How to insulate yourself from static methods

Sometimes your code needs to call a static method in a 3rd party library and unit testing  suddenly becomes difficult, particularly when that static method requires context in order to work.

Enter the insulating class (also known as a Facade).

1. Create a new interface that declares a method with the same signature as the static method you want to delegate to
2. Create a new Facade class that implements the interface
3. Replace the calls to the static method with calls to your new Facade class

Let’s assume that the method you need to call looks like this

 public class ThirdPartyValueComputer {
   public static int computeValue(SomeObject param, SomeOtherObject param){
     ....
   }
 }

If you put this call into another class with an interface then you will be able to use mock objects to test your code.

 public interface IValueComputer {
   int computeValue(SomeObject p1, SomeOtherObject p2);
 }
 
 public class ValueComputerFacade implements IValueComputer {
   @Override
   public int computeValue(SomeObject p1, SomeOtherObject p2){
     return ThirdPartyValueComputer.computeValue(SomeObject p1, SomeOtherObject p2);
   }
 }

 public interface ISomeService {
   int calculate(SomeObject p1, SomeOtherObject p2);
 }
 
 public class SomeService implements ISomeService {
   private IValueComputer valueComputer;
   public void setValueComputer(IValueComputer c){
     this.valueComputer = c;
   }

   @Override
   public int calculate(SomeObject p1, SomeOtherObject p2) {
     int computedValue = valueComputer.computeValue(p1, p2);
     // Now perform the logic you need to test
     int result = ...;
     return result;
   }
 }

The service is configured in Spring as follows

Now that you’ve created and configured your facade, you can write tests for the service class as follows

 Mockery context = new Mockery();

 @Test
 public void testCalculate() throws Exception {
   final SomeObject p1 = new SomeObject();
   final SomeOtherObject p2 = new SomeOtherObject();

   IValueComputer valueComputer = context.mock(IValueComputer.class);
   SomeService service = new SomeService();
   service.setValueComputer(valueComputer);
  
   context.checking(new Expectations() {
     oneOf(valueComputer).computeValue(p1, p2))
     will(Expectations.returnValue(1));
   });

   int result = service.calculate(p1, p2);
   assertEquals(1, result);
 }