Java threads

A thread is a single independent stream that runs within a program. Java™ is a multithreaded programming language, so more than one thread may be running within the Java virtual machine at one time. Java threads provide a way for a Java program to perform multiple tasks at the same time. A thread is essentially a flow of control in a program.

Threads are a modern programming construct that are used to support concurrent programs and to improve the performance and scalability of applications. Most programming languages support threads through the use of add-in programming libraries. Java supports threads as built-in application programming interfaces (APIs).

Threads are the mechanism for waiting on long running interactions, while still allowing the program to handle other work. Threads have the ability to support multiple flows through the same code stream. They are sometimes called lightweight processes. The Java language includes direct support for threads. But, by design, it does not support asynchronous non-blocking input and output with interrupts or multiple wait.

Threads allow the development of parallel programs that scale well in an environment where a machine has multiple processors. If properly constructed, they also provide a model for handling multiple transactions and users.

You can use threads in a Java program for a number of situations. Some programs must be able to engage in multiple activities and still be able to respond to additional input from the user. For example, a Web browser should be able to respond to user input while playing a sound.

Threads can also use asynchronous methods. When you call a second method, you do not have to wait for the first method to complete before the second method continues with its own activity.

There are also many reasons not to use threads. If a program uses inherently sequential logic, one thread can accomplish the entire sequence. Using multiple threads in such a case results in a complex program with no benefits. There is considerable work in creating and starting a thread. If an operation involves only a few statements, it is faster to handle it in a single thread. This can be true even when the operation is conceptually asynchronous. When multiple threads share objects, the objects must synchronize to coordinate thread access and maintain consistency. Synchronization adds complexity to a program, is difficult to tune for optimal performance, and can be a source of programming errors.

For more threads information, see Developing multithreaded applications.