Extending from base class with TestNG with java

I am writing some test classes and extending from base test class. But the problem is even though I lock the isInited variable it runs once for each class. It should be run once and initialize it after that it should not be called again but it calls 3 times since I have 3 classes that extends from base class. Please see below.

Java 1.8 and TestNG

public class BaseTest(){
private static isInited;
@BeforeClass
  public void init(){
  synchronized (BaseTest.class) {
 //here even though I lock and initialize the variable this code is still called once for each class. I do not understand why this happens?
      if (!isInited) {
        //do some init 
        isInited=true;
     }
  }
}

public class TestClass1 extends BaseTest{

@BeforeClass
  public void setup(){
      //setup somethings
  }

  //test methods
}


public class TestClass2 extends BaseTest{

@BeforeClass
  public void setup(){
      //setup somethings
  }

  //test methods
}

public class TestClass3 extends BaseTest{

@BeforeClass
  public void setup(){
      //setup somethings
  }

  //test methods
}