Does synchronized (this) lock only the synchronized block or all the "this" code?

Question

public class ObjectCounter {
    private static long numOfInstances = 0;
    public ObjectCounter(){
        synchronized(this){
        numOfInstances++;
        }
    }
    **public static synchronized long getCount(){
        return numOfInstances;
    }**
//vs//
    **public static long getCount(){
        return numOfInstances;
    }**
}

if I'll run few threads, some of them call the static function getCount() and some of them create new instances. I want to get in each call to getCount() the real number of instances at the time.

1. Is there a difference between the two options in the code?
2. If I lock "this" shouldn't it mean that I can't call getCount() until the constructor exits the synchronized block (lets say if I don't write synchronize on the getCount()).
3. if I do a synchronized block in some place in the code, does it lock only the synchronized block or all the "this" code?
4. From here down EDIT: thank you all, it was very helpful, but I have a few more questions following your answers.
5. If I understand correctly, the synchronized(this) block doesn't effect (or connected to) the static synchronized function (in lock terms not the numOfInstances increment)?
6. is there a better option to make the increment and the getCount() function Thread-safe? (like open a static object and do synchronized(obj) instead synchronized(this) - friend suggested).
7. If I had a f1() method (non-static) in ObjectCounter class, while one thread is in the synchronized(this) can other thread enter f1() block (not a synchronized class or have synchronized block inside)?
8. If I had a f1() method (non-static) and f2() method (non-static) in ObjectCounter, in f1() I have synchronized(this) block. while one thread is in the synchronized(this) block, can other thread enter f1() block (not a synchronized class or have synchronized block inside)? (lets say both of the threads "working" on the same instance)

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Answer 1

Using synchronized means in order for a thread to execute that block or method, it has to acquire a lock referenced (explicitly or implicitly) by that block or method. For the static synchronized methods, that lock is the monitor on the class object. For the synchronized(this) block, the lock used is the monitor on the current instance. Sharing of locks between multiple methods or blocks is what enforces atomicity and memory visibility of updates, also the shared lock provides a shared communication path through which waiting and notification can take place.

Since the static synchronized blocks use a different lock from that used by the block in the constructor, entering a static synchronized block is not blocked by another thread's accessing the block that requires acquiring the lock on the current instance, and the synchronized block in the constructor has no effect on anything, the lock acquisition will always be uncontended. More importantly here, changes made by one thread in the constructor may not get seen by other threads using the getter. Synchronization affects both locking and memory visibility.

This changed version would work:

public class ObjectCounter {
    private static long numOfInstances = 0;
    public ObjectCounter(){
        synchronized(ObjectCounter.class){
            numOfInstances++;
        }
    }
    public static synchronized long getCount(){
        return numOfInstances;
    }
}

because the getter and the incrementing block are using the same lock. Making the different threads acquire the same monitor ensures that the change to the counter gets safely published so that another thread accessing the getter can see the updated value.

The synchronized keyword says, "you have to acquire a lock before you can enter", where for the method the lock is assumed: with the static keyword on the method it's the monitor on the class, without a static keyword it's the monitor on the current instance. For locking to work correctly the different blocks and methods need to use the same lock. There is arguably too much syntax sugar and too much making things convenient in how Java was designed: allowing implicit choice of locks and putting the monitor on java.lang.Object can cause confusion.

WRT your question #6: For what you're doing here you'd be better off with an AtomicLong. Use synchronized blocks for coordinating multiple changes that need to take place without interference from other threads.

Questions #3, #7 and #8 seem very similar: If a method/block isn't attempting to acquire a lock, nothing prevents threads from executing that method/block. The object as a whole doesn't get any protection, using the synchronized methods or blocks to enforce locking is what does the protecting. Think less in terms of "using the synchronized keyword" and more in terms of what lock threads need to acquire.

Answer 2

1. Yes there is a difference in the options. In the above option, two threads cannot call getCount() at the same time, in the below one they can.

2. Yes, that is correct. There can only be one thread at the same time holding a lock on an object.

3. Each object has its own lock. So it lock all synchronized (this) block of that object.

Note, however, that each object has a lock of its own and also each class has a lock of its own. In the constructor you use the object lock to access a static (class) variable, while in getCount() you use the class lock. That means that your code is not thread-safe!

Answer 3

synchronized steps:

1. Check if object lock is already acquired. If so, proceed into synchronized block/method
2. Attempt to aquire lock. If the lock has already been acquired by another thread, then the thread will wait for the lock to be released, at which point it will go through the cycle (2.) again

Answer 4

Is there a difference between the two options in the code?

Yes, there is clear difference. In first, you are synchronizing threads access to getCount() method on the class object of ObjectCounter. While in second you are not.

If I lock "this" shouldn't it means that I can't call getCount() until the contractor exit the synchronized block (lets say if I don't write synchronize on the getCount()).

Since there is only one lock of an object (class locks are different which are held through using static keyword along with synchronized), so if some other thread is acquiring that lock either because of synchronized(this){ or because of this synchronized long getCount(){ then the new thread trying to acquire the lock has to wait until the previous thread has released the lock.

Now since in your case, you are doing, static synchronized long getCount(){, so, its locking becomes different from synchronized(this){. Which means that if some thread is acquiring a lock because of synchronized(this){ and some other thread is trying to invoke getCount() then that thread will not be blocked.

if I do a synchronized block in some place in the code, does it locks the only the synchronized block or all the "this" code?

1. Non-static synchronization: If you do synchronized block in some place in the code and it is non-static public synchronized long getCount(){, then also the lock of your object will be held, so new thread trying to acquire the lock has to wait until the previous thread has released the lock.

2. Static synchronization: If you do synchronized block in some place in the code and it is static public static synchronized long getCount(){, then it will have no effect on lock of non-static sychronization.

Bottom line:

• There is only and only one lock for one object, and if that lock is acquired by some thread then other threads has to wait until that lock released.

• Then there is a class lock, which is held if static keyword is used along with synchronized keyword.