Fork me on GitHub

Installation

Requirements

  • EasyMock requires Java 1.5.0 and above
  • Objenesis (2.0+) must be in the classpath to perform class mocking

Using Maven

EasyMock is available in the Maven central repository. Just add the following dependency to your pom.xml:

<dependency>
  <groupId>org.easymock</groupId>
  <artifactId>easymock</artifactId>
  <version>3.4</version>
  <scope>test</scope>
</dependency>

You can obviously use any other dependency tool compatible with the Maven repository.

Standalone

  • Download the EasyMock zip file
  • It contains the easymock-3.4.jar to add to your classpath
  • To perform class mocking, also add Objenesis to your classpath.
  • The bundle also contains jars for the javadoc, the tests, the sources and the samples

Android Since 3.2

EasyMock can be used on Android VM (Dalvik). Just add EasyMock and Dexmaker as dependencies of the apk project used to test your application. It is a good idea to exclude Cglib since Dexmaker is used instead. If you use Maven, the final required dependencies will look like this:

<dependency>
  <groupId>org.easymock</groupId>
  <artifactId>easymock</artifactId>
  <version>3.4</version>
</dependency>
<dependency>
  <groupId>com.google.dexmaker</groupId>
  <artifactId>dexmaker</artifactId>
  <version>1.1</version>
</dependency>
        

Mocking

The first Mock Object

We will now build a test case and toy around with it to understand the functionalities of EasyMock. You can also have a look at the samples and the Getting Started.

Our first test should check whether the removal of a non-existing document does not lead to a notification of the collaborator. Here is the test without the definition of the Mock Object:

import org.junit.*;

public class ExampleTest {

  private ClassUnderTest classUnderTest;

  private Collaborator mock;

  @Before
  public void setUp() {
    classUnderTest = new ClassUnderTest();
    classUnderTest.setListener(mock);
  }

  @Test
  public void testRemoveNonExistingDocument() {
    // This call should not lead to any notification
    // of the Mock Object:
    classUnderTest.removeDocument("Does not exist");
  }
}

For many tests using EasyMock, we only need a static import of methods of org.easymock.EasyMock.

import static org.easymock.EasyMock.*;
import org.junit.*;

public class ExampleTest {
  private ClassUnderTest classUnderTest;
  private Collaborator mock;
}

To get a Mock Object, we need to

  1. create a Mock Object for the interface we would like to simulate
  2. record the expected behavior
  3. switch the Mock Object to replay state

Here is a first example:

@Before
public void setUp() {
  mock = mock(Collaborator.class); // 1
  classUnderTest = new ClassUnderTest();
  classUnderTest.setListener(mock);
}

@Test
public void testRemoveNonExistingDocument() {
  // 2 (we do not expect anything)
  replay(mock); // 3
  classUnderTest.removeDocument("Does not exist");
}

After activation in step 3, mock is a Mock Object for the Collaborator interface that expects no calls. This means that if we change our ClassUnderTest to call any of the interface's methods, the Mock Object will throw an AssertionError:

java.lang.AssertionError:
  Unexpected method call documentRemoved("Does not exist"):
    at org.easymock.internal.MockInvocationHandler.invoke(MockInvocationHandler.java:29)
    at org.easymock.internal.ObjectMethodsFilter.invoke(ObjectMethodsFilter.java:44)
    at $Proxy0.documentRemoved(Unknown Source)
    at org.easymock.samples.ClassUnderTest.notifyListenersDocumentRemoved(ClassUnderTest.java:74)
    at org.easymock.samples.ClassUnderTest.removeDocument(ClassUnderTest.java:33)
    at org.easymock.samples.ExampleTest.testRemoveNonExistingDocument(ExampleTest.java:24)
      ...

