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Concurrency
Deadlock
Deadlock describes a situation where two or more threads are blocked
forever, waiting for each other. Here's an example.
Alphonse and Gaston are friends, and great believers in courtesy. A
strict rule of courtesy is that when you bow to a friend, you must
remain bowed until your friend has a chance to return the bow.
Unfortunately, this rule does not account for the possibility that two
friends might bow to each other at the same time. This example
application,
Deadlock
public class Deadlock {
static class Friend {
private final String name;
public Friend(String name) {
this.name = name;
}
public String getName() {
return this.name;
}
public synchronized void bow(Friend bower) {
System.out.format("%s: %s has bowed to me!%n",
this.name, bower.getName());
bower.bowBack(this);
}
public synchronized void bowBack(Friend bower) {
System.out.format("%s: %s has bowed back to me!%n",
this.name, bower.getName());
}
}
public static void main(String[] args) {
final Friend alphonse = new Friend("Alphonse");
final Friend gaston = new Friend("Gaston");
new Thread(new Runnable() {
public void run() { alphonse.bow(gaston); }
}).start();
new Thread(new Runnable() {
public void run() { gaston.bow(alphonse); }
}).start();
}
}
When
Deadlock runs, it's extremely likely that both
threads will block when they attempt to invoke
bowBack.
Neither block will ever end, because each thread is waiting for the
other to exit
bow.
