How to partially mock a class and its private methods using PowerMock

Earlier, in the how-to series we had seen how-to mock constructors.  In this post we will have a look at how-to partially mock classes and even their private methods using PowerMocks - Mocks on Steroids!

Lets first look at the code to test before I start with my rambling

Code under test:

We want to write a test case to test method1 of ServiceA class.  ServiceA's method1 invokes method2.  method2 performs some complicated business logic - in this case creates an instance of ServiceB and invoked doBusinessOperation on that instance.

There are two approaches to test ServiceA's method1.

• Write the test case to mock out the ServiceB's constructor and assert that doBusinessOperation method was invoked on the mocked ServiceB object.
• Stub the method2 call!  Only verify that from method1 a call was made to the method2.

We already know how to write tests for the first approach.  Today we are more interested in looking at how do we write tests for the second approach.

Why would we ever choose the second approach over the first approach?  May be because method2 was already tested in some other test case.  Hence, we just want to make sure that method1 calls method2 with the right arguments.

So how do we partially mock ServiceA?

How will we test it - How do they do it!:

PowerMockito class provides a method called spy to create partial mock of a class.  This method takes an instance of the class that you want to partially mock as argument (in this case ServiceA).  Why? Because we want to invoke actual implementation of method1 and mock only method2.

Syntax

Then, we want to stubb out the method2 and return some dummy value.  This how its done

Notice the difference in the syntax then the regular when-theReturn syntax.  As, if we use the when-thenReturn syntax actual call to method2 will be made!  Why? Because PowerMocks does not know whether you are setting up an expectation on a partial mock or actually invoking the method.  Hence, be careful with partial mocks and choose the right syntax.

How to verify that the method was actually called?

No surprises here.

The full test case would look like:

A variation of the above example, what if the method2 was private?  Can we still stub it?  Of-course the right answer is YES!

Code under test:

How will we test it - How do they do it!:

Now, the above doReturn-when-method2 syntax wont work.  As method2 is private and is not accessible outside the scope of the class.  How do we stub it then?

Only change is, we pass in the name of the private method to stub as string argument to when method.  when method also accepts the var args array of arguments to the method2.

To verify that the private method2 was invoked use

Here also, instead of verify we use verifyPrivate and call a invoke method which accepts, name of the private method and its arguments.

Full test case would look like this:

Yes!  Thats it.  We have achieved what we intended to do!  PowerMock's is really a valuable tool to have in once arsenal!

How to mock constructors using PowerMock

In the how-to series today I will show how-to mock constructors using PowerMocks - Mocks on Steroids!.  In my previous post we saw how can we write unit-tests for mocking final methods and classes.

Requirement:

Again, taking a fictitious example, lets say that ServiceA creates an instance of ServiceB and invokes a businessOperation on ServiceB's instance.  ServiceB's constructor is evil and it throws a Exception.  We want to test this code.  Lets look at the code under test

As we can see, ServiceA's doSomething method create an instance of ServiceB class (by invoking a constructor with no arguments) and then invokes doBusinessOperation() method.  Looks like a simple class, but wait, have a look at ServiceB's constructor, its evil, it throws an exception!  Now how do we mock the constructor so that we can write a good tests case for ServiceA's doSomething method?  Simple, Using PowerMocks (Mockito api) :)

How will we test it - How do they do it!:

We have to use @PrepareForTest annotation with ServiceA class as, it instanciates ServiceB and we actually want to inject a stub on ServiceB so that we can verify that doBusinessOperation method was actually invoked.

Then, we use the whenNew method of PowerMockito to actually return a stub of ServiceB when a new instance of ServiceB is created using a no argument constructor.

Just read the above line in and then compare the syntax below

We can actually read the code in English and it does make a lot of sense!

Lets look at the test code now:

Piece of cake isn't!

Testing the Untestable code using PowerMock (Mockito api)

Taking inspiration from this excellent post by Johan Haleby, today I am going to jump a few gears and show you the real power of PowerMocks.  I will show you how can we write good unit-tests to test the untestable code!

We will continue the How-To series using PowerMocks (Mockito api) but today lets look at something more complicated.

You must be thinking, if the code is untestable then how can we write unit-tests?  But, that my friend is the real power of PowerMocks, we can write unit-tests for testing untestable code as well!

So What are we testing today.

Code to Test:

The code example is taken from one of the code samples of JMockit unit-testing framework (which by the way is another powerful unit-testing framework).  At first glance the code might not look as complicated, but make a note of the following points and then think how will you ever write a unit-test for doBusinessOperationXyz method?

• The ServiceA class is final
• It invokes a static method on Database class called find which returns a list
• It then creates a new instance of ServiceB class and invokes computeTotal method, passing it items retrieved from Database.find call
• ServiceB class is final as well
• Invokes another static method on Database class called save, passing it the EntityX instance

Wow! it seems like a really tough class to test.  Lets move ahead and write some good unit-tests to test the above code!

How will we test it - How do they do it!:

You will be surprised to know that, we could write multiple unit-tests for the above code depending on what exactly we want to test!  How awesome!

Test Requirement 1:

Lets say we want to test that, the ServiceB was invoked to compute total.

Test Code:

As usual we have to annotate the TestCase with @RunWith annotation (go here know more on why we have to do so).

We can verify that computeTotal method was invoked on ServiceB instance via a mocked ServiceB object. But if you think about this for a moment, how can we do that?  Because ServiceA's doBusinessOperationXyz is actually instantiating ServiceB.

How can we suppress this behavior and inject our mocked ServiceB object?  Simple! via byte-code manipulation of ServiceA class :)

No no, don't worry, we don't have to do byte-code manipulation on our own, PowerMocks does that for us.  All we need to tell PowerMocks is for which classes we want to perform byte-code manipulation using the @PrepareForTest annotation (go here to know more about @PrepareForTest annotation).