Using annotations Since 3.2

There is a nice and shorter way to create your mocks and inject them to the tested class. Here is the example above, now using annotations:

import static org.easymock.EasyMock.*;
import org.easymock.EasyMockRunner;
import org.easymock.TestSubject;
import org.easymock.Mock;
import org.junit.Test;
import org.junit.runner.RunWith;

@RunWith(EasyMockRunner.class)
public class ExampleTest {

  @TestSubject
  private ClassUnderTest classUnderTest = new ClassUnderTest(); // 2

  @Mock
  private Collaborator mock; // 1

  @Test
  public void testRemoveNonExistingDocument() {
    replay(mock);
    classUnderTest.removeDocument("Does not exist");
  }
}

The mock is instantiated by the runner at step 1. It is then set by the runner, to the listener field on step 2. The setUp method can be removed since all the initialization was done by the runner.

On top of that, since EasyMock 3.3, if you need to use another runner on you tests, a JUnit rule is also available to you. Both have the exact same behavior. Choosing one of the other is a matter of taste.

import static org.easymock.EasyMock.*;
import org.easymock.EasyMockRule;
import org.easymock.TestSubject;
import org.easymock.Mock;
import org.junit.Rule;
import org.junit.Test;

public class ExampleTest {

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

  @TestSubject
  private ClassUnderTest classUnderTest = new ClassUnderTest();

  @Mock
  private Collaborator mock;

  @Test
  public void testRemoveNonExistingDocument() {
    replay(mock);
    classUnderTest.removeDocument("Does not exist");
  }
}

The annotation has an optional element, 'type', to refine the mock as a 'nice' mock or a 'strict' mock. Another optional annotation, 'name', allows setting of a name for the mock that will be used in the mock() call, which will appear in expectation failure messages for example. Finally, an optional element, "fieldName", allows specifying the target field name where the mock should be injected. Mocks are injected to any field in any @TestSubject that is of compatible type. If more than one mock can be assigned to the same field then this is considered an error. The fieldName qualifier can be used in this scenario to disambiguate the assignments.

@Mock(type = MockType.NICE, name = "mock", fieldName = "someField")
private Collaborator mock;

@Mock(type = MockType.STRICT, name = "anotherMock", fieldName = "someOtherField")
private Collaborator anotherMock;

EasyMockSupport

EasyMockSupport is a class that exist to help you keeping track of your mock. Your test cases should extend or delegate to it. It will automatically registers all created mocks and replay, reset or verify them in batch instead of explicitly. Here's an example:

public class SupportTest extends EasyMockSupport {

  private Collaborator firstCollaborator;
  private Collaborator secondCollaborator;
  private ClassTested classUnderTest;

  @Before
  public void setup() {
    classUnderTest = new ClassTested();
  }

  @Test
  public void addDocument() {
    // creation phase
    firstCollaborator = mock(Collaborator.class);
    secondCollaborator = mock(Collaborator.class);
    classUnderTest.addListener(firstCollaborator);
    classUnderTest.addListener(secondCollaborator);

    // recording phase
    firstCollaborator.documentAdded("New Document");
    secondCollaborator.documentAdded("New Document");
    replayAll(); // replay all mocks at once

    // test
    classUnderTest.addDocument("New Document", new byte[0]);
    verifyAll(); // verify all mocks at once
  }
}

Strict Mocks

On a Mock Object returned by a EasyMock.mock(), the order of method calls is not checked. If you would like a strict Mock Object that checks the order of method calls, use EasyMock.strictMock() to create it.

If an unexpected method is called on a strict Mock Object, the message of the exception will show the method calls expected at this point followed by the first conflicting one. verify(mock) shows all missing method calls.

Nice Mocks

On a Mock Object returned by mock() the default behavior for all methods is to throw an AssertionError for all unexpected method calls. If you would like a "nice" Mock Object that by default allows all method calls and returns appropriate empty values (0, null or false), use niceMock() instead.

Partial mocking

Sometimes you may need to mock only some methods of a class and keep the normal behavior of others. This usually happens when you want to test a method that calls some others in the same class. So you want to keep the normal behavior of the tested method and mock the others.

In this case, the first thing to do is to consider a refactoring since most of the time this problem caused by a bad design. If it's not the case or if you can't do otherwise because of some development constraints, here's the solution.

