Java Get Thread State

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A thread can exist in number of different states. You can get the current state of a thread by calling the getState() method defined by Thread. Here is the way to get the state of a thread:

Thread.State getState()

It returns a value of the type Thread.State which indicates the state of the thread at the time at which the call was made. State is an enumeration defined by the Thread. (An enumerations is nothing, it is a list of named constants. You will learn about enumerations in next chapter).

Here this table lists the values that can be returned by the method getState() :

Value State
BLOCKED A thread that has suspended execution because it is waiting to gain a lock
NEW A thread that has no begun execution
RUNNABLE A thread that either is currently executing or will execute when it gains access to CPU
TERMINATED A thread that has completed execution
TIMED_WAITING A thread that has suspended execution for specified period of time. Such as when it has called sleep(). This state also entered when a timeout version of wait() or join() is called
WAITING A thread that has suspended execution because it is waiting for some action to occur. For instance, it is waiting because of a call to a non-timeout version of wait() or join()

Java Thread States

Following figure shows how the various thread states relates :

java get thread state

Given a Thread instance, you can use getState() method to get the state of a thread. For instance, the below sequence determines if a thread called thrd is in the RUNNABLE state at the time the getState() is called :

Thread.State ts = thrd.getState();
if(ts == Thread.State.RUNNABLE)  // ...

It is important to understand that a thread's state may change after the call to the getState(). Therefore, depending on the circumstances, the state got by calling the method getState() may not reflect the actual state of the thread only a moment later. For this reason and other reasons, the method getState() is not intended to provide a means of synchronizing threads. It is primarily uses is for debugging/profiling a thread's run-time characteristics.

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