What’s a monitor in Java?

by

A monitor is mechanism to control concurrent access to an object.

This allows you to do:

Thread 1:

public void a()
{
    synchronized(someObject) {
        // do something (1)
    }
}

Thread 2:

public void b()
{
    synchronized(someObject) {
        // do something else (2)
    }
}

This prevents Threads 1 and 2 accessing the monitored (synchronized) section at the same time. One will start, and monitor will prevent the other from accessing the region before the first one finishes.

It’s not a special object. It’s synchronization mechanism placed at class hierarchy root: java.lang.Object.

There are also wait and notify methods that will also use object’s monitor to communication among different threads.

More Related Contents:

  1. What’s the difference between Thread start() and Runnable run()
  2. Why is creating a Thread said to be expensive?
  3. Why is i++ not atomic?
  4. Synchronization vs Lock
  5. Java Wait and Notify: IllegalMonitorStateException
  6. Are static variables shared between threads?
  7. Prime numbers by Eratosthenes quicker sequential than concurrently?
  8. Is iterating ConcurrentHashMap values thread safe?
  9. How to solve the “Double-Checked Locking is Broken” Declaration in Java?
  10. Executors.newCachedThreadPool() versus Executors.newFixedThreadPool()
  11. Is it safe to get values from a java.util.HashMap from multiple threads (no modification)?
  12. Java’s Fork/Join vs ExecutorService – when to use which?
  13. What is mutex and semaphore in Java ? What is the main difference?
  14. CountDownLatch vs. Semaphore
  15. Java threads and number of cores
  16. Parallel execution of directed acyclic graph of tasks
  17. Forcing multiple threads to use multiple CPUs when they are available
  18. Simplest and understandable example of volatile keyword in Java
  19. ThreadPoolExecutor Block When Queue Is Full?
  20. Why does Java not see the updated value from another thread?
  21. Programmatic deadlock detection in java
  22. Why does java.util.concurrent.ArrayBlockingQueue use ‘while’ loops instead of ‘if’ around calls to await()?
  23. What is adaptive spinning w.r.t lock acquisition?
  24. Run Java Threads sequentially
  25. WAITING at sun.misc.Unsafe.park(Native Method)
  26. Java : Does wait() release lock from synchronized block
  27. How Threadpool re-use Threads and how it works
  28. What is the most frequent concurrency issue you’ve encountered in Java? [closed]
  29. Does a breakpoint halt all threads?
  30. Are 64 bit assignments in Java atomic on a 32 bit machine?

Leave a Comment