ToMock mock = partialMockBuilder(ToMock.class)
  .addMockedMethod("mockedMethod").createMock();

In this case only the methods added with addMockedMethod(s) will be mocked (mockedMethod() in the example). The others will still behave as they used to. One exception: abstract methods are conveniently mocked by default.

partialMockBuilder returns a IMockBuilder interface. It contains various methods to easily create a partial mock. Have a look at the javadoc.

Remark: EasyMock provides a default behavior for Object's methods (equals, hashCode, toString, finalize). However, for a partial mock, if these methods are not mocked explicitly, they will have their normal behavior instead of EasyMock default's one.

Self testing

It is possible to create a mock by calling one of its constructor. This can be handy when a class method needs to be tested but the class other methods, mocked. For that you should do something like

ToMock mock = partialMockBuilder(ToMock.class)
  .withConstructor(1, 2, 3); // 1, 2, 3 are the constructor parameters

See the ConstructorCalledMockTest for an example.

Replace default class instantiator

For some reason (usually an unsupported JVM), it is possible that EasyMock isn't able to mock a class mock in your environment. Under the hood, class instantiation is implemented with a factory pattern. In case of failure, you can replace the default instantiator with:

  • The good old DefaultClassInstantiator which works well with Serializable classes and otherwise tries to guess the best constructor and parameters to use.
  • You own instantiator which only needs to implement IClassInstantiator.

You set this new instantiator using ClassInstantiatorFactory.setInstantiator(). You can set back the default one with setDefaultInstantiator().

Important:The instantiator is kept statically so it will stick between your unit tests. Make sure you reset it if needed.

Serialize a class mock

A class mock can also be serialized. However, since it extends a serializable class, this class might have defined a special behavior using for instance writeObject. These methods will still be called when serializing the mock and might fail. The workaround is usually to call a constructor when creating the mock.

Also, de-serializing the mock in a different class loader than the serialization might fail. It wasn't tested.

Class Mocking Limitations

  • To be coherent with interface mocking, EasyMock provides a built-in behavior for equals(), toString(), hashCode() and finalize() even for class mocking. It means that you cannot record your own behavior for these methods. This limitation is considered to be a feature that prevents you from having to care about these methods.
  • Final methods cannot be mocked. If called, their normal code will be executed.
  • Private methods cannot be mocked. If called, their normal code will be executed. During partial mocking, if your method under test is calling some private methods, you will need to test them as well since you cannot mock them.
  • Class instantiation is performed using Objenesis. Supported JVMs are listed here.

Naming Mock Objects

Mock Objects can be named at creation using mock(String name, Class<T> toMock), strictMock(String name, Class<T> toMock) or niceMock(String name, Class<T> toMock). The names will be shown in exception failures.

Behavior

A second test

Let us write a second test. If a document is added on the class under test, we expect a call to mock.documentAdded() on the Mock Object with the title of the document as argument:

@Test
public void testAddDocument() {
  mock.documentAdded("New Document"); // 2
  replay(mock); // 3
  classUnderTest.addDocument("New Document", new byte[0]);
}

So in the record state (before calling replay), the Mock Object does not behave like a Mock Object, but it records method calls. After calling replay, it behaves like a Mock Object, checking whether the expected method calls are really done.

If classUnderTest.addDocument("New Document", new byte[0]) calls the expected method with a wrong argument, the Mock Object will complain with an AssertionError:

java.lang.AssertionError:
  Unexpected method call documentAdded("Wrong title"):
    documentAdded("New Document"): expected: 1, actual: 0
      at org.easymock.internal.MockInvocationHandler.invoke(MockInvocationHandler.java:29)
      at org.easymock.internal.ObjectMethodsFilter.invoke(ObjectMethodsFilter.java:44)
      at $Proxy0.documentAdded(Unknown Source)
      at org.easymock.samples.ClassUnderTest.notifyListenersDocumentAdded(ClassUnderTest.java:61)
      at org.easymock.samples.ClassUnderTest.addDocument(ClassUnderTest.java:28)
      at org.easymock.samples.ExampleTest.testAddDocument(ExampleTest.java:30)
      ...

