同步方法而不是Java中的同步语句

Question

I have a method with a synchronization statement on an "non-this" object

class Some_Class { 
  public A s = new A();
  public void method_A() { 
    synchronized(s) { 
      ....
    } 
  }
}

Can I instead extend class A and synchronize as follows:

class B extends A {  
  public A a;
  public B(A a) {  
    this.a = a; 
  }  
  public synchronized void some_m() {  
    ...  
  }  
}   
class Some_Class {  
  public A s = new A();
  public void method_A() {
    B b = new B(s);
    b.some_m();
  }  
} 

Are these two synchronizations equivalent?

Answer 1

No, they're not equivalent. This method here:

public synchronized void some_m() {  
  ...  
}

Does the same as this one:

public void some_m() {  
    synchronized(this) {
        ...  
    }
}  

Or in other words

1. Your first code option synchronises on an instance of A in Some_Class (a class member, visible to everyone).
2. Your second code option synchronises on the instance of B within Some_Class.method_A() (a local variable, invisible to the outside of that method)

Answer 2

No, they are not equivalent. In second case you actually don't have synchronization at all. Because some_m method synchronized on instance of B . So you create local instance of B and call method on it. This method is synchronized only on this local instance of B and other threads don't care about it and can do whatever they want with s because it's not synchronized.
Can you describe what you want to achieve?

Answer 3

Synchronized block synchronizes the whole object while synchronized method synchronizes just that method. In the second case, some thread can still access other non-synchronized methods of the object.