Thread State

Thread State Throughout its life, a Java thread is in one of several states. A thread’s state indicates what the Thread is doing and what it is capable of doing at that time of its life: is it running? Is it sleeping? Is it dead?

The above diagram illustrates the various states that a Java thread can be in at any point during its life. It also illustrates which method calls cause a transition to another State.

New Thread

The following statement creates a new thread but does not start it, thereby leaving the thread in the “New Thread” state.

Thread myThread = new MyThreadClass();

When a thread is in the “New Thread” state, it is merely an empty Thread object. No system resources have been allocated for it yet. Thus when a thread is in this state, we can only start the thread or stop it. Calling any method besides start() or stop() when a thread is in this state makes no sense and causes an IllegalThreadstateException.

Runnable

Now consider these two lines of code:

Thread myThread = new MyThreadClass(); myThread.start();

The start() method creates the system resources necessary to run the thread, schedules the thread to run, and calls the thread’s run() method. At this point the thread is in the “Runnable” state. This state is called “Runnable” rather than “Running” because the thread might not actually be running when it is in this state. Many computers have a single processor, making it impossible to run all “Runnable” threads at the same time. So, the Java runtime system must implement a scheduling scheme that shares the processor between all “Runnable” threads.

Not Runnable

A thread enters the “Not Runnable” state when one of these four events occurs:

1. Someone invokes its sleep() method.

2. Someone invokes its suspend() method.

3. The thread uses its wait() method to wait on a condition variable.

4. The thread is blocking on I/O.

For example,

The bold line in the following code snippet puts the current thread to sleep for 10 seconds (10,000 milliseconds):

try { Thread.sleep(10000); } catch (InterruptedException e) { }

During the 10 seconds that myThread is asleep; even if the processor becomes available myThread does not run. After the 10 seconds are up, myThread becomes “Runnable” again and, if the processor becomes available, runs.

If a thread has been put to sleep, then the specified number of milliseconds must elapse before the thread becomes “Runnable” again. Calling resume() on a sleeping thread has no effect.

The following indicates the escape route for every entrance into the “Not Runnable” state.

1. If a thread has been put to sleep, then the specified number of milliseconds must elapse.

2. If a thread has been suspended, then someone must call its resume() method.

3. If a thread is waiting on a condition variable, whatever object owns the variable must relinquish it by calling either notify() or notifyAll().

4. If a thread is blocked on I/O, then the I/O must complete.

Dead

A thread can die in two ways: either from natural causes, or by being killed (stopped). A thread dies naturally when its run() method exits normally.

For example,

The while loop in this method is a finite loop–it will iterate 100 times and then exit.

public void run() { int i = 0; while (i < 100) { i++; System.out.println(“i = ” + i); } }

A thread with this run() method will die naturally after the loop and the run() method completes.

We can also kill a thread at any time by calling its stop() method.

The following code snippet creates and starts myThread then puts the current thread to sleep for 10 seconds. When the current thread wakes up, the bold line in the code segment kills myThread.

Thread myThread = new MyThreadClass(); myThread.start(); try { Thread.sleep(10000); } catch (InterruptedException e) { } myThread.stop();

The stop() method throws a ThreadDeath object at the thread to kill it. Thus when a thread is killed in this manner it dies asynchronously. The thread will die when it actually receives the ThreadDeath exception.

IllegalThreadStateException

The runtime system throws an IllegalThreadStateException when we call a method on a thread and that thread’s state does not allow for that method call.

For example,

IllegalThreadStateException is thrown when we invoke suspend() on a thread that is not “Runnable”.

The isAlive() Method

A final word about thread state: the programming interface for the Thread class includes a method called isAlive().

The isAlive() method returns true if the thread has been started and not stopped.

Thus, if the isAlive() method returns false we know that the thread is either a “New Thread” or “Dead”.

If the isAlive() method returns true, we know that the thread is either “Runnable” or “Not Runnable”.

We cannot differentiate between a “New Thread” and a “Dead” thread; nor can we differentiate between a “Runnable” thread and a “Not Runnable” thread.

Thread Priority

A thread’s priority tells the Java thread scheduler when this thread should run in relation to other threads.

Some points

1. Most computers have only one CPU, thus threads must share the CPU with other threads.     The execution of multiple threads on a single CPU, in some order, is called scheduling.     The Java runtime supports a very simple, deterministic scheduling algorithm known as     fixed priority scheduling.

2. Each Java thread is given a numeric priority between MIN_PRIORITY and MAX_PRIORITY     (constants defined in class Thread). At any given time, when multiple threads are ready     to be executed, the thread with the highest priority will be chosen for execution. Only     when that thread stops, or is suspended for some reason, will a lower priority thread start     executing.

3. Scheduling of the CPU is fully preemptive. If a thread with a higher priority than the     currently executing thread needs to execute, the higher priority thread is immediately     scheduled.

4. The Java runtime will not preempt the currently running thread for another thread of the     same priority. In other words, the Java runtime does not time-slice. However, the system     implementation of threads underlying the Java Thread class may support time-slicing.     Do not write code that relies on time-slicing.

5. In addition, a given thread may, at any time, give up its right to execute by calling the     yield() method. Threads can only yield the CPU to other threads of the same priority–attempts to yield to a lower priority thread are ignored.

6. When all the “Runnable” threads in the system have the same priority, the scheduler     chooses the next thread to run in a simple, non-preemptive, round-robin scheduling     order.

Daemon Threads

Daemon threads are those that provide a service for other threads in the system. Any Java thread can be a daemon thread.