All missed expectations are shown, as well as all fulfilled expectations for the unexpected call (none in this case). If the method call is executed too often, the Mock Object complains, too:

java.lang.AssertionError:
  Unexpected method call documentAdded("New Document"):
    documentAdded("New Document"): expected: 1, actual: 2
      at org.easymock.internal.MockInvocationHandler.invoke(MockInvocationHandler.java:29)
      at org.easymock.internal.ObjectMethodsFilter.invoke(ObjectMethodsFilter.java:44)
      at $Proxy0.documentAdded(Unknown Source)
      at org.easymock.samples.ClassUnderTest.notifyListenersDocumentAdded(ClassUnderTest.java:62)
      at org.easymock.samples.ClassUnderTest.addDocument(ClassUnderTest.java:29)
      at org.easymock.samples.ExampleTest.testAddDocument(ExampleTest.java:30)
      ...

Changing Behavior for the Same Method Call

It is also possible to specify a changing behavior for a method. The methods times, andReturn, and andThrow may be chained. As an example, we define voteForRemoval("Document") to

  • return 42 for the first three calls,
  • throw a RuntimeException for the next four calls,
  • return -42 once.
expect(mock.voteForRemoval("Document"))
  .andReturn((byte) 42).times(3)
  .andThrow(new RuntimeException(), 4)
  .andReturn((byte) -42);

Altering EasyMock default behavior

EasyMock provides a property mechanisim allowing to alter its behavior. It mainly aims at allowing to use a legacy behavior on a new version. Currently supported properties are:

easymock.notThreadSafeByDefault
If true, a mock won't be thread-safe by default. Possible values are "true" or "false". Default is false
easymock.enableThreadSafetyCheckByDefault
If true, thread-safety check feature will be on by default. Possible values are "true" or "false". Default is false
easymock.disableClassMocking
Do not allow class mocking (only allow interface mocking). Possible values are "true" or "false". Default is false.

Properties can be set in two ways.

  • In an easymock.properties file set in the classpath default package
  • By calling EasyMock.setEasyMockProperty. Constants are available in the EasyMock class. Setting properties in the code obviously override any property set in easymock.properties

Object Methods

The behavior for the four Object methods equals(), hashCode(), toString() and finalize() cannot be changed for Mock Objects created with EasyMock, even if they are part of the interface for which the Mock Object is created.

Using Stub Behavior for Methods

Sometimes, we would like our Mock Object to respond to some method calls, but we do not want to check how often they are called, when they are called, or even if they are called at all. This stub behavoir may be defined by using the methods andStubReturn(Object value), andStubThrow(Throwable throwable), andStubAnswer(IAnswer<T> answer) and asStub(). The following code configures the MockObject to answer 42 to voteForRemoval("Document") once and -1 for all other arguments:

expect(mock.voteForRemoval("Document")).andReturn(42);
expect(mock.voteForRemoval(not(eq("Document")))).andStubReturn(-1);

Reusing a Mock Object

Mock Objects may be reset by reset(mock).

If needed, a mock can also be converted from one type to another by calling resetToNice(mock), resetToDefault(mock) ou resetToStrict(mock).

Verification

A first verification

There is one error that we have not handled so far: If we specify behavior, we would like to verify that it is actually used. The current test would pass if no method on the Mock Object is called. To verify that the specified behavior has been used, we have to call verify(mock):

@Test
public void testAddDocument() {
  mock.documentAdded("New Document"); // 2
  replay(mock); // 3
  classUnderTest.addDocument("New Document", new byte[0]);
  verify(mock);
}

If the method is not called on the Mock Object, we now get the following exception:

java.lang.AssertionError:
  Expectation failure on verify:
    documentAdded("New Document"): expected: 1, actual: 0
      at org.easymock.internal.MocksControl.verify(MocksControl.java:70)
      at org.easymock.EasyMock.verify(EasyMock.java:536)
      at org.easymock.samples.ExampleTest.testAddDocument(ExampleTest.java:31)
      ...

The message of the exception lists all missed expectations.

