A Java class library for developer testing with "mock methods". JMockit consists of a single class with a small set of static methods, which allow arbitrary methods and constructors of any other class to be replaced by mock implementations at runtime. This facility can be used for writing unit or integration tests where the code under test is isolated from other parts of the codebase on which calls are made. This approach is an alternative to the now conventional use of "mock objects", as provided by tools such as EasyMock and jMock. The advantage is that no particular design must be followed by code under test. Legacy code can be unit tested without the need for any adaptation. In short, JMockit makes testability much less of an issue, or even a non-issue. Since JMockit depends on the JVM class redefinition mechanism exposed by java.lang.instrumentation, JUnit (or TestNG) tests that use it must be run under a Java SE 5 VM. However, application and test code can still be compiled to older versions of the language.