thread safety ?

[Edward Chen]

In the coding below, is it thread safety ? My answer is yes. because the method is synchronized and the variable is private so that no other way to change X value.

What do you think ?

public class Test extends Thread{
  private static int x;
  public synchronized void doit(){ x=200;}
  public void run(){ doit();};
}

[Henry Wong]

What about two different threads call the doit() methods of two different Test objects at the same time?

Henry

[Alex Hurtt]

It is NOT thread safe. You are synchronizing an instance method that modifies a static variable. Each instance of the class Test has its own lock ensuring that no 2 threads will access the doit() method on the same single instance of Test simultaneously. But nothing stops somebody from constructing multiple instances of Test and starting all of them up. In this case they will all be accessing the same x because it is static and you are not synchronizing on the Test class lock. You are synchronizing on an instances lock when you synchronize an instance method.

[Suhrid Karthik]

Perhaps think about what you can change in your code to synchronize on the class lock instead of the instance lock.

[Lester Burnham]

Not a direct response, but be sure to read http://faq.javaranch.com/java/ExtendingThreadVsImplementingRunnable

[Campbell Ritchie]

Too difficult a question for beginners. Moving thread.