What's the difference between:
public synchronized void test(){} and
public void test() { synchronized(Sample.class){} } 
- 1docs.oracle.com/javase/tutorial/essential/concurrency/…QuakeCore– QuakeCore2014-12-07 07:33:52 +00:00Commented Dec 7, 2014 at 7:33
- possible duplicate of Synchronized block lock object questionJatin– Jatin2014-12-07 07:34:26 +00:00Commented Dec 7, 2014 at 7:34
Add a comment |
3 Answers
To make the difference more clear, the first can be rewritten as:
public void test() { synchronized(this){ } } The difference is that the first is synchronized on the instance of the class, whereas the second is synchronized on the class itself.
In the first case, two threads can simultaneously be executing test() on two instances of your class. In the second, they can't.
Comments
Declaring a an instance method synchronized is just syntactic sugar that's equivalent to having a synchronized (this) block. In other words, only a single thread can execute the method on this instance at a single point of time.
synchronized (Sample.class) means that all instances of this class share a single lock object (the class object itself), and only a single thread can execute this method on any instance at a single point in time.
Comments
To complete @NPE's answer -
a synchronized method is actually a method that is synchronized on the object the method "belongs" to. Whether it's an instance object or the class object itself.
Therefore:
class Sample { public synchronized void test(){} } is equivalent to
class Sample { public void test() { synchronized(this) {} } } while
class Sample { public void test() { synchronized(Sample.class){} } } is equivalent to:
class Sample { public static synchronized void test(){} } 2 Comments
ward
Thanks for replying. But why static?
Elist
When you synchronize on the
Sample.class object from within the Sample class, it's the same as synchronizing a static method in the Sample class. That's the point of my answer - a synchronized method is actually a method that is synchronized on the object the method "belongs" to. Whether it's an instance object or the class object itself. Are you familiar with the static concept?