Another point is we want to mock static methods in Database class as well.  Hence we have to use @PrepareForTest annotation for Database class as well.

And finally, note that since we want to mock ServiceB class, which is a final class we have to use @PrepareForTest annotation for ServiceB class as well.

Enough fluff, show me the stuff!

The unit-test code looks like this

Thats all folks!  We have successfully tested the Untestable code for our first requirement


Thats all folks!  We have successfully tested the Untestable code for our first requirement

Test Requirement 2:

Lets say we want to test that, the Database was used to save EntityX.

Test Code:

Now that you have read the post so far, this one must look straightforward to you!

The test-case looks like

Thats it!

Test Requirement 3:

Database find was invoked to get the list of EntityY

Test Code:

The test-case looks a lot similar to the previous example

I promise its not much after this.

Test Requirement 4:

Should verify that EntityY list returned from Database.find was used to computeTotal

Test Code:

This is the final one.

Test Requirement 5:

Should verify that computed total was set on EntityX instance

Test code:

Done!  Its pretty evident from the above examples that testing the Untestable code is pretty easy using PowerMocks (Mockito api)

Feedback, questions and comments are welcome as usual!

How to mock final methods and classes using PowerMock

This is the second post in the how-to series of using PowerMockito to write beautiful unit-tests. In my previous post we saw how can we write unit-tests for mocking static methods.

Today, I will demostrate how we can mock final class and final methods using PowerMocks (Mockito api) - Mocks on steroids!

Requirement:

I am going to take a ficticious example, were AnotherService class's invokeFinal() method invokes a final method on the instance of FinalService class which is a final class.

Code under test:

The final method doSomething in the final Class FinalService does nothing, just throws an exception.  AnotherService's invokeFinalMethod invokes doSomething method.

Most common mocking framework in the java world cannot mock final classes/methods because they are typically based on creating proxies.  Creating proxies for final classes is not possible as we cannot subclass a final classes.

So how we do we it in PowerMocks (Mockito api)?

How will we test it - How do they do it!:

Aha! Its tad simple.  We don't need to do anything special here.

We have to use @PrepareForTest annotation with FinalService so that PowerMocks can actually mock FinalService (Go Here for explanation on @PrepareForTest).  Next up, we need to create the stub of FinalService using the following code

Then, we have two approaches

• We can either stub doSomething method and assert that the return value is what we expect (State based testing)
• We can verify that the doSomething method was actually invoked (Interaction based testing)

I will show both the approaches

State based testing:

Interaction based testing:

Thats it!

It is needless to say that things look pretty easy with PowerMocks (Mockito api).  They look easy because they are easy with PowerMocks!  Yes! you can with PowerMocks (Mockito api)!

Feedback, questions and comments are always welcome.

How to mock static methods using PowerMock

OK, what are we up to today!  Today I will demonstrate how can we write unit test for a class that invokes a static method on another class to get some work done using PowerMocks (Mockito api) - Mocks on steroids! 

Yeah, Yeah I know, I know implementing functionality using static methods is bad and stuff like that.  But sometimes you just can't help it as its not in your control.  Hence, as all wise man (woman included) say, better deal with it!

We have not yet seen how we could mock normal classes and interface and directly looking at how to mock static methods.  Well as Mr. IronMan said

Sometimes you gotta run before you can walk.

Currently, I am working on a project that uses Spring Roo.  Roo uses a lot of AspetJ goodness.  It adds a lot of static methods to domain objects via AspectJ, making the domain objects richer.  For e.g. In a domain Class Customer it would add methods like findCustomer, findAllCustomers, countCustomers etc.  If you are interested in exploring more about Spring Roo, start here.

Spring Roo uses Spring MVC as the web layer.  Hence, I am going to take example of CustomerController (a controller that does CRUD on Customer domain object).  Enough history, lets see the real stuff!

Requirement:

When user requests to see the customer information, CustomerController should find the requested Customer and return that in the model.

Controller Code:

The controller code would look like

Not showing the Spring MVC specific annotations.  Here, findCustomer is the method added by Spring Roo to the Customer domain class.  This is a static method which  is invoked by CustomerController to get the requested Customer instance.  This is what we want to test. 

How will we test it - How do they do it!:

As I explained in my previous post, PowerMocks uses custom class-loader and byte-code manipulation to enable mocking of static methods.  PowerMocks does this by using the following annotations

• @RunWith -- PowerMocks uses a custom Runner called PowerMockRunner.  This is done so that 
• PowerMocks could show informative message if an exception is thrown from the test
• PowerMocks could notify listeners of certain events.  For e.g. before the test methods start executing an event BeforeTestMethod is triggered.  Don't worry, if you don't understand this.  Its a little advanced stuff.  Just remember, that we need to use a Custom runner.
• @PrepareForTest -- This annotation tells PowerMock to prepare certain classes for testing. Classes needed to be defined using this annotation are typically those that needs to be byte-code manipulated. This includes final classes, classes with final, private, static or native methods that should be mocked

That's about all the annotations you will need!  Next we will have to tell PowerMocks that we want to mock static methods of which class.  This is done using

We are saying PowerMocks (using the Mockito extension api) that we want to mock static methods in Customer class.

Next up, we have to stub the method we are interested in (we will be doing state based testing.  PowerMocks also supports interaction based testing, using the verify syntax.  Will not be showing it in this post though).  That's done like this

And then we can simply assert the customer is returned in the modelMap.  That's all!  Here's the complete test case code.

And we are done!  As we can see, PowerMocks (using the Mockito extension api) uses English like syntax, because of this test cases look a lot compact and are easier to read.

As usual, feedback, comments and complaints are always welcome!