticleRead(){
    ArticleReader mockArticleReader = mock(ArticleReader.class);
    BaeldungReader baeldungReader
      = new BaeldungReader(mockArticleReader);

    expect(mockArticleReader.next()).andReturn(null);
    replay(mockArticleReader);

    baeldungReader.readNext();

    verify(mockArticleReader);
}

In the sample code above, we stick strictly to the 4-step procedure and mock the ArticleReader class.

Although we really don’t care what mockArticleReader.next() returns, we still need to specify a return value for mockArticleReader.next() by using expect(…).andReturn(…).

With expect(…), EasyMock is expecting the method to return a value or throw an Exception.

If we simply do:

mockArticleReader.next();
replay(mockArticleReader);

EasyMock will complain about this, as it requires a call on expect(…).andReturn(…) if the method returns anything.

If it’s a void method, we can expect its action using expectLastCall() like this:

mockArticleReader.someVoidMethod();
expectLastCall();
replay(mockArticleReader);

5.2. Replay Order

If we need actions to be replayed in a specific order, we can be more strict:

@Test
public void whenReadNextAndSkimTopics_thenAllAllowed(){
    ArticleReader mockArticleReader
      = strictMock(ArticleReader.class);
    BaeldungReade baeldungReader
      = new BaeldungReader(mockArticleReader);

    expect(mockArticleReader.next()).andReturn(null);
    expect(mockArticleReader.ofTopic("easymock")).andReturn(null);
    replay(mockArticleReader);

    baeldungReader.readNext();
    baeldungReader.readTopic("easymock");

    verify(mockArticleReader);
}

In this snippet, we use strictMock(…) to check the order of method calls. For mocks created by mock(…) and strictMock(…), any unexpected method calls would cause an AssertionError.

To allow any method call for the mock, we can use niceMock(…):

@Test
public void whenReadNextAndOthers_thenAllowed(){
    ArticleReader mockArticleReader = niceMock(ArticleReader.class);
    BaeldungReade baeldungReader = new BaeldungReader(mockArticleReader);

    expect(mockArticleReader.next()).andReturn(null);
    replay(mockArticleReader);

    baeldungReader.readNext();
    baeldungReader.readTopic("easymock");

    verify(mockArticleReader);
}

Here we didn’t expect the baeldungReader.readTopic(…) to be called, but EasyMock won’t complain. With niceMock(…), EasyMock now only cares if the target object performed expected action or not.

5.3. Mocking Exception Throws

Now, let’s continue with mocking the interface IArticleWriter, and how to handle expected Throwables:

@Test
public void whenWriteMaliciousContent_thenArgumentIllegal() {
    // mocking and initialization

    expect(mockArticleWriter
      .write("easymock","<body onload=alert('baeldung')>"))
      .andThrow(new IllegalArgumentException());
    replay(mockArticleWriter);

    // write malicious content and capture exception as expectedException

    verify(mockArticleWriter);
    assertEquals(
      IllegalArgumentException.class, 
      expectedException.getClass());
}

In the snippet above, we expect the articleWriter is solid enough to detect XSS(Cross-site Scripting) attacks.

So when the reader tries to inject malicious code into the article content, the writer should throw an IllegalArgumentException. We recorded this expected behavior using expect(…).andThrow(…).

6. Mock with Annotation

EasyMock also supports injecting mocks using annotations. To use them, we need to run our unit tests with EasyMockRunner so that it processes @Mock and @TestSubject annotations.

Let’s rewrite previous snippets:

@RunWith(EasyMockRunner.class)
public class BaeldungReaderAnnotatedTest {

    @Mock
    ArticleReader mockArticleReader;

    @TestSubject
    BaeldungReader baeldungReader = new BaeldungReader();

    @Test
    public void whenReadNext_thenNextArticleRead() {
        expect(mockArticleReader.next()).andReturn(null);
        replay(mockArticleReader);
        baeldungReader.readNext();
        verify(mockArticleReader);
    }
}

Equivalent to mock(…), a mock will be injected into fields annotated with @Mock. And these mocks will be injected into fields of the class annotated with @TestSubject.

In the snippet above, we didn’t explicitly initialize the articleReader field in baeldungReader. When calling baeldungReader.readNext(), we can inter that implicitly called mockArticleReader.

That was because mockArticleReader was injected to the articleReader field.

Note that if we want to use another test runner instead of EasyMockRunner, we can use the JUnit test rule EasyMockRule:

public class BaeldungReaderAnnotatedWithRuleTest {

    @Rule
    public EasyMockRule mockRule = new EasyMockRule(this);

    //...

    @Test
    public void whenReadNext_thenNextArticleRead(){
        expect(mockArticleReader.next()).andReturn(null);
        replay(mockArticleReader);
        baeldungReader.readNext();
        verify(mockArticleReader);
    }

}

7. Mock with EasyMockSupport

Sometimes we need to introduce multiple mocks in a single test, and we have to repeat manually:

replay(A);
replay(B);
replay(C);
//...
verify(A);
verify(B);
verify(C);

This is ugly, and we need an elegant solution.

Luckily, we have a class EasyMockSupport in EasyMock to help deal with this. It helps keep track of mocks, such that we can replay and verify them in a batch like this:

//...
public class BaeldungReaderMockSupportTest extends EasyMockSupport{

    //...

    @Test
    public void whenReadAndWriteSequencially_thenWorks(){
        expect(mockArticleReader.next()).andReturn(null)
          .times(2).andThrow(new NoSuchElementException());
        expect(mockArticleWriter.write("title", "content"))
          .andReturn("BAEL-201801");
        replayAll();

        // execute read and write operations consecutively
 
        verifyAll();
 
        assertEquals(
          NoSuchElementException.class, 
          expectedException.getClass());
        assertEquals("BAEL-201801", articleId);
    }

}

Here we mocked both articleReader and articleWriter. When setting these mocks to “replay” mode, we used a static method replayAll() provided by EasyMockSupport, and used verifyAll() to verify their behaviors in batch.

We also introduced times(…) method in the expect phase. It helps specify how many times we expect the method to be called so that we can avoid introducing duplicate code.

We can also use EasyMockSupport through delegation:

EasyMockSupport easyMockSupport = new EasyMockSupport();

@Test
public void whenReadAndWriteSequencially_thenWorks(){
    ArticleReader mockArticleReader = easyMockSupport
      .createMock(ArticleReader.class);
    IArticleWriter mockArticleWriter = easyMockSupport
      .createMock(IArticleWriter.class);
    BaeldungReader baeldungReader = new BaeldungReader(
      mockArticleReader, mockArticleWriter);

    expect(mockArticleReader.next()).andReturn(null);
    expect(mockArticleWriter.write("title", "content"))
      .andReturn("");
    easyMockSupport.replayAll();

    baeldungReader.readNext();
    baeldungReader.write("title", "content");

    easyMockSupport.verifyAll();
}

Previously, we used static methods or annotations to create and manage mocks. Under the hood, these static and annotated mocks are controlled by a global EasyMockSupport instance.

Here, we explicitly instantiated it and take all these mocks under our own control, through delegation. This may help avoid confusion if there’s any name conflicts in our test code with EasyMock or be there any similar cases.

8. Conclusion

In this article, we briefly introduced the basic usage of EasyMock, about how to generate mock objects, record and replay their behaviors, and verify if they behaved correctly.

In case you may be interested, check out this article for a comparison of EasyMock, Mocket, and JMockit.

As always, the full implementation can be found over on Github.

I just announced the new Spring 5 modules in REST With Spring:

>> CHECK OUT THE LESSONS

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