Java Thread States – Life Cycle of Java Threads

In the following state transition diagram we show the various states for a Java thread and the events that cause the thread to jump from one state to another.

Thread States in Java – Understanding Thread Life Cycle of Java Threads

As you begin you journey into Java Development there will come a time in your life where you will want to take advantage of multithreading for performance reasons. When you do, you will inevitably need to become familiar with the life cycle of java threads.

Constructed/Born

A newly created Thread in Java will be in this state just after constructing but before calling the start() method.

Thread t1 = new Thread(new BankAccount(1));

Ready-To-Run

We must call the start() method in order to schedule the thread to begin running. It is this that puts a thread into the “Ready to Run” state. When the thread’s run() method is called, the thread goes into “Running” state.

t1.start();

Warning

Please note, a common mistake of novice programmers is to call the run() method like this, t1.run() We do not call run() method, instead always make sure you start the thread using the start() method.

Running

Once running, a thread may go back to the “Ready to Run” state when one of the following occurs:

  • the thread is blocking on some I/O
  • thread’s sleep() method is called
  • the allotted time slice ends (OS dependent)
  • thread’s yield() method is called
  • the object’s wait() method is called
java thread states

Waiting

A Running thread will go into a waiting state when an object’s wait() method is called. This mechanism is used as a communication means between threads in order to avoid polling. Waiting threads are put into the object’s queue. When another thread associated with the object calls notify(), then the first thread in object’s wait queue go into “Ready-To-Run” state. If notifyAll() is called, then all threads in the object’s wait queue go into the “Ready-To-Run” state.

Sleeping

A Running thread will go into the “Sleeping” state when the thread.sleep() method is called. The thread goes back into the “Ready-To-Run” state after the specified time has elapsed or if interrupted.

try {
  Thread.sleep(1000);
} catch (InterruptedException e) {
  e.printStackTrace();
}

Blocking

A Running thread enters a “Blocking” state when it requests some type of I/O and is waiting for the I/O to complete or become available. The thread goes back to “Ready-To-Run” state when the I/O becomes available.

Dead

A Running thread enters the “Dead” state when the condition is met such that it exits its run() method normally or completes normally.

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