Mockito - Spying

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Mockito provides option to create spy on real objects. When spy is called, then actual method of real object is called.

Syntax

//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);

Example

Step 1 − Create an interface called CalculatorService to provide mathematical functions

File: CalculatorService.java

public interface CalculatorService {
   public double add(double input1, double input2);
   public double subtract(double input1, double input2);
   public double multiply(double input1, double input2);
   public double divide(double input1, double input2);
}

Step 2 − Create a JAVA class to represent MathApplication

File: MathApplication.java

public class MathApplication {
   private CalculatorService calcService;

   public void setCalculatorService(CalculatorService calcService){
      this.calcService = calcService;
   }
   
   public double add(double input1, double input2){
      return calcService.add(input1, input2);		
   }
   
   public double subtract(double input1, double input2){
      return calcService.subtract(input1, input2);
   }
   
   public double multiply(double input1, double input2){
      return calcService.multiply(input1, input2);
   }
   
   public double divide(double input1, double input2){
      return calcService.divide(input1, input2);
   }
}

Step 3 − Test the MathApplication class

Let's test the MathApplication class, by injecting in it a mock of calculatorService. Mock will be created by Mockito.

Here we've added one mock method calls, add() to the mock object via when(). However during testing, we've called subtract() before calling add(). When we create a mock object using Mockito.createStrictMock(), the order of execution of the method does matter.

File: MathApplicationTester.java

import static org.mockito.Mockito.spy;

import org.junit.Assert;
import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.runners.MockitoJUnitRunner;

// @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!");
      }
   }
}

Step 4 − Execute test cases

Create a java class file named TestRunner in C:\> Mockito_WORKSPACE to execute Test case(s).

File: TestRunner.java

import org.junit.runner.JUnitCore;
import org.junit.runner.Result;
import org.junit.runner.notification.Failure;

public class TestRunner {
   public static void main(String[] args) {
      Result result = JUnitCore.runClasses(MathApplicationTester.class);
      
      for (Failure failure : result.getFailures()) {
         System.out.println(failure.toString());
      }
      
      System.out.println(result.wasSuccessful());
   }
}

Step 5 − Verify the Result

Compile the classes using javac compiler as follows −

C:\Mockito_WORKSPACE>javac CalculatorService.java MathApplication.
   java MathApplicationTester.java TestRunner.java

Now run the Test Runner to see the result −

C:\Mockito_WORKSPACE>java TestRunner

Verify the output.

true
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