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The Java™ Tutorials
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Concurrency
Processes and Threads
Thread Objects
Defining and Starting a Thread
Pausing Execution with Sleep
Interrupts
Joins
The SimpleThreads Example
Synchronization
Thread Interference
Memory Consistency Errors
Synchronized Methods
Intrinsic Locks and Synchronization
Atomic Access
Liveness
Deadlock
Starvation and Livelock
Guarded Blocks
Immutable Objects
A Synchronized Class Example
A Strategy for Defining Immutable Objects
High Level Concurrency Objects
Lock Objects
Executors
Executor Interfaces
Thread Pools
Fork/Join
Concurrent Collections
Atomic Variables
Concurrent Random Numbers
For Further Reading
Questions and Exercises
Trail: Essential Classes
Lesson: Concurrency
Section: Liveness
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# 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``, models this possibility:

    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`.

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