Avoid synchronized(this) in Java?

Question

Whenever a question pops up on SO about Java synchronization, some people are very eager to point out that synchronized(this) should be avoided. Instead, they claim, a lock on a private reference is to be preferred.

Some of the given reasons are:

• some evil code may steal your lock (very popular this one, also has an "accidentally" variant)
• all synchronized methods within the same class use the exact same lock, which reduces throughput
• you are (unnecessarily) exposing too much information

Other people, including me, argue that synchronized(this) is an idiom that is used a lot (also in Java libraries), is safe and well understood. It should not be avoided because you have a bug and you don't have a clue of what is going on in your multithreaded program. In other words: if it is applicable, then use it.

I am interested in seeing some real-world examples (no foobar stuff) where avoiding a lock on this is preferable when synchronized(this) would also do the job.

Therefore: should you always avoid synchronized(this) and replace it with a lock on a private reference?

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Some further info (updated as answers are given):

• we are talking about instance synchronization
• both implicit (synchronized methods) and explicit form of synchronized(this) are considered
• if you quote Bloch or other authorities on the subject, don't leave out the parts you don't like (e.g. Effective Java, item on Thread Safety: Typically it is the lock on the instance itself, but there are exceptions.)
• if you need granularity in your locking other than synchronized(this) provides, then synchronized(this) is not applicable so that's not the issue

Answer 1

I think there is a good explanation on why each of these are vital techniques under your belt in a book called Java Concurrency In Practice by Brian Goetz. He makes one point very clear - you must use the same lock "EVERYWHERE" to protect the state of your object. Synchronised method and synchronising on an object often go hand in hand. E.g. Vector synchronises all its methods. If you have a handle to a vector object and are going to do "put if absent" then merely Vector synchronising its own individual methods isn't going to protect you from corruption of state. You need to synchronise using synchronised (vectorHandle). This will result in the SAME lock being acquired by every thread which has a handle to the vector and will protect overall state of the vector. This is called client side locking. We do know as a matter of fact vector does synchronised (this) / synchronises all its methods and hence synchronising on the object vectorHandle will result in proper synchronisation of vector objects state. Its foolish to believe that you are thread safe just because you are using a thread safe collection. This is precisely the reason ConcurrentHashMap explicitly introduced putIfAbsent method - to make such operations atomic.

In summary

1. Synchronising at method level allows client side locking.
2. If you have a private lock object - it makes client side locking impossible. This is fine if you know that your class doesn't have "put if absent" type of functionality.
3. If you are designing a library - then synchronising on this or synchronising the method is often wiser. Because you are rarely in a position to decide how your class is going to be used.
4. Had Vector used a private lock object - it would have been impossible to get "put if absent" right. The client code will never gain a handle to the private lock thus breaking the fundamental rule of using the EXACT SAME LOCK to protect its state.
5. Synchronising on this or synchronised methods do have a problem as others have pointed out - someone could get a lock and never release it. All other threads would keep waiting for the lock to be released.
6. So know what you are doing and adopt the one that's correct.
7. Someone argued that having a private lock object gives you better granularity - e.g. if two operations are unrelated - they could be guarded by different locks resulting in better throughput. But this i think is design smell and not code smell - if two operations are completely unrelated why are they part of the SAME class? Why should a class club unrelated functionalities at all? May be a utility class? Hmmmm - some util providing string manipulation and calendar date formatting through the same instance?? ... doesn't make any sense to me at least!!

Answer 2

As already said here synchronized block can use user-defined variable as lock object, when synchronized function uses only "this". And of course you can manipulate with areas of your function which should be synchronized and so on.

But everyone says that no difference between synchronized function and block which covers whole function using "this" as lock object. That is not true, difference is in byte code which will be generated in both situations. In case of synchronized block usage should be allocated local variable which holds reference to "this". And as result we will have a little bit larger size of function (not relevant if you have only few number of functions).

