Life Cycle of A Thread

When you are programming with threads, understanding the life cycle of thread is very valuable. While a thread is alive, it is in one of several states. By invoking start() method, it doesn?t mean that the thread has access to CPU and start executing straight away. Several factors determine how it will proceed.

Different states of a thread are :

1. New state ? After the creations of Thread instance the thread is in this state but before the start() method invocation. At this point, the thread is considered not alive.
    
2. Runnable (Ready-to-run) state ? A thread start its life from Runnable state. A thread first enters runnable state after the invoking of start() method but a thread can return to this state after either running, waiting, sleeping or coming back from blocked state also. On this state a thread is waiting for a turn on the processor.
    
3. Running state ? A thread is in running state that means the thread is currently executing. There are several ways to enter in Runnable state but there is only one way to enter in Running state: the scheduler select a thread from runnable pool.
    
4. Dead state ? A thread can be considered dead when its run() method completes. If any thread comes on this state that means it cannot ever run again.
5. Blocked - A thread can enter in this state because of waiting the resources that are hold by another thread.

Different states implementing Multiple-Threads are:

As we have seen different states that may be occur with the single thread. A running thread can enter to any non-runnable state, depending on the circumstances. A thread cannot enters directly to the running state from non-runnable state, firstly it goes to runnable state. Now lets understand the some non-runnable states which may be occur handling the multithreads.

• Sleeping ? On this state, the thread is still alive but it is not runnable, it might be return to runnable state later, if a particular event occurs. On this state a thread sleeps for a specified amount of time. You can use the method sleep( ) to stop the running state of a thread.
  
     static void sleep(long millisecond) throws InterruptedException
• Waiting for Notification ? A thread waits for notification from another thread. The thread sends back to runnable state after sending notification from another thread.
  
     final void wait(long timeout) throws InterruptedException
     final void wait(long timeout, int nanos) throws InterruptedException
     final void wait() throws InterruptedException
   
• Blocked on I/O ? The thread waits for completion of blocking operation. A thread can enter on this state because of waiting I/O resource. In that case the thread sends back to runnable state after availability of resources.
   
• Blocked for joint completion ? The thread can come on this state because of waiting the completion of another thread.
    
• Blocked for lock acquisition ? The thread can come on this state because of waiting to acquire the lock of an object.

 Methods that can be applied apply on a Thread:

Some Important Methods defined in java.lang.Thread are shown in the table:

Method	Return Type	Description
currentThread( )	Thread	Returns an object reference to the thread in which it is invoked.
getName( )	String	Retrieve the name of the thread object or instance.
start( )	void	Start the thread by calling its run method.
run( )	void	This method is the entry point to execute thread, like the main method for applications.
sleep( )	void	Suspends a thread for a specified amount of time (in milliseconds).
isAlive( )	boolean	This method is used to determine the thread is running or not.
activeCount( )	int	This method returns the number of active threads in a particular thread group and all its subgroups.
interrupt( )	void	The method interrupt the threads on which it is invoked.
yield( )	void	By invoking this method the current thread pause its execution temporarily and allow other threads to execute.
join( )	void	This method and  join(long millisec) Throws InterruptedException.  These two methods are invoked on a thread. These are not returned until either the thread has completed or it is timed out respectively.

The following diagram shows the life cycle of a Java Thread. Thread has following states : Start, Stop, Blocked, Running, Runnable. These states can be changed to each other as shown in the diagram. Thread can sleep, notify, resume, start, stop, suspend, yield and wait. These activities lead the thread from one state to move to another one.

More Information:

http://www.iam.ubc.ca/guides/javatut99/essential/threads/lifecycle.html