Java provides a monitor-like concurrency mechanism for thread synchronization. Every object in Java has associated with it a single lock. When a method is declared to be synchronized, calling the method requires owning the lock for the object. We declare a synchronized method by placing the synchronized keyword in the method definition. The following defines safeMethod() as synchronized, for example:
public class SimpleClass {
...
public synchronized void safeMethod() {
...
/* Implementation of safeMethod() */
...
}
}
Next, we create an object instance of SimpleClass, such as the following:
SimpleClass sc = new SimpleClass();
Invoking sc.safeMethod() method requires owning the lock on the object instance sc. If the lock is already owned by another thread, the thread calling the synchronized method blocks and is placed in the entry setfor the object’s lock. The entry set represents the set of threads waiting for the lock to become available. If the lock is available when a synchronized method is called, the calling thread becomes the owner of the object’s lock and can enter the method. The lock is released when the thread exits the method. A thread from the entry set is then selected as the new owner of the lock
Java also provides wait() and notify() methods, which are similar in function to the wait() and signal() statements for a monitor. The Java API provides support for semaphores, condition variables, and mutex locks (among other concurrency mechanisms) in the java.util.concurrent package.