Expecting an Explicit Number of Calls

Up to now, our test has only considered a single method call. The next test should check whether the addition of an already existing document leads to a call to mock.documentChanged() with the appropriate argument. To be sure, we check this three times (hey, it is an example ;-)):

@Test
public void testAddAndChangeDocument() {
  mock.documentAdded("Document");
  mock.documentChanged("Document");
  mock.documentChanged("Document");
  mock.documentChanged("Document");
  replay(mock);
  classUnderTest.addDocument("Document", new byte[0]);
  classUnderTest.addDocument("Document", new byte[0]);
  classUnderTest.addDocument("Document", new byte[0]);
  classUnderTest.addDocument("Document", new byte[0]);
  verify(mock);
}

To avoid the repetition of mock.documentChanged("Document"), EasyMock provides a shortcut. We may specify the call count with the method times(int times) on the object returned by expectLastCall(). The code then looks like:

@Test
public void testAddAndChangeDocument() {
  mock.documentAdded("Document");
  mock.documentChanged("Document");
  expectLastCall().times(3);
  replay(mock);
  classUnderTest.addDocument("Document", new byte[0]);
  classUnderTest.addDocument("Document", new byte[0]);
  classUnderTest.addDocument("Document", new byte[0]);
  classUnderTest.addDocument("Document", new byte[0]);
  verify(mock);
}

If the method is called too often, we get an exception that tells us that the method has been called too many times. The failure occurs immediately at the first method call exceeding the limit:

java.lang.AssertionError:
  Unexpected method call documentChanged("Document"):
    documentChanged("Document"): expected: 3, actual: 4
      at org.easymock.internal.MockInvocationHandler.invoke(MockInvocationHandler.java:29)
      at org.easymock.internal.ObjectMethodsFilter.invoke(ObjectMethodsFilter.java:44)
      at $Proxy0.documentChanged(Unknown Source)
      at org.easymock.samples.ClassUnderTest.notifyListenersDocumentChanged(ClassUnderTest.java:67)
      at org.easymock.samples.ClassUnderTest.addDocument(ClassUnderTest.java:26)
      at org.easymock.samples.ExampleTest.testAddAndChangeDocument(ExampleTest.java:43)
      ...

If there are too few calls, verify(mock) throws an AssertionError:

java.lang.AssertionError:
  Expectation failure on verify:
    documentChanged("Document"): expected: 3, actual: 2
      at org.easymock.internal.MocksControl.verify(MocksControl.java:70)
      at org.easymock.EasyMock.verify(EasyMock.java:536)
      at org.easymock.samples.ExampleTest.testAddAndChangeDocument(ExampleTest.java:43)
      ...

Specifying Return Values

For specifying return values, we wrap the expected call in expect(T value) and specify the return value with the method andReturn(Object returnValue) on the object returned by expect(T value).

As an example, we check the workflow for document removal. If ClassUnderTest gets a call for document removal, it asks all collaborators for their vote for removal with calls to byte voteForRemoval(String title) value. Positive return values are a vote for removal. If the sum of all values is positive, the document is removed and documentRemoved(String title) is called on all collaborators:

@Test
public void testVoteForRemoval() {
  mock.documentAdded("Document");
  // expect document addition
  // expect to be asked to vote for document removal, and vote for it
  expect(mock.voteForRemoval("Document")).andReturn((byte) 42);
  mock.documentRemoved("Document");
  // expect document removal
  replay(mock);
  classUnderTest.addDocument("Document", new byte[0]);
  assertTrue(classUnderTest.removeDocument("Document"));
  verify(mock);
}

@Test
public void testVoteAgainstRemoval() {
  mock.documentAdded("Document");
  // expect document addition
  // expect to be asked to vote for document removal, and vote against it
  expect(mock.voteForRemoval("Document")).andReturn((byte) -42);
  replay(mock);
  classUnderTest.addDocument("Document", new byte[0]);
  assertFalse(classUnderTest.removeDocument("Document"));
  verify(mock);
}

The type of the returned value is checked at compile time. As an example, the following code will not compile, as the type of the provided return value does not match the method's return value:

expect(mock.voteForRemoval("Document")).andReturn("wrong type");

Instead of calling expect(T value) to retrieve the object for setting the return value, we may also use the object returned by expectLastCall(). Instead of

expect(mock.voteForRemoval("Document")).andReturn((byte) 42);

we may use

mock.voteForRemoval("Document");
expectLastCall().andReturn((byte) 42);

This type of specification should only be used if the line gets too long, as it does not support type checking at compile time.

