在JUnit的方法中模拟变量

Question

How can I mock clientInstance.execute if I am writing Junit for the following method?

public class ClassName{
    public static <type> fun(){
        HttpClient clientInstance = null;
        // clientInstance initialised here 

        response = clientInstance.execute();
        //I am trying to mock clientInstance.execute 
    }
}

Answer 1

As kryger pointed out, you'd better redesign your code for testability:

public class ClassName {
    public static <type> fun(HttpClient clientInstance){
        response = clientInstance.execute();
        //I am trying to mock response.execute 
    }
}

This is called dependency injection . Which is a fancy term for putting the burden on the caller of this code. JBRainsberger wrote an excellent post about it.

If you can't modify the signature of fun() because too many other code depends on it, then you can overload it:

public class ClassName {
    /* this method is *not* going to be tested */
    public static <type> fun(HttpClient clientInstance){
        HttpClient clientInstance = null;
        // clientInstance initialised here 
        return fun(clientInstance);
    }

    /* this method is going to be tested */
    public static <type> fun(HttpClient clientInstance){
        response = clientInstance.execute();
        // mocking clientInstance.execute() will be easy now
    }
}

What if the HttpClient initialisation is tricky and you want to test it? You can refactor it into another method:

public class ClassName {
    /**
      * Package-private for testing reasons.
      * Do not call it from within the class in production code!
    */
    static HttpClient createHttpClient() {
        HttpClient clientInstance = null;
        // clientInstance initialised here 
        return clientInstance;
    }

    /* this method is *not* going to be tested */
    public static <type> fun(HttpClient clientInstance){
        HttpClient clientInstance = createHttpClient();
        return fun(clientInstance);
    }

    /* this method is going to be tested */
    public static <type> fun(HttpClient clientInstance){
        response = clientInstance.execute();
        // mocking clientInstance.execute() will be easy now
    }
}

If everything else fails, you can use PowerMock to mock constructors. I've never seen a case where it was worth the trouble. Anyway, this Q&A explains how to use it.

Answer 2

You can create a mock instance for clientInstance using Mockito.mock and then define what it returns using Mockito.when , like so:

HttpClient clientInstance = Mockito.mock(HttpClient.class); // initialize mock
Object mockReturn = new Object(); // define your return object
Mockito.when(clientInstance.execute()).thenReturn(mockReturn); // define what mock returns
Object response = clientInstance.execute(); // call your mock