An expressive Mock Object library for Test Driven Development



Oli Bye Steve Freeman Nat Pryce Duncan McGregor Ittai Zeidman spand Parth Mehrotra Nick Howard Nat Pryce James Mead Stefan Beller Jericho James Elford

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Build Status

Upgrading to 2.8.X

Are you seeing NPEs?

We have had to make a breaking change to with(). Tests using with(any(matcher)) for method signatures that require native types will throw NullPointerException.


should be changed to:


This is due to a compiler change in Java 1.7. The 2.6.0 release was compiled with Java 1.6 so it did not suffer this problem.

Advantages of jMock 2 over jMock 1

  • Uses real method calls, not strings: can refactor more easily and autocomplete in the IDE.
  • Customisation by delegation, not by inheritance
  • Many more plugin-points for customisation
  • Independent of any testing framework: compatability with the testing framework is a plugin-point.
  • Can mock concrete classes without calling their constructors (if you really want to).
  • Uses Hamcrest matchers, so can use a large and ever-growing library of matchers in expectations.
  • Expectations match in FIFO order, so tests are easier to understand

How to get up and running

You will need to add the jmock-2.0.0.jar and to your classpath. Also add the appropriate integration JAR to your classpath to integrate jMock 2 with the test framework you use. For example, if you use JUnit 4, add jmock-junit4-2.0.0.jar to your classpath.

You will also need to add hamcrest-api-1.0.0.jar and hamcrest-lib-1.0.0.jar to your classpath.

Package Structure

jMock 2 is organised into published and internal packages. We guarantee to maintain backward compatability of types in published packages within the same major version of jMock. There are no guarantees about backward compatability for types in internal packages.

Types defined in published packages may themselves define public methods that accept or return types from internal packages or inherit methods from types in internal packages. Such methods have no compatability guarantees and should not be considered as part of the published interface.

Published packages

org.jmock DSL-style API


org.jmock.lib Convenient classes that implement the APIs in the core, are used by the DSL-style API, and can be used in user-defined APIs

org.jmock.integration Classes integrating jMock with different testing APIs, such as JUnit 3.x, JUnit 4.x and TestNG.

Packages of example code

Lib classes that rely on clever hacks or otherwise cannot be guaranteed to always work in all JVMs. There are no compatability guarantees with these classes. Use at your own risk.

Internal packages

org.jmock.internal Internal implementation details

org.jmock.test Tests for jMock itself

Plug-in Points

Matcher: Controls the matching of invocations to expectations Action: Performs an action in response to an invocation Imposteriser: Wraps mock objects in an adapter of the correct type Expectation: Matches an invocation and fakes its behaviour ExpectationErrorTranslator: Translates expectation errors into error type used by a specific testing framework. MockObjectNamingScheme: Creates names for mock objects based on the mocked type.