BLOCKS AND STATEMENTS

The

synchronized

 Statement

14.17

14.17   The

synchronized

 Statement

A

synchronized

 statement acquires a mutual exclusion lock ( 17.13) on behalf

of the executing thread, executes a block, then releases the lock. While the execut 

ing thread owns the lock, no other thread may acquire the lock.

SynchronizedStatement:

synchronized (

Expression

 )

Block

The type of

Expression

 must be a reference type, or a compile time error occurs.

A

synchronized

 statement is executed by first evaluating the

Expression

.

If evaluation of the

Expression

 completes abruptly for some reason, then the

synchronized

 statement completes abruptly for the same reason.

Otherwise, if the value of the

Expression

 is

null

, a

NullPointerException

is thrown.

Otherwise, let the non 

null

 value of the

Expression

 be

V

. The executing

thread locks the lock associated with

V

. Then the

Block

is executed. If execution

of the

Block

 completes normally, then the lock is unlocked and the

synchronized

statement completes normally. If execution of the

Block

 completes abruptly for

any reason, then the lock is unlocked and the

synchronized

 statement then com 

pletes abruptly for the same reason.

Acquiring the lock associated with an object does not of itself prevent other

threads from accessing fields of the object or invoking unsynchronized methods

on the object. Other threads can also use

synchronized

 methods or the

synchronized

statement in a conventional manner to achieve mutual exclusion.

The locks acquired by

synchronized

 statements are the same as the locks

that are acquired implicitly by

synchronized

 methods; see  8.4.3.5. A single

thread may hold a lock more than once. The example:

class Test {

public static void main(String[] args) {

Test t = new Test();

synchronized(t) {

synchronized(t) {

System.out.println("made it!");

}

}

}

}

prints:

made it!

This example would deadlock if a single thread were not permitted to lock a lock

more than once.

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