Google Guava – Synchronization with Monitor

Bill BejeckNovember 19th, 2012Last Updated: November 18th, 2012

1 80 2 minutes read

The Google Guava project is a collection of libraries that every Java developer should become familiar with. The Guava libraries cover I/O, collections, string manipulation, and concurrency just to name a few. In this post I am going to cover the Monitor class. Monitor is a synchronization construct that can be used anywhere you would use a ReentrantLock. Only one thread can occupy a monitor at any time. The Monitor class has operations of entering and leaving which are semantically the same as the lock and unlock operations in ReentrantLock. Additionally, the Monitor supports waiting on boolean conditions.
Comparing Monitor and ReentrantLock

For starters it would be helpful to do a side by side comparison the Monitor and ReentrantLock.

public class ReentrantLockSample {
    private List<String> list = new ArrayList<String>();
    private static final int MAX_SIZE = 10;

    private ReentrantLock rLock = new ReentrantLock();
    private Condition listAtCapacity = rLock.newCondition();

    public void addToList(String item) throws InterruptedException {
        rLock.lock();
        try {
            while (list.size() == MAX_SIZE) {
                listAtCapacity.await();
            }
            list.add(item);
        } finally {
            rLock.unlock();
        }
    }
}

public class MonitorSample {
    private List<String> list = new ArrayList<String>();
    private static final int MAX_SIZE = 10;

    private Monitor monitor = new Monitor();
    private Monitor.Guard listBelowCapacity = new Monitor.Guard(monitor) {
        @Override
        public boolean isSatisfied() {
            return (list.size() < MAX_SIZE);
        }
    };

    public void addToList(String item) throws InterruptedException {
        monitor.enterWhen(listBelowCapacity);
        try {
            list.add(item);
        } finally {
            monitor.leave();
        }
    }
}

As you can see from the example, both have virtually the same number of lines of code. The Monitor adds some complexity around the Guard object compared to the ReentrantLock Condition. However, the clarity of the addToList method in Monitor more than makes up for it. It could just be my personal preference, but I have always found the

  while(something==true){
     condition.await()
}

to be a little awkward.

Usage Guidelines

It should be noted that enter methods that return void should always take the form of:

monitor.enter()
try{
    ...work..
}finally{
  monitor.leave();
}

and enter methods that return a boolean should look like:

if(monitor.enterIf(guard)){
  try{
       ...work..
  }finally{
      monitor.leave();
  }
}else{
   .. monitor not available..
}

Boolean Conditions

There too many enter methods on the Monitor class to effectively cover in one post, so I am going to pick my top three and present then in order from minimal blocking to the maximum.

tryEnterIf – threads will not wait to enter the monitor and will only enter if the guard condition returns true.
enterIf – threads will wait to enter the monitor but only if the guard condition returns true. There are also enterIf method signatures that allow for specifying a timeout as well as an enterIfInterruptibly version.
enterWhen – threads will wait indefinitely for the monitor and the condition to return true, but can be interrupted. Likewise there are options to specify timeouts as well as an enterWhenUniterruptibly version.

Conclusion

I have not yet had the chance to use the Monitor at work, but I can see the usefulness in the granularity of the boolean guard conditions. I have written some basic example code and an accompanying unit test, that demonstrate some of the functionality covered in this post. They are available here. As always your comments/suggestions are welcome. In my next post I will cover more of what can be found in Guava concurrency.

Resources

Reference: Google Guava – Synchronization with Monitor from our JCG partner Bill Bejeck at the Random Thoughts On Coding blog.