deadlock.java

These are the examples from the book Java Examples in a Nutshell, 2nd Edition, by David Flanagan.

/*
 * Copyright (c) 2000 David Flanagan.  All rights reserved.
 * This code is from the book Java Examples in a Nutshell, 2nd Edition.
 * It is provided AS-IS, WITHOUT ANY WARRANTY either expressed or implied.
 * You may study, use, and modify it for any non-commercial purpose.
 * You may distribute it non-commercially as long as you retain this notice.
 * For a commercial use license, or to purchase the book (recommended),
 * visit http://www.davidflanagan.com/javaexamples2.
 */
package com.davidflanagan.examples.thread;

/**
 * This is a demonstration of how NOT to write multi-threaded programs.
 * It is a program that purposely causes deadlock between two threads that
 * are both trying to acquire locks for the same two resources.
 * To avoid this sort of deadlock when locking multiple resources, all threads
 * should always acquire their locks in the same order.
 **/
public class Deadlock {
    public static void main(String[] args) {
        // These are the two resource objects we'll try to get locks for
        final Object resource1 = "resource1";
        final Object resource2 = "resource2";
        // Here's the first thread.  It tries to lock resource1 then resource2
        Thread t1 = new Thread() {
		public void run() {
		    // Lock resource 1
		    synchronized(resource1) {
			System.out.println("Thread 1: locked resource 1");
			
			// Pause for a bit, simulating some file I/O or
			// something.  Basically, we just want to give the
			// other thread a chance to run.  Threads and deadlock
			// are asynchronous things, but we're trying to force
			// deadlock to happen here...
			try { Thread.sleep(50); }
			catch (InterruptedException e) {}
			
			// Now wait 'till we can get a lock on resource 2
			synchronized(resource2) {
			    System.out.println("Thread 1: locked resource 2");
			}
		    }
		}
	    };
        
        // Here's the second thread.  It tries to lock resource2 then resource1
        Thread t2 = new Thread() {
		public void run() {
		    // This thread locks resource 2 right away
		    synchronized(resource2) {
			System.out.println("Thread 2: locked resource 2");
			
			// Then it pauses, just like the first thread.
			try { Thread.sleep(50); }
			catch (InterruptedException e) {}
			
			// Then it tries to lock resource1.  But wait!  Thread
			// 1 locked resource1, and won't release it 'till it
			// gets a lock on resource2.  This thread holds the
			// lock on resource2, and won't release it 'till it
			// gets resource1.  We're at an impasse. Neither
			// thread can run, and the program freezes up.
			synchronized(resource1) {
			    System.out.println("Thread 2: locked resource 1");
			}
		    }
		}
	    };
        
        // Start the two threads. If all goes as planned, deadlock will occur, 
        // and the program will never exit.
        t1.start(); 
        t2.start();
    }
}