Working with Exceptions

For specifying exceptions (more exactly: Throwables) to be thrown, the object returned by expectLastCall() and expect(T value) provides the method andThrow(Throwable throwable). The method has to be called in record state after the call to the Mock Object for which it specifies the Throwable to be thrown.

Unchecked exceptions (that is, RuntimeException, Error and all their subclasses) can be thrown from every method. Checked exceptions can only be thrown from the methods that do actually throw them.

Creating Return Values or Exceptions

Sometimes we would like our mock object to return a value or throw an exception that is created at the time of the actual call. Since EasyMock 2.2, the object returned by expectLastCall() and expect(T value) provides the method andAnswer(IAnswer answer) which allows to specify an implementation of the interface IAnswer that is used to create the return value or exception.

Inside an IAnswer callback, the arguments passed to the mock call are available via EasyMock.getCurrentArguments(). If you use these, refactorings like reordering parameters may break your tests. You have been warned.

An alternative to IAnswer are the andDelegateTo and andStubDelegateTo methods. They allow to delegate the call to a concrete implementation of the mocked interface that will then provide the answer. The pros are that the arguments found in EasyMock.getCurrentArguments() for IAnswer are now passed to the method of the concrete implementation. This is refactoring safe. The cons are that you have to provide an implementation which is kind of doing a mock manually... Which is what you try to avoid by using EasyMock. It can also be painful if the interface has many methods. Finally, the type of the concrete class can't be checked statically against the mock type. If for some reason, the concrete class isn't implementing the method that is delegated, you will get an exception during the replay only. However, this case should be quite rare.

To understand correctly the two options, here is an example:

List<String> l = mock(List.class);

// andAnswer style
expect(l.remove(10)).andAnswer(new IAnswer<String>() {
  public String answer() throws Throwable {
    return getCurrentArguments()[0].toString();
  }
});

// andDelegateTo style
expect(l.remove(10)).andDelegateTo(new ArrayList<String>() {
  @Override
  public String remove(int index) {
    return Integer.toString(index);
  }
});

Checking Method Call Order Between Mocks

Up to this point, we have seen a mock object as a single object that is configured by static methods on the class EasyMock. But many of these static methods just identify the hidden control of the Mock Object and delegate to it. A Mock Control is an object implementing the IMocksControl interface.

So instead of

IMyInterface mock = strictMock(IMyInterface.class);
replay(mock);
verify(mock);
reset(mock);

we may use the equivalent code:

IMocksControl ctrl = createStrictControl();
IMyInterface mock = ctrl.createMock(IMyInterface.class);
ctrl.replay();
ctrl.verify();
ctrl.reset();

The IMocksControl allows to create more than one Mock Object, and so it is possible to check the order of method calls between mocks. As an example, we set up two mock objects for the interface IMyInterface, and we expect the calls mock1.a() and mock2.a() ordered, then an open number of calls to mock1.c() and mock2.c(), and finally mock2.b() and mock1.b(), in this order:

IMocksControl ctrl = createStrictControl();
IMyInterface mock1 = ctrl.createMock(IMyInterface.class);
IMyInterface mock2 = ctrl.createMock(IMyInterface.class);
mock1.a();
mock2.a();
ctrl.checkOrder(false);
mock1.c();
expectLastCall().anyTimes();
mock2.c();
expectLastCall().anyTimes();
ctrl.checkOrder(true);
mock2.b();
mock1.b();
ctrl.replay();

Relaxing Call Counts

To relax the expected call counts, there are additional methods that may be used instead of times(int count):

times(int min, int max)
to expect between min and max calls,
atLeastOnce()
to expect at least one call, and
anyTimes()
to expected an unrestricted number of calls.

If no call count is specified, one call is expected. If we would like to state this explicitely, once() or times(1) may be used.

