How to mock/stub void static methods using PowerMock

In my previous post we saw how we could mock/stub void instance methods.
In this post we will try our hands at mocking/stubbing another exotic species called void static methods

Yes, I know void static methods should be avoided at all costs!

But sometimes you just cant help it!

So lets look at the code under test

In our example we want to test the upgradeAllCustomers method of the CustomerController.

This method upgrades all customers by invoking a void static method upgradeAll on Customer class. Currently implementation of upgradeAll method of Customer class throws and exception.  

What do we want to test:

• upgradeAll void static method of Customer class was invoked
• true is returned after the upgrade

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

The syntax that we had seen in the earlier post will now work anymore

If we notice persist is an instance method on the Customer class (and the above syntax works for instance methods only).

But in this case we want to stub the void static method. How do we do it?

We have to do the following things

• Use the annotation @RunWith(PowerMockRunner.class) to bootstrap PowerMock itself
• Use the annotation @PrepareForTest(Customer.class) to be able to stub static methods of Customer class.
• Stub the void static method using the following lines of code

Notice that the syntax is a bit like record-playback-verify style.

We have to first tell PowerMock to do nothing when a static method is invoked on Customer class.

PowerMock should do nothing on which void static method is specified in the next line by invoking the void static upgradeAll method on the Customer class.

Its effectively recording the fact that, when static void upgradeAll method is invoked on Customer class nothing is to be done!

Before moving, lets see how can we verify that a void static method was actually invoked?

We can easily see that the syntax to verify that a static method was invoked also follows the record-playback-verify pattern.

First we have to inform PowerMock that we are going to verify a static method invocation, and in the next line we actually verify the call to the upgradeAll method.

Enough explanation lets look at the complete test code:

Yes thats how we do it!

For the first time I have to say, and believe me I hate to do so but, I don't like this syntax that much!

But that's just me and hats of to the excellent job that PowerMock and Mockito guys have done so far!

How to mock/stub void methods using PowerMock

Ok, I had though that the previous post would be the last post in the how-to series, but there are so many features in PowerMock that I am tempted to write more about it :)

So, here comes another post on how to mock/stub void methods using PowerMock.

One would think why are void methods so special, that we need a post to explain them?

Well thats because they are void methods. They don't return anything. If you remember, from my earlier post, the syntax of stubbing any method with a non void return type is as follows

In the above example doSomething method of ServiceA instance returns a String.

But imagine what will be the syntax, if doSomething method's return type was void!

May be something like this?

But, if you think for a moment about the above syntax, we would realize that  

methods in java are not very happy to accept void as the argument type!

Effectively, in the above syntax the when method is taking void as one of the argument!

Well this is a problem. This cannot happen!  Hence, we need a special syntax to mock/stub void methods.

So without wasting any more time lets look at the code under test:

In our example we want to test the persistCustomer method of the CustomerController. This method simply calls persist method on the Customer instance passed as argument. Assuming that the persist method of the Customer class returns nothing if everything is fine and throws a RuntimeException if something horribly goes wrong.

We want to write the test case for both these scenarios. So how do we do it?

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

Well, there is always a way with PowerMock!

Our first scenario is to verify that the persistCustomer method returns true if everything goes fine.

PowerMock uses the following syntax to stub the void method to do nothing.


If we read the syntax in pure English it says,

Do nothing when persist method is invoked on customer instance

As simple as that! We are telling the mock customer object to do nothing when persist method is invoked.

Lets look at the test case:


Now the other scenario, we want to make sure that false is returned when an exception is thrown from the persist method of customer instance, but for that to happen we have to stub the persist method of customer to throw an exception.

This is how we do it:

Syntax in English:

Throw RuntimeException when persist method of customer instance is invoked.

Lets look at the test case:

Thats it! Well you see there always is an easy solution with PowerMock