More detailed explanation of the difference you can find here: http://www.artima.com/insidejvm/ed2/threadsynchP.html

Also usage of synchronized block is not good due to following point of view:

The synchronized keyword is very limited in one area: when exiting a synchronized block, all threads that are waiting for that lock must be unblocked, but only one of those threads gets to take the lock; all the others see that the lock is taken and go back to the blocked state. That's not just a lot of wasted processing cycles: often the context switch to unblock a thread also involves paging memory off the disk, and that's very, very, expensive.

For more details in this area I would recommend you read this article: http://java.dzone.com/articles/synchronized-considered

Answer 3

Well, firstly it should be pointed out that:

public void blah() {
  synchronized (this) {
    // do stuff
  }
}

is semantically equivalent to:

public synchronized void blah() {
  // do stuff
}

which is one reason not to use synchronized(this). You might argue that you can do stuff around the synchronized(this) block. The usual reason is to try and avoid having to do the synchronized check at all, which leads to all sorts of concurrency problems, specifically the double checked-locking problem, which just goes to show how difficult it can be to make a relatively simple check threadsafe.

A private lock is a defensive mechanism, which is never a bad idea.

Also, as you alluded to, private locks can control granularity. One set of operations on an object might be totally unrelated to another but synchronized(this) will mutually exclude access to all of them.

synchronized(this) just really doesn't give you anything.

Answer 4

No, you shouldn't always. However, I tend to avoid it when there are multiple concerns on a particular object that only need to be threadsafe in respect to themselves. For example, you might have a mutable data object that has "label" and "parent" fields; these need to be threadsafe, but changing one need not block the other from being written/read. (In practice I would avoid this by declaring the fields volatile and/or using java.util.concurrent's AtomicFoo wrappers).

Synchronization in general is a bit clumsy, as it slaps a big lock down rather than thinking exactly how threads might be allowed to work around each other. Using synchronized(this) is even clumsier and anti-social, as it's saying "no-one may change anything on this class while I hold the lock". How often do you actually need to do that?

I would much rather have more granular locks; even if you do want to stop everything from changing (perhaps you're serialising the object), you can just acquire all of the locks to achieve the same thing, plus it's more explicit that way. When you use synchronized(this), it's not clear exactly why you're synchronizing, or what the side effects might be. If you use synchronized(labelMonitor), or even better labelLock.getWriteLock().lock(), it's clear what you are doing and what the effects of your critical section are limited to.

Answer 5

I'll cover each point separately.

1. Some evil code may steal your lock (very popular this one, also has an "accidentally" variant)

   I'm more worried about accidentally. What it amounts to is that this use of this is part of your class' exposed interface, and should be documented. Sometimes the ability of other code to use your lock is desired. This is true of things like Collections.synchronizedMap (see the javadoc).

2. All synchronized methods within the same class use the exact same lock, which reduces throughput

   This is overly simplistic thinking; just getting rid of synchronized(this) won't solve the problem. Proper synchronization for throughput will take more thought.

3. You are (unnecessarily) exposing too much information

   This is a variant of #1. Use of synchronized(this) is part of your interface. If you don't want/need this exposed, don't do it.

Answer 6

While you are using synchronized(this) you are using the class instance as a lock itself. This means that while lock is acquired by thread 1 the thread 2 should wait

Suppose the following code

public void method1() {
    do something ...
    synchronized(this) {
        a ++;      
    }
    ................
}


public void method2() {
    do something ...
    synchronized(this) {
        b ++;      
    }
    ................
}

Method 1 modifying the variable a and method 2 modifying the variable b, the concurrent modification of the same variable by two threads should be avoided and it is. BUT while thread1 modifying a and thread2 modifying b it can be performed without any race condition.

Unfortunately, the above code will not allow this since we are using the same reference for a lock; This means that threads even if they are not in a race condition should wait and obviously the code sacrifices concurrency of the program.

The solution is to use 2 different locks for two different variables.

  class Test {
        private Object lockA = new Object();
        private Object lockB = new Object();

public void method1() {
    do something ...
    synchronized(lockA) {
        a ++;      
    }
    ................
}


public void method2() {
    do something ...
    synchronized(lockB) {
        b ++;      
    }
    ................
 }

The above example uses more fine grained locks (2 locks instead one (lockA and lockB for variables a and b respectively) and as a result allows better concurrency, on the other hand it became more complex than the first example ...