The Java TM Tutorial

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Trail: Essential Java Classes
Lesson: Doing Two or More Tasks At Once: Threads

## Synchronizing Threads

> So far, this lesson has contained examples with independent, asynchronous threads. That is, each thread contained all of the data and methods required for its execution and didn't require any outside resources or methods. In addition, the threads in those examples ran at their own pace without concern over the state or activities of any other concurrently running threads.

> However, there are many interesting situations where separate, concurrently running threads do share data and must consider the state and activities of other threads. One such set of programming situations are known as producer/consumer scenarios where the producer generates a stream of data which then is consumed by a consumer.

> For example, imagine a Java application where one thread (the producer) writes data to a file while a second thread (the consumer) reads data from the same file. Or, as you type characters on the keyboard, the producer thread places key events in an event queue and the consumer thread reads the events from the same queue. Both of these examples use concurrent threads that share a common resource: the first shares a file, the second shares an event queue. Because the threads share a common resource, they must be synchronized in some way.

### The Producer/Consumer Example

> This lesson teaches you about Java thread synchronization through a simple producer/consumer example.

### Locking an Object

> The code segments within a program that access the same object from separate, concurrent threads are called critical sections. In the Java language, a critical section can be a block or a method and are identified with the `synchronized` keyword. The Java platform then associates a lock with every object that has synchronized code.

### Using the `notifyAll` and `wait` Methods

> This section investigates the code in `CubbyHole`'s `put` and `get` methods that helps the `Producer` and `Consumer` coordinate their activities.

### Avoid Starvation and Deadlock

> If you write a program in which several concurrent threads are competing for resources, you must take precautions to ensure fairness. A system is fair when each thread gets enough access to limited resource to make reasonable progress. A fair system prevents starvation and deadlock. Starvation occurs when one or more threads in your program is blocked from gaining access to a resource and thus cannot make progress. Deadlock is the ultimate form of starvation; it occurs when two or more threads are waiting on a condition that cannot be satisfied. Deadlock most often occurs when two (or more) threads are each waiting for the other(s) to do something.

> This section uses the dining philosophers problem to illustrate deadlock. It also discusses ways you can prevent deadlock.

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