Locks

A Simple Lock

Let's start out by looking at a synchronized block of Java code:

public class Counter{

  private int count = 0;

  public int inc(){
    synchronized(this){
      return ++count;
    }
  }
}

Notice the synchronized(this) block in the inc() method. This block makes sure that only one thread can execute the return ++count at a time. The code in the synchronized block could have been more advanced, but the simple ++count suffices to get the point across.

The Counter class could have been written like this instead, using a Lock instead of a 

synchronized block:

public class Counter{

  private Lock lock = new Lock();
  private int count = 0;

  public int inc(){
    lock.lock();
    int newCount = ++count;
    lock.unlock();
    return newCount;
  }
}

The lock() method locks the Lock instance so that all threads calling lock() are blocked until unlock() is executed.

Here is a simple Lock implementation:

public class Lock{

  private boolean isLocked = false;

  public synchronized void lock()
  throws InterruptedException{
    while(isLocked){
      wait();
    }
    isLocked = true;
  }

  public synchronized void unlock(){
    isLocked = false;
    notify();
  }
}

Notice the while(isLocked) loop, which is also called a "spin lock"