Java在对象的副本而不是实际的对象上进行同步

Question

I am looking at the bluetooth chat example and I don't understand why in the code bellow it is syncronzed on a copy of the local ConnectedThread object. Surely r & mConnectedThread point to the same object?

public synchronized void write(byte[] out)
{
    // Create temporary object
    ConnectedThread r;

    // Synchronize a copy of the ConnectedThread
    synchronized (this)
    {

        if (mState != ICommsService.STATE_CONNECTED)
            return;

        r = mConnectedThread;
    }

    // Perform the write unsynchronized
    r.write(out);
}

Answer 1

It's not synchronized on the ConnectedThread object. It's synchronized on this , so that no other thread can modify mState and mConnectedThread when the current thread needs them.

The comment who is probably confusing you says that's it's synchronizing a copy of the ConnectedThread (that is, it makes the copy of the reference to the ConnectedThread and that the copy process is synchronized). The comment does not say that it is synchronizing on the copy of the ConnectedThread .

And the reason why the function puts the reference to the ConnectedThread object in a local variable r before using it is to be able to not synchronize the whole object when calling write , which can be a long action.