[FIXED] Does Junit reinitialize the class with each test method invocation?

Issue

When i run the below code, both test cases come true:

import static junit.framework.Assert.assertEquals;

import org.junit.Test;

public class MyTest{
    private int count;

    @Before
    public void before(){
        count=1;
    }

    @Test
    public void test1(){
        count++;
        assertEquals(2, count); 
    }

    @Test
    public void test2(){
        count++;
        assertEquals(2, count); 
    }
}

EXPECTED BEHAVIOUR

1. test1 - success
2. test2 - fail(as expected that count will become 3)

ACTUAL BEHAVIOUR

1. test1 - success
2. test2 - success

Why junit is reinitializing class/variable with each test method invocation. It is a bug in junit or is provided intentionally.

Solution

New Instance of MyTest for each test method

For each test method a new instance of MyTest will be created this is the behavior of Junit.

So in your case for both methods the variable count will have value 1, and thus the value of count++ will be 2 for both the test methods and hence the test cases pass.

public class MyTest{
   public MyTest(){
      // called n times
      System.out.println("Constructor called for MyTest");
   }

   @Before //called n times
   public void setUp(){
      System.out.println("Before called for MyTest");
   }
    
   //n test methods
}

If you execute the code above with 2 test methods:

Output will be:

Constructor called for MyTest
Before called for MyTest
//test execution
Constructor called for MyTest
Before called for MyTest

Edit:

Isolation from the F.I.R.S.T principle of testing

Test frameworks help you in doing the right thing, a very important property of unit tests is isolation.

By creating a new instance every test method, the dirty SUT is thrown away. So that we have a fresh state for every test.

Read about F.I.R.S.T principle of testing.

Answered By - Narendra Pathai
Answer Checked By - David Goodson (JavaFixing Volunteer)