Java JMockIt mocks via Spring DI

How to use Dependency Injected mock objects to allow Integration Testing of a Java application.

When working with some legacy code bases, the introduction of Dependency Injection has limitations. Thus, various forms of ‘instrumentation’ will be required to reach the “last mile”. A major difficulty with legacy code is instantiation of objects with the “new” operator.

One form of instrumentation is a modern Mocking framework, like JMockIt, another is the use of AOP with, for example, AspectJ.

As part of an evaluation of a possible approach I looked into using DI for data driven Integration Testing. Is it possible to use declarative specification of JMockIt mock objects? That is, can they be specified in a Spring XML bean configuration file and loaded via Spring Framework DI? This is in lieu of Using AspectJ to dependency inject domain objects with Spring or some other framework.

Short answer, yes. Useful technique? Probably not. But …

I was able to solve this by looking at the JMockIt source code for the mockit.internal.startup.JMockitInitialization class.

From JMockIt source repository:

final class JMockitInitialization
  ... snipped ...
  private void setUpStartupMocksIfAny()
      for (String mockClassName : config.mockClasses) {
         Class<?> mockClass = ClassLoad.loadClass(mockClassName);

         //noinspection UnnecessaryFullyQualifiedName
         if (mockit.MockUp.class.isAssignableFrom(mockClass)) {
         else {
            new MockClassSetup(mockClass).setUpStartupMock();
  ... snipped ...

See the “Using mocks and stubs over entire test classes and suites” in the tutorial for further info.

Turned out to be easy to create a Spring XML configuration and a simple mocked “hello world” proof of concept. The web server was instrumented dynamically!

One thing I could not figure out was how to destroy existing Mocks that were created in the Tomcat app server. Calling Mockit.tearDownMocks() had no effect. Admittedly, JMockIt is for use in JUnit based tests, so this may be pushing it.

Further reading

  1. Hermetic Servers This shows that the above idea is useful and not unique.

3 thoughts on “Java JMockIt mocks via Spring DI”

  1. Is Spring Dependency Injection involved here in any way?

    Usage of the @UsingMocksAndStubs annotation will with Spring DI as the @ContextConfiguration loader will break while trying to initialize the bean class being tested – if we do not declare or provide the injected bean.

    It would be of great help if you actually provided an example snippet of the configuration you used.

    1. The configuration file was something as follows (I cannot include the actual file of course):

      <bean id="mockLoader" class="com.......MockLoader" init-method="init">
      <property name="classList">

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