EasyMock 2.0_Pre-Release Readme

Documentation for release 2.0_Pre-Release (August 21 2005)
© 2001-2005 OFFIS, .

EasyMock is a class library that provides an easy way to use Mock Objects for given interfaces. EasyMock is available under the terms of the MIT license.

Mock Objects simulate parts of the behavior of domain code, and are able to check whether they are used as defined. Domain classes can be tested in isolation by simulating their collaborators with Mock Objects.

Writing and maintaining Mock Objects often is a tedious task that may introduce errors. EasyMock generates Mock Objects dynamically - no need to write them, and no generated code!

EasyMock 2 Benefits

EasyMock 2 Drawbacks

EasyMock by default supports the generation of Mock Objects for interfaces only. For those who would like to generate Mock Objects for classes, there is an extension available at the EasyMock home page.

Installation

  1. Java 2 (at least 5.0) is required.
  2. Unzip the EasyMock zip file (easymock2.0_Pre-Release.zip). It contains a directory easymock2.0_Pre-Release. Add the EasyMock jar file (easymock.jar) from this directory to your classpath.

To execute the EasyMock tests, add tests.zip and JUnit (at least 3.8.1) to your class path and start 'java org.easymock.tests.AllTests'.

The source code of EasyMock is stored in the zip file src.zip.

Usage

Most parts of a software system do not work in isolation, but collaborate with other parts to get their job done. In a lot of cases, we do not care about using collaborators in unit testing, as we trust these collaborators. If we do care about it, Mock Objects help us to test the unit under test in isolation. Mock Objects replace collaborators of the unit under test.

The following examples use the interface Collaborator: 

package org.easymock.samples;

public interface Collaborator {
    void documentAdded(String title);
    void documentChanged(String title);
    void documentRemoved(String title);
    byte voteForRemoval(String title);
    byte[] voteForRemovals(String[] title);
}

Implementors of this interface are collaborators (in this case listeners) of a class named ClassUnderTest: 

public class ClassUnderTest {
    // ...    
    public void addListener(Collaborator listener) {
        // ... 
    }
    public void addDocument(String title, byte[] document) { 
        // ... 
    }
    public boolean removeDocument(String title) {
        // ... 
    }
    public boolean removeDocuments(String[] titles) {
        // ... 
    }
}

The code for both the class and the interface may be found in the package org.easymock.samples in samples.zip.

The following examples assume that you are familiar with the JUnit testing framework.

The first Mock Object

We will now build a test case and toy around with it to understand the functionality of the EasyMock package. samples.zip contains a modified version of this test. 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: 

package org.easymock.samples;

import junit.framework.TestCase;

public class ExampleTest extends TestCase {

    private ClassUnderTest classUnderTest;
    private Collaborator mock;

    protected void setUp() {
        classUnderTest = new ClassUnderTest();
        classUnderTest.addListener(mock);
    }

    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 2, we only need a static import of methods of org.easymock.EasyMock. This is the only non-internal, non-deprecated class of EasyMock 2. 

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

public class ExampleTest extends TestCase {

    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, and
  3. switch the Mock Object to replay state.

Here is a first example: 

    protected void setUp() {
        mock = createMock(Collaborator.class); // 1
        classUnderTest = new ClassUnderTest();
        classUnderTest.addListener(mock);
    }

    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)
    ...

Adding Behavior

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: 

    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: 1 (+1)
    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)
    ...

Verifying Behavior

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): 

    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 ;-)): 

    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: 

    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: 3 (+1)
	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)
    ...

Strict Mocks

On a Mock Object returned by a EasyMock.createMock(), 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.createStrictMock() 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.

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, true).

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).

Specifying Return Values

For specifying return values, the object returned by expectLastCall() provides the method andReturn(Object returnValue). The method has to be called in record state after the call to the Mock Object for which it specifies the return 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: 

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

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

Instead of calling expectLastCall() to retrieve the object for setting the return value, there is also a method expect(Object ignored). It allows to define the expected call and the return value in one line of code. Instead of 

mock.voteForRemoval("Document");
expectLastCall().andReturn(42);
we may use 
expect(mock.voteForRemoval("Document")).andReturn(42);

Working with Exceptions

For specifying exceptions (more exact: Throwables) to be thrown, the object returned by expectLastCall() and expect(Object ignored) 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.

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 

    expect(mock.voteForRemoval("Document"))
        .andReturn(42).times(3)
        .andThrow(new RuntimeException(), 4)
        .andReturn(-42);

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 it is requested to make this explicit, once() or times(1) may be used.

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(), anyShort()
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 eaual to the given value according to Arrays.equals(). Available for primitive and object arrays.
isNull()
Matches if the actual value is null. Available for objects.
notNull()
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. Available for objects.
lt(X value), leq(X value), geq(X value), gt(X value)
Matches if the actual value is less than/less or equal/greter or equal/greater than the given value. Available for all numeric primitive types.
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.

Defining 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())
                ßß 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. A first attempt may look like: 

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

However, this only works if the method logThrowable in the exampe 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 <T extends Throwable> T eqException(T in) {
    reportMatcher(new ThrowableEquals(in));
    return in;
}

Reusing a Mock Object

Mock Objects may be reset by reset(mock).

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) 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);

Nice Mocks

On a Mock Object returned by createMock() 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 createNiceMock() instead.

Object Methods

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

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 control of the Mock Object and delegate to it. A Mock Control is an object implementing the IMocksControl interface.

So instead of 

    IMyInterface mock = createStrictMock(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);

    mock1.b();
    mock2.b();

    ctrl.replay();

Callbacks

There are a few cases in which it is desirable to run a piece of code whenever a mock object reacts to a method call. For these cases, a Runnable may be provided: 

    expect(mock.a()).callback(runnable).andReturn(false);

EasyMock Development

EasyMock 1.0 has been developed by Tammo Freese at OFFIS. The development of EasyMock is now hosted on SourceForge to allow other developers and companies to contribute.

Thanks to the people who gave feedback: Nascif Abousalh-Neto, Dave Astels, Francois Beausoleil, George Dinwiddie, Shane Duan, Wolfgang Frech, Steve Freeman, Oren Gross, John D. Heintz, Dale King, Brian Knorr, Dierk Koenig, Chris Kreussling, Robert Leftwich, Johannes Link, Rex Madden, David McIntosh, Karsten Menne, Stephan Mikaty, Ivan Moore, Ilja Preuss, Justin Sampson, Richard Scott, Joel Shellman, Shaun Smith, Marco Struck, Ralf Stuckert, Victor Szathmary, Henri Tremblay, Bill Uetrecht, Frank Westphal, Chad Woolley, Bernd Worsch, and numerous others.

Please check the EasyMock home page for new versions, and send bug reports and suggestions to the EasyMock Yahoo!Group. If you would like to subscribe to the EasyMock Yahoo!Group, send a message to easymock-subscribe@yahoogroups.com.

EasyMock Version 2.0_Pre-Release (August 21 2005)

Changes since 1.2: