JUnit: @Before only for some test methods?

Just move out the tests that don't need the setup code into a separate test class. If you have some other code common to the tests that would be helpful to keep, move that out into a helper class.

@Nested + @BeforeEach

Totally agree with the point of moving the related code to an inner class. So here what I have done.

1. Create an inner class inside your test class
2. Annotate the inner class with @Nested
3. Move all the test methods you want to use in the inner class
4. Write the init code inside the inner class and annotate it with @ForEach

Here is the code:

class Testing {

    @Test
    public void testextmethod1() {
    
      System.out.println("test ext method 1");
    
    }
    
    @Nested
    class TestNest{
    
       @BeforeEach
       public void init() {
          System.out.println("Init");
       }
    
       @Test
       public void testmethod1() {
          System.out.println("This is method 1");
       }
    
       @Test
       public void testmethod2() {
          System.out.println("This is method 2");
       }
    
       @Test
       public void testmethod3() {
          System.out.println("This is method 3");
       }
        
     }

     @Test
     public void testextmethod2() {
    
         System.out.println("test ext method 2");
    
     }

}

Here is the output

test ext method 1
test ext method 2
Init
This is method 1
Init
This is method 2
Init
This is method 3

Note: I am not sure if this is supported in Junit4. I am doing this in JUnit5

It is possible to achieve also via Assume from JUnit. And then you can check the method name for which you want to process @Before.

public class MyTest {
     @Rule
     public TestName testName = new TestName();

     @Before
     public void setUp() {
       assumeTrue(testName.getMethodName().equals("myMethodName"));
       // setup follows
     }
}

Check the topic for more insights about @Rule.

Or use TestNG. It gives you finer grained control over tests.