如何有条件地禁用Java中的同步块？

Question

Can I do something like that:

synchronized(isSynchronized ? myLock : null) {

}

I want to disable/enable synchronization through a flag. Is it possible?

Answer 1

You could use a lock ( https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/locks/Lock.html ) and lock it manually.

if (shouldSync) {
  lock.lock();
}
try {
  // do your stuff
} finally { 
  if (shouldSync) {
    lock.unlock();
  }
}

Answer 2

The parameter passed into the synchronized block is not a statement but rather a object to synchronize upon (the mutex). To disable synchronization for whatever reason you should encase the statement in a if-condition as such:

if(condition){
    synchronized(myLock){
        // Critical segment
    }
}

note however, if the evaluation of condition can be dependent on several threads (ie multiple writes to a boolean from different threads) you may need to use an existing thread-safe mechanism like AtomicBoolean as such:

AtomicBoolean condition = ... // defined elsewhere

if(condition.get()){
    synchronized(myLock){
        // Critical segment
    }
}

if you need conditional synchronization upon a synchronized method, remove the synchronized keyword from the method declaration and move it into the body:

public synchronized void foo(){
    // Critical segment
}

to

public void foo(){
    if(condition){
        synchronized(this){
            // Critical segment
        }
    }
}

Answer 3

Sure. Use an if before hand. Also, make sure the variable isSynchronized is marked volatile .

if (isSynchronized) {
  synchronized(myLock) {
    // ...
  }
}

of course, that won't be synchronized when isSynchronized is false. And that doesn't sound like a great idea, if it's thread-safe it shouldn't be synchronized. If it isn't thread safe, it should be synchronized.

Answer 4

You can't synchronize of null. So if you have another mutex, then definately you could do something like:

synchronized(isSynchronized ? myLock : myLock2) {
}

else you could check and enter the critical section like:

if (isSynchronized) {
    synchronized(myLock) {..}
}

Answer 5

How about this for starters:

if (isSynchronized) {
    synchronized(lock) { return doStuff(...); }
} else {
    return doStuff(...);
}

private MyType doStuff(...) {
    ...
}