Switching Order Checking On and Off

Sometimes, it is necessary to have a Mock Object that checks the order of only some calls. In record phase, you may switch order checking on by calling checkOrder(mock, true) and switch it off by calling checkOrder(mock, false).

There are two differences between a strict Mock Object and a normal Mock Object:

  1. A strict Mock Object has order checking enabled after creation.
  2. A strict Mock Object has order checking enabled after reset (see Reusing a Mock Object).

Flexible Expectations with Argument Matchers

To match an actual method call on the Mock Object with an expectation, Object arguments are by default compared with equals(). This may lead to problems. As an example, we consider the following expectation:

String[] documents = new String[] { "Document 1", "Document 2" };
expect(mock.voteForRemovals(documents)).andReturn(42);

If the method is called with another array with the same contents, we get an exception, as equals() compares object identity for arrays:

java.lang.AssertionError:
  Unexpected method call voteForRemovals([Ljava.lang.String;@9a029e):
    voteForRemovals([Ljava.lang.String;@2db19d): expected: 1, actual: 0
    documentRemoved("Document 1"): expected: 1, actual: 0
    documentRemoved("Document 2"): expected: 1, actual: 0
      at org.easymock.internal.MockInvocationHandler.invoke(MockInvocationHandler.java:29)
      at org.easymock.internal.ObjectMethodsFilter.invoke(ObjectMethodsFilter.java:44)
      at $Proxy0.voteForRemovals(Unknown Source)
      at org.easymock.samples.ClassUnderTest.listenersAllowRemovals(ClassUnderTest.java:88)
      at org.easymock.samples.ClassUnderTest.removeDocuments(ClassUnderTest.java:48)
      at org.easymock.samples.ExampleTest.testVoteForRemovals(ExampleTest.java:83)
      ...

To specify that only array equality is needed for this call, we may use the method aryEq that is statically imported from the EasyMock class:

String[] documents = new String[] { "Document 1", "Document 2" };
expect(mock.voteForRemovals(aryEq(documents))).andReturn(42);

If you would like to use matchers in a call, you have to specify matchers for all arguments of the method call.

There are a couple of predefined argument matchers available.

eq(X value)
Matches if the actual value is equals the expected value. Available for all primitive types and for objects.
anyBoolean(), anyByte(), anyChar(), anyDouble(), anyFloat(), anyInt(), anyLong(), anyObject(), anyObject(Class clazz), anyShort(), anyString()
Matches any value. Available for all primitive types and for objects.
eq(X value, X delta)
Matches if the actual value is equal to the given value allowing the given delta. Available for float and double.
aryEq(X value)
Matches if the actual value is equal to the given value according to Arrays.equals(). Available for primitive and object arrays.
isNull(), isNull(Class clazz)
Matches if the actual value is null. Available for objects.
notNull(), notNull(Class clazz)
Matches if the actual value is not null. Available for objects.
same(X value)
Matches if the actual value is the same as the given value. Available for objects.
isA(Class clazz)
Matches if the actual value is an instance of the given class, or if it is in instance of a class that extends or implements the given class. Null always return false. Available for objects.
lt(X value), leq(X value), geq(X value), gt(X value)
Matches if the actual value is less/less or equal/greater or equal/greater than the given value. Available for all numeric primitive types and Comparable.
startsWith(String prefix), contains(String substring), endsWith(String suffix)
Matches if the actual value starts with/contains/ends with the given value. Available for Strings.
matches(String regex), find(String regex)
Matches if the actual value/a substring of the actual value matches the given regular expression. Available for Strings.
and(X first, X second)
Matches if the matchers used in first and second both match. Available for all primitive types and for objects.
or(X first, X second)
Matches if one of the matchers used in first and second match. Available for all primitive types and for objects.
not(X value)
Matches if the matcher used in value does not match.
cmpEq(X value)
Matches if the actual value is equals according to Comparable.compareTo(X o). Available for all numeric primitive types and Comparable.
cmp(X value, Comparator<X> comparator, LogicalOperator operator)
Matches if comparator.compare(actual, value) operator 0 where the operator is <,<=,>,>= or ==. Available for objects.
capture(Capture<T> capture), captureXXX(Capture<T> capture)
Matches any value but captures it in the Capture parameter for later access. You can do and(someMatcher(...), capture(c)) to capture a parameter from a specific call to the method. You can also specify a CaptureType telling that a given Capture should keep the first, the last, all or no captured values.

