What is the difference between synchronized on lockObject and using this as the lock?

Question

I know the difference between synchronized method and synchronized block but I am not sure about the synchronized block part.

Assuming I have this code

class Test {
  private int x=0;
  private Object lockObject = new Object();

  public void incBlock() {
    synchronized(lockObject) {
      x++;
    }
    System.out.println("x="+x);
  }

  public void incThis() {  // same as synchronized method
    synchronized(this) {
      x++;
    }
    System.out.println("x="+x);
  }
}

In this case what is the difference between using lockObject and using this as the lock? It seems to be the same to me..

When you decide to use synchronized block, how do you decide which object to be the lock?

Answer 1

Personally I almost never lock on "this". I usually lock on a privately held reference which I know that no other code is going to lock on. If you lock on "this" then any other code which knows about your object might choose to lock on it. While it's unlikely to happen, it certainly could do - and could cause deadlocks, or just excessive locking.

There's nothing particularly magical about what you lock on - you can think of it as a token, effectively. Anyone locking with the same token will be trying to acquire the same lock. Unless you want other code to be able to acquire the same lock, use a private variable. I'd also encourage you to make the variable final - I can't remember a situation where I've ever wanted to change a lock variable over the lifetime of an object.

Answer 2

I had this same question when I was reading Java Concurrency In Practice, and I thought I'd add some added perspective on the answers provided by Jon Skeet and spullara.

Here's some example code which will block even the "quick" setValue(int)/getValue() methods while the doStuff(ValueHolder) method executes.

public class ValueHolder {
    private int value = 0;

    public synchronized void setValue(int v) {
        // Or could use a sychronized(this) block...
        this.value = 0;
    }

    public synchronized int getValue() {
        return this.value;
    }
}

public class MaliciousClass {

    public void doStuff(ValueHolder holder) {
        synchronized(holder) {
            // Do something "expensive" so setter/getter calls are blocked
        }
    }
}

The downside of using this for synchronization is other classes can synchronize on a reference to your class (not via this, of course). Malicious or unintentional use of the synchronized keyword while locking on your object's reference can cause your class to behave poorly under concurrent usage, as an external class can effectively block your this-synchronized methods and there is nothing you can do (in your class) to prohibit this at runtime. To avoid this potential pitfall, you would synchronize on a private final Object or use the Lock interface in java.util.concurrent.locks.

For this simple example, you could alternately use an AtomicInteger rather than synchronizing the setter/getter.

Answer 3

Item 67 of Effective Java Second Edition is Avoid excessive synchronization, thus I would synchronize on a private lock object.

Answer 4

Every object in Java can act as a monitor. Choosing one is dependent on what granularity you want. Choosing 'this' has the advantage and disadvantage that other classes could also synchronize on the same monitor. My advice though is to avoid using the synchronize keyword directly and instead use constructs from the java.util.concurrency library which are higher level and have well defined semantics. This book has a lot of great advice in it from very notable experts:

Java Concurrency in Practice http://amzn.com/0321349601

Answer 5

In this case it does not matter which object you choose for lock. But you must consistently use the same object for locking to achieve correct synchronization. Above code does not ensure proper synchronization as you once use the 'this' object as lock and next the 'lockObject' as lock.