Callback

Mockito 提供了一个 Answer 接口，它允许使用通用接口进行stubbing。

语法

//add the behavior to add numbers
when(calcService.add(20.0,10.0)).thenAnswer(new Answer<Double>() {
   @Override
   public Double answer(InvocationOnMock invocation) throws Throwable {
      //get the arguments passed to mock
      Object[] args = invocation.getArguments();
      //get the mock 
      Object mock = invocation.getMock();	
      //return the result
      return 30.0;
   }
});

举一个使用的例子：

// @RunWith attaches a runner with the test class to initialize the test data
@RunWith(MockitoJUnitRunner.class)
public class MathApplicationTester {
	
   private MathApplication mathApplication;
   private CalculatorService calcService;

   @Before
   public void setUp(){
      mathApplication = new MathApplication();
      calcService = mock(CalculatorService.class);
      mathApplication.setCalculatorService(calcService);
   }

   @Test
   public void testAdd(){

      //add the behavior to add numbers
      when(calcService.add(20.0,10.0)).thenAnswer(new Answer<Double>() {

         @Override
         public Double answer(InvocationOnMock invocation) throws Throwable {
            //get the arguments passed to mock
            Object[] args = invocation.getArguments();
			
            //get the mock 
            Object mock = invocation.getMock();	
			
            //return the result
            return 30.0;
         }
      });

      //test the add functionality
      Assert.assertEquals(mathApplication.add(20.0, 10.0),30.0,0);
   }
}

我们通过在其中注入一个calculatorService 的模拟来测试MathApplication 类。 Mock 将由 Mockito 创建。我们通过回调来实现一个接口来模拟方法的返回。

Spy

Mockito 提供了在真实对象上创建间谍的选项。当调用 spy 时，会调用真实对象的实际方法。

语法：

//create a spy on actual object
calcService = spy(calculator);

//perform operation on real object
//test the add functionality
Assert.assertEquals(mathApplication.add(20.0, 10.0),30.0,0);

还是用一个例子来说明：

// @RunWith attaches a runner with the test class to initialize the test data
@RunWith(MockitoJUnitRunner.class)
public class MathApplicationTester {
	
   private MathApplication mathApplication;
   private CalculatorService calcService;

   @Before
   public void setUp(){
      mathApplication = new MathApplication();
      Calculator calculator = new Calculator();
      calcService = spy(calculator);
      mathApplication.setCalculatorService(calcService);	     
   }

   @Test
   public void testAdd(){

      //perform operation on real object
      //test the add functionality
      Assert.assertEquals(mathApplication.add(20.0, 10.0),30.0,0);
   }

   class Calculator implements CalculatorService {
      @Override
      public double add(double input1, double input2) {
         return input1 + input2;
      }

      @Override
      public double subtract(double input1, double input2) {
         throw new UnsupportedOperationException("Method not implemented yet!");
      }

      @Override
      public double multiply(double input1, double input2) {
         throw new UnsupportedOperationException("Method not implemented yet!");
      }

      @Override
      public double divide(double input1, double input2) {
         throw new UnsupportedOperationException("Method not implemented yet!");
      }
   }
}

看过代码后很容易理解，mock一般是对一个接口，而spy是对一个对象实例；这里提出一个问题，它们之间有什么区别呢？