'Implicit' synchronization in java?

Question

when I have a method like

public void unsynchronizedMethod(){
  callSynchronizedMethod();
  //dosomestuff after this line
}

does it mean that all content, after calling callSynchronizedMethod() in the unsynchronizedMethod()-Method, is implicitly synchronized?

Answer 1

No, there wont be an implicit synchronisation. Synchronized works within a block or function scope. Anything outside a synchronized block is not synchronized.

The following example code shows that. If the methods where synchronized it would always print 0.

class example extends Thread{
   //Global value updated by the example threads
   public static volatile int value= 0;
    public void run(){
    while(true)//run forever
       unsynchMethod();

   }
   public void unsynchMethod(){
     synchronizedMethod();
    //Count value up and then back down to 0
    for(int i =0; i < 20000;++i)
       value++;//reads increments and updates value (3 steps)
    for(int i = 0; i < 20000;++i)
       value--;//reads decrements and updates value (3 steps)
      //Print value 
      System.out.println(value);
   }
   //Classlevel synchronized function
   public static synchronized void synchronizedMethod(){
      //not important
   }
   public static void main(String... args){
    example a = new example();
    example b = new example();
    a.start();
    b.start();
   }

}

My Results, should have been 0 if synchronized:

4463
6539
-313
-2401
-3012
...

Answer 2

Synchronization is defined in a straightforward manner:

• For a synchronized method: within the execution of that method
• For a synchronized block: within the execution of that block

Here are the relevant excerpts from Java Language Specification 3rd Edition:

JLS 14.19 The synchronized Statement

A synchronized statement:

1. acquires a mutual-exclusion lock on behalf of the executing thread,
2. executes a block,
3. then releases the lock.

While the executing thread owns the lock, no other thread may acquire the lock.

synchronized methods is semantically identical to a synchronized statement applied to the whole method (§JLS 8.4.3.6. The lock is obtained from either this (if it's an instance method) or the Class object associated with the method's class (if it's a static method; you can't refer to this in a static context).

So to answer the original question, given this snippet:

public void unsynchronizedMethod(){
  callSynchronizedMethod();

  doSomeUnsynchronizedStuff(); // no "implicit synchronization" here
}

Note that this is by design: you should always strive to minimize synchronization to only critical sections. Outside of those critical sections, there is no lingering effect from earlier synchronization.

See also

• Effective Java 2nd Edition, Item 67: Avoid excessive synchronization

Related questions

• What is the difference between a synchronized method and synchronized block in Java ?
• synchronized block vs synchronized method?

Answer 3

No, it's not.

Synchronized means that only one thread at a time will execute the code.

But after callSynchronizedMethod() the threads could run again over the code in different order, all at at the same time.

Answer 4

No. The lock is released at the end of callSynchronizedMethod().