Defining your own Argument Matchers

Sometimes it is desirable to define own argument matchers. Let's say that an argument matcher is needed that matches an exception if the given exception has the same type and an equal message. It should be used this way:

IllegalStateException e = new IllegalStateException("Operation not allowed.")
expect(mock.logThrowable(eqException(e))).andReturn(true);

Two steps are necessary to achieve this: The new argument matcher has to be defined, and the static method eqException has to be declared.

To define the new argument matcher, we implement the interface org.easymock.IArgumentMatcher. This interface contains two methods: matches(Object actual) checks whether the actual argument matches the given argument, and appendTo(StringBuffer buffer) appends a string representation of the argument matcher to the given string buffer. The implementation is straightforward:

import org.easymock.IArgumentMatcher;

public class ThrowableEquals implements IArgumentMatcher {

  private Throwable expected;

  public ThrowableEquals(Throwable expected) {
    this.expected = expected;
  }

  public boolean matches(Object actual) {
    if (!(actual instanceof Throwable)) {
      return false;
    }

    String actualMessage = ((Throwable) actual).getMessage();
    return expected.getClass().equals(actual.getClass()) &amp;&amp; expected.getMessage().equals(actualMessage);
  }

  public void appendTo(StringBuffer buffer) {
    buffer.append("eqException(");
    buffer.append(expected.getClass().getName());
    buffer.append(" with message \"");
    buffer.append(expected.getMessage());
    buffer.append("\"")");
  }
}

The method eqException must create the argument matcher with the given Throwable, report it to EasyMock via the static method reportMatcher(IArgumentMatcher matcher), and return a value so that it may be used inside the call (typically 0, null or false). A first attempt may look like:

public static Throwable eqException(Throwable in) {
  EasyMock.reportMatcher(new ThrowableEquals(in));
  return null;
}

However, this only works if the method logThrowable in the example usage accepts Throwables, and does not require something more specific like a RuntimeException. In the latter case, our code sample would not compile:

IllegalStateException e = new IllegalStateException("Operation not allowed.")
expect(mock.logThrowable(eqException(e))).andReturn(true);

Java 5.0 to the rescue: Instead of defining eqException with a Throwable as parameter and return value, we use a generic type that extends Throwable:

public static&lt;T extends Throwable&gt;T eqException(T in) {
  reportMatcher(new ThrowableEquals(in));
  return null;
}

Advanced

Serializing Mocks

Mocks can be serialized at any time during their life. However, there are some obvious constraints:

  • All used matchers should be serializable (all genuine EasyMock ones are)
  • Recorded parameters should also be serializable

Multithreading

During recording, a mock is not thread-safe. So a giving mock (or mocks linked to the same IMocksControl) can only be recorded from a single thread. However, different mocks can be recorded simultaneously in different threads.

During the replay phase, mocks are by default thread-safe. This can be change for a given mock if makeThreadSafe(mock, false) is called during the recording phase. This can prevent deadlocks in some rare situations.

Finally, calling checkIsUsedInOneThread(mock, true) on a mock will make sure the mock is used in only one thread and throw an exception otherwise. This can be handy to make sure a thread-unsafe mocked object is used correctly.

OSGi

EasyMock jar can be used as an OSGi bundle. It exports org.easymock, org.easymock.internal and org.easymock.internal.matchers packages. However, to import the two latter, you need to specify the poweruser attribute at true (poweruser=true). These packages are meant to be used to extend EasyMock so they usually don't need to be imported.

Backward Compatibility

EasyMock 3 still has a Class Extension project (although deprecated) to allow an easier migration from EasyMock 2 to EasyMock 3. It is a source not a binary compatibility. So the code will need to be recompiled.

EasyMock 2.1 introduced a callback feature that has been removed in EasyMock 2.2, as it was too complex. Since EasyMock 2.2, the IAnswer interface provides the functionality for callbacks.