Marcus mock exam #42

[Anonymous]

Which of the following statements about threading are true
1) You can only obtain a mutually exclusive lock on methods in a class that extends Thread or implements runnable
2) You can obtain a mutually exclusive lock on any object
3) A thread can obtain a mutually exclusive lock on a method declared with the keyword synchronized
4) Thread scheduling algorithms are platform dependent
The right answers are 2, 3, 4
I would like to know why answer 2 is right as i feel that u need to have snchronized method or code to get a lock.
thanks
maala

[Ajith Kallambella]

Maala,
Though using a synchronized method is a popular practice, one can achieve a finer granularity of thread-safetyness by synchronizing only a part of the code. Here you will use the synchronize(object) syntax

      public void someMethod()
      {
         // lots of code here
         synchronized ( SomeObject )
         {
            // Synchronized code.
         }
         // lots of other code her.
      }

Occasionally, it is useful to lock an object and obtain exclusive access to it for just a few instructions without writing a whole new synchronized method. Synchronized blocks are slightly more efficient than synchronized methods as they save an additional method call.
Hope this helps,
Ajith

[kevin jia]

Hi,Ajith
I have two questions as follow:
No1: I think answer 3) is not right. The concept of "block" is belong to object, not belong to method. Could you explain it.
No2: For the code you provide, here I write it again
public void someMethod() {
//lots of code here
synchronized(someObject) {
//synchronized code
}
//lots of other code here
}
If someObject means "this", I can understand. But if it means someObject of other type, I want to know why we do like this,
we obtain lock on one object, but we operate the code on another object. Could you give me a practical example.
Thanks Ajith

[Anonymous]

Thanks ajith!!! for ur explanation
maala