Generic JUnit test class

I wrote an interface MyInterface, which is going to be implemented by different implementors.

I also wrote a class MyInterfaceTest, which contains generic test methods that all implementors should be able to use to test their implementations.

I just don't know how to make it work as a JUnit test.

Currently, I have something like this:

public class MyInterfaceTest {
    private static MyInterface theImplementationToTest = null;

    @BeforeClass public static void setUpBeforeClass() throws Exception {
                // put your implementation here:
        theImplementationToTest = new Implementation(...);
    }

    @AfterClass public static void tearDownAfterClass() throws Exception { 
        theImplementationToTest = null;
    }

    @Test public void test1() { /* uses theImplementationToTest */ }    
    @Test public void test2() { /* uses theImplementationToTest */ }    
}

I use the static method setUpBeforeClass because the initialization of each implementation takes a lot of time, so I want to initialize it once for all tests.

With this version of the test, implementors have to change the code of setUpBeforeClass and put their own implementation.

I am sure that there is another way to write MyInterfaceTest, so that implementors will only have to inherit it or send it a parameter, and not change the code. However, I am not experienced enough in JUnit to make it work. Can you please show me how to do this?