Thread states/ Thread life cycle in java


Contents of page :
  • Thread states/Thread life cycle >
  • Thread have following states >
  • Thread states in diagram >
  • Thread states in detail >


Thread states/Thread life cycle >
Thread states/Thread life cycle is very basic question, before going deep into concepts we must understand Thread life cycle. This post contains in depth explanation of thread methods explaining which method puts thread from which state to which state.


Thread have following states >
  • New
  • Runnable
  • Running
  • Waiting/blocked/sleeping
  • Terminated (Dead)

Thread states in diagram >


Thread states in detail >

New : When instance of thread is created using new operator it is in new state, but the start() method has not been invoked on the thread yet, thread is not eligible to run yet.
>Thread object is considered alive but thread is not alive yet.

Runnable :  When start() method is called on thread it enters runnable state.
>As soon as Thread enters runnable state it is eligible to run, but not running. (Thread scheduler has not scheduled the Thread execution yet, Thread has not entered in run() method yet)
>A thread first enters the runnable state when the start() method is invoked, but a thread can also return to the runnable state after either running or coming back from a
blocked, waiting, or sleeping state.
>Thread is considered alive in runnable state.
>Thread is in Runnable pool.

Running : Thread scheduler selects thread to go from runnable to running state. In running state Thread starts executing by entering run() method.
>Thread scheduler selects thread from the runnable pool on basis of priority, if priority of two threads is same, threads are scheduled in unpredictable manner. Thread scheduler behaviour is completely unpredictable.
>When threads are in running state, yield() method can make thread to go in Runnable state.

Waiting/blocked/sleeping : In this state a thread is not eligible to run.
>Thread is still alive, but currently it’s not eligible to run. In other words.

> How can Thread go from running to waiting state ?
  By calling wait() method thread go from running to waiting state. In waiting state it will wait for other threads to release object monitor/lock.
> How can Thread return from waiting to runnable state ?
  Once notify() or notifyAll() method is called object monitor/lock becomes available and thread can again return to runnable state.


> How can Thread go from running to sleeping state ?
  By calling sleep() method thread go from running to sleeping state. In sleeping state it will wait for sleep time to get over.
> How can Thread return from sleeping to runnable state ?
  Once specified sleep time is up thread can again return to runnable state.

Suspend() method can be used to put thread in waiting state and resume() method is the only way which could put thread in runnable state.

Thread also may go from running to waiting state if it is waiting for some I/O operation to take place. Once input is available thread may return to running state.


Terminated (Dead) : A thread is considered dead when its run() method completes.
>Once thread is dead it cannot be started again doing so will throw runtimeException i.e. IllegalThreadStateException.

destroy() method puts thread directly into dead state.


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RELATED LINKS>

Thread basics >

What is thread in java

Thread states/ Thread life cycle in java

Difference between Process and Thread in java

Implementing Threads in java by implementing Runnable interface and extending Thread class

Threads implement their own stack - demonstration using program and diagram

Differences between implementing Runnable interface and extending Thread class

Thread behaviour is unpredictable

When threads are not lightweight process in java, in fact they become heavy weight process


run() and Start() methods >

What will happen if we don’t override run method of thread?

What will happen if we override start method of thread?

Difference between starting thread with run() and start() methods in java


Can we start Thread again?

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