Threads - Java Examples in a Nutshell - page 76

Example 4−3: Deadlock.java
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");
}
}