Introduction to Multithreading

Introduction to Multithreading

A program can be divided into a number of small processes. Each small process can be addressed as a single thread (a lightweight process). Multithreaded programs contain two or more threads that can run concurrently. This means that a single program can perform two or more tasks simultaneously. For example, one thread is writing content on a file at the same time another thread is performing spelling check.

In Java, the word thread means two different things.

• An instance of Thread class.
• or, A thread of execution.

An instance of Thread class is just an object, like any other object in java. But a thread of execution means an individual "lightweight" process that has its own call stack. In java each thread has its own call stack.

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The main thread

Even if you don't create any thread in your program, a thread called main thread is still created. Although the main thread is automatically created, you can control it by obtaining a reference to it by calling currentThread() method.

Two important things to know about main thread are,

• It is the thread from which other threads will be produced.
• main thread must be always the last thread to finish execution.

class MainThread
{
 public static void main(String[] args)
 {
  Thread t=Thread.currentThread();
  t.setName("MainThread");
  System.out.println("Name of thread is "+t);
 }
}

Output :

Name of thread is Thread[MainThread,5,main]

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Life cycle of a Thread

1. New : A thread begins its life cycle in the new state. It remains in this state until the start() method is called on it.
2. Runable : After invocation of start() method on new thread, the thread becomes runable.
3. Running : A method is in running thread if the thread scheduler has selected it.
4. Waiting : A thread is waiting for another thread to perform a task. In this stage the thread is still alive.
5. Terminated : A thread enter the terminated state when it complete its task.

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Thread Priorities

Every thread has a priority that helps the operating system determine the order in which threads are scheduled for execution. In java thread priority ranges between,

• MIN-PRIORITY (a constant of 1)
• MAX-PRIORITY (a constant of 10)

By default every thread is given a NORM-PRIORITY(5). The main thread always have NORM-PRIORITY.