Question

My java code looks like this:

class xHandler
{
    private Channel _channel;

    // Methods...

    void init()
    {
        _channel = new Channel(...);

        synchronized (_channel)
        {
            // Do some stuff here...e.g.
            _channel.send("...");
        }
    }
}

In other files(threads) I create instances of Channel and send stuff with the use of the object, but only in the init method mentioned above I need it to be sync’ed and no other thread should open a channel and send something during this period.

FindBugs gives me a warning called: Method synchronizes on an updated field

This method synchronizes on an object referenced from a mutable field.
This is unlikely to have useful semantics, since different threads may be synchronizing on different objects.

How can I get this issue right? Is it possible to trigger this issue easily with a simple test?

Answer 1

_channel = new Channel(...);
synchronized (_channel)

I need it to be sync’ed and no other thread should open a channel and
send something during this period.

Seems like your code is not doing what you are thinking that it is doing. Since you are creating a new Channel object per xHandler instance, and acquiring this specific Channel lock, two threads may run concurrently.

1. xHandler instance 1 creates Channel instance 1 and acquires it’s lock
2. xHandler instance 2 creates Channel instance 2 and acquires it’s lock
3. Both run concurrently

What you really need to do is either:

1. Share the specific Channel object that you want to sync for multiple xHandler instances (Example: receive a instance of Channel on xHandler constructor and assign it to _channel, remove the _channel = new Channel(...); line and keep your sync code as it is).
2. Sync on Channel.class which will mean that two different instances of xHandler will never be able to use any Channel simultaneously.

Example of sharing a specific Channel for two different xHandlers:

class xHandler
{
    private final Channel _channel;

    public xHandler(Channel channel)
    {
        this._channel = channel  
    }

    // Methods...

    void init()
    {           
        synchronized (_channel)
        {
            // Do some stuff here...e.g.
            _channel.send("...");
        }
    }

            public static void main(String[] args) 
            {
                Channel myChannel = new Channel(...);
                xHandler xHand1 = new xHandler(myChannel);
                xHandler xHand2 = new xHandler(myChannel);
                // Code to create / start your threads. 
                // xHand1 and xHand2 will not use myChannel simultaneously  
            }

}