Threads - Overview

• on an operating system a running program is known as a process
• a single process can have seperate runnable tasks called threads
• a thread is a single sequential flow of control within a process
• a thread is also referred to as a lightweight process

• with a single-processor CPU, only one thread is executing at any given time
• the CPU quickly switches between active threads giving the illusion that they are all executing at the same time (logical concurrency)
• on multi-processor systems several threads are actually executing at the same time (physical concurrency)
• multi-programming occurs when multiple programs or processes are executed
• multi-threading occurs when concurrency exists amoung threads running in a single process (also referred to as multi-tasking)

• Java provides support for multi-threading as a part of the language
• support centers on the:
  • java.lang.Thread class
  • java.lang.Runnable interface
  • java.lang.Object methods wait(), notify(), and notifyAll
  • synchronized keyword

• every Java program has at least one thread which is executed when main() is invoked
• all user-level threads are explicitly constructed and started from the main thread or by a thread originally started from main()
• when the last user thread completes any daemon threads are stopped and the application stops
• a thread's default daemon status is the same as that of thread creating it
• you can check the daemon status using isDaemon()
• you can set the daemon status using setDaemon().

  You cannot change a thread's status after it has been started

• main() daemon status is false
• if you want all your threads to quit when main() completes you can set their status to daemon using setDaemon(true)

• there are two basic ways to create and run threads
  1. by subclassing the Thread class
  2. by implementing the Runnable interface

Also see

• Sun Tutorial on Threads
• IBM Redbook: Java Thin-Client Programming - Introduction to Threads
• SunTech Tip: Why Use Threads?
• Beware the daemons
• Exploring Java, Chapter 6, Threads

Example Code

• Source code to check daemon status of main()