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 points one (somebody else using your lock) and two (all methods using the same lock needlessly) can happen in any fairly large application. Especially when there's no good communication between developers.

It's not cast in stone, it's mostly an issue of good practice and preventing errors.

Answer 2

A lock is used for either visibility or for protecting some data from concurrent modification which may lead to race.

When you need to just make primitive type operations to be atomic there are available options like AtomicInteger and the likes.

But suppose you have two integers which are related to each other like x and y co-ordinates, which are related to each other and should be changed in an atomic manner. Then you would protect them using a same lock.

A lock should only protect the state that is related to each other. No less and no more. If you use synchronized(this) in each method then even if the state of the class is unrelated all the threads will face contention even if updating unrelated state.

class Point{
   private int x;
   private int y;

   public Point(int x, int y){
       this.x = x;
       this.y = y;
   }

   //mutating methods should be guarded by same lock
   public synchronized void changeCoordinates(int x, int y){
       this.x = x;
       this.y = y;
   }
}

In the above example I have only one method which mutates both x and y and not two different methods as x and y are related and if I had given two different methods for mutating x and y separately then it would not have been thread safe.

This example is just to demonstrate and not necessarily the way it should be implemented. The best way to do it would be to make it IMMUTABLE.

Now in opposition to Point example, there is an example of TwoCounters already provided by @Andreas where the state which is being protected by two different locks as the state is unrelated to each other.

The process of using different locks to protect unrelated states is called Lock Striping or Lock Splitting

Answer 3

While I agree about not adhering blindly to dogmatic rules, does the "lock stealing" scenario seem so eccentric to you? A thread could indeed acquire the lock on your object "externally"(synchronized(theObject) {...}), blocking other threads waiting on synchronized instance methods.

If you don't believe in malicious code, consider that this code could come from third parties (for instance if you develop some sort of application server).

The "accidental" version seems less likely, but as they say, "make something idiot-proof and someone will invent a better idiot".

So I agree with the it-depends-on-what-the-class-does school of thought.

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Edit following eljenso's first 3 comments:

I've never experienced the lock stealing problem but here is an imaginary scenario:

Let's say your system is a servlet container, and the object we're considering is the ServletContext implementation. Its getAttribute method must be thread-safe, as context attributes are shared data; so you declare it as synchronized. Let's also imagine that you provide a public hosting service based on your container implementation.

I'm your customer and deploy my "good" servlet on your site. It happens that my code contains a call to getAttribute.

A hacker, disguised as another customer, deploys his malicious servlet on your site. It contains the following code in the init method:

synchronized (this.getServletConfig().getServletContext()) {
   while (true) {}
}

Assuming we share the same servlet context (allowed by the spec as long as the two servlets are on the same virtual host), my call on getAttribute is locked forever. The hacker has achieved a DoS on my servlet.

This attack is not possible if getAttribute is synchronized on a private lock, because 3rd-party code cannot acquire this lock.

I admit that the example is contrived and an oversimplistic view of how a servlet container works, but IMHO it proves the point.

So I would make my design choice based on security consideration: will I have complete control over the code that has access to the instances? What would be the consequence of a thread's holding a lock on an instance indefinitely?

Answer 4

If you've decided that:

• the thing you need to do is lock on the current object; and
• you want to lock it with granularity smaller than a whole method;

then I don't see the a taboo over synchronizezd(this).

Some people deliberately use synchronized(this) (instead of marking the method synchronized) inside the whole contents of a method because they think it's "clearer to the reader" which object is actually being synchronized on. So long as people are making an informed choice (e.g. understand that by doing so they're actually inserting extra bytecodes into the method and this could have a knock-on effect on potential optimisations), I don't particularly see a problem with this. You should always document the concurrent behaviour of your program, so I don't see the "'synchronized' publishes the behaviour" argument as being so compelling.

As to the question of which object's lock you should use, I think there's nothing wrong with synchronizing on the current object if this would be expected by the logic of what you're doing and how your class would typically be used. For example, with a collection, the object that you would logically expect to lock is generally the collection itself.

Answer 5

There seems a different consensus in the C# and Java camps on this. The majority of Java code I have seen uses:

// apply mutex to this instance
synchronized(this) {
    // do work here
}

whereas the majority of C# code opts for the arguably safer:

// instance level lock object
private readonly object _syncObj = new object();

...

// apply mutex to private instance level field (a System.Object usually)
lock(_syncObj)
{
    // do work here
}

The C# idiom is certainly safer. As mentioned previously, no malicious / accidental access to the lock can be made from outside the instance. Java code has this risk too, but it seems that the Java community has gravitated over time to the slightly less safe, but slightly more terse version.

That's not meant as a dig against Java, just a reflection of my experience working on both languages.

Answer 6

Short answer: You have to understand the difference and make choice depending on the code.

Long answer: In general I would rather try to avoid synchronize(this) to reduce contention but private locks add complexity you have to be aware of. So use the right synchronization for the right job. If you are not so experienced with multi-threaded programming I would rather stick to instance locking and read up on this topic. (That said: just using synchronize(this) does not automatically make your class fully thread-safe.) This is a not an easy topic but once you get used to it, the answer whether to use synchronize(this) or not comes naturally.