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This chapter is from the book

Thread Properties

Thread States

Threads can be in one of four states:

  • new
  • runnable
  • blocked
  • dead

Each of these states is explained in the sections that follow.

New threads

When you create a thread with the new operator—for example, new Ball() —the thread is not yet running. This means that it is in the new state. When a thread is in the new state, the program has not started executing code inside of it. A certain amount of bookkeeping needs to be done before a thread can run.

Runnable threads

Once you invoke the start method, the thread is runnable. A runnable thread may not yet be running. It is up to the operating system to give the thread time to run. When the code inside the thread begins executing, the thread is running. (The Java platform documentation does not call this a separate state, though. A running thread is still in the runnable state.)


The runnable state has nothing to do with the Runnable interface.

Once a thread is running, it doesn't necessarily keep running. In fact, it is desirable if running threads are occasionally interrupted so that other threads have a chance to run. The details of thread scheduling depend on the services that the operating system provides. If the operating system doesn't cooperate, then the Java thread implementation does the minimum to make multithreading work. An example is the so-called green threads package that is used by the Java 1.x platform on Solaris. It keeps a running thread active until a higher-priority thread awakes and takes control. Other thread systems (such as Windows 95 and Windows NT) give each runnable thread a slice of time to perform its task. When that slice of time is exhausted, the operating system gives another thread an opportunity to work. This approach is called time slicing. Time slicing has an important advantage: an uncooperative thread can't prevent other threads from running. Current releases of the Java platform on Solaris can be configured to allow use of the native Solaris threads, which also perform time-slicing.

Always keep in mind that a runnable thread may or may not be running at any given time. (This is why the state is called "runnable" and not "running.") See Figure 1–4.

Figure 1–4: Time-slicing on a single CPU

Blocked threads

A thread enters the blocked state when one of the following actions occurs:

  1. Someone calls the sleep() method of the thread.

  2. The thread calls an operation that is blocking on input/output, that is, an operation that will not return to its caller until input and output operations are complete.

  3. The thread calls the wait() method.

  4. The thread tries to lock an object that is currently locked by another thread. We will discuss object locks later in this chapter.

  5. Someone calls the suspend() method of the thread. However, this method is deprecated, and you should not call it in your code. We will explain the reason later in this chapter.

Figure 1–5 shows the states that a thread can have and the possible transitions from one state to another. When a thread is blocked (or, of course, when it dies), another thread is scheduled to run. When a blocked thread is reactivated (for example, because it has slept the required number of milliseconds or because the I/O it waited for is complete), the scheduler checks to see if it has a higher priority than the currently running thread. If so, it preempts the current thread and picks a new thread to run. (On a machine with multiple processors, each processor can run a thread, and you can have multiple threads run in parallel. On such a machine, a thread is only preempted if a higher priority thread becomes runnable and there is no available processor to run it.)

Figure 1–5: Thread states

For example, the run method of the BallThread blocks itself after it has completed a move, by calling the sleep method.

This gives other threads (in our case, other balls and the main thread) the chance to run.

If the computer has multiple processors, then more than one thread has a chance to run at the same time.

Moving Out of a Blocked State

The thread must move out of the blocked state and back into the runnable state, using the opposite of the route that put it into the blocked state.

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

  2. If a thread is waiting for the completion of an input or output operation, then the operation must have finished.

  3. If a thread called wait, then another thread must call notifyAll or notify. (We cover the wait and notifyAll/notify methods later in this chapter.)

  4. If a thread is waiting for an object lock that was owned by another thread, then the other thread must relinquish ownership of the lock. (You will see the details later in this chapter.)

  5. If a thread has been suspended, then someone must call its resume method. However, since the suspend method has been deprecated, the resume method has been deprecated as well, and you should not call it in your own code.


A blocked thread can only reenter the runnable state through the same route that blocked it in the first place. For example, if a thread is blocked on input, you cannot call its resume method to unblock it.

If you invoke a method on a thread that is incompatible with its state, then the virtual machine throws an IllegalThreadStateException. For example, this happens when you call sleep on a thread that is currently blocked.

Dead Threads

A thread is dead for one of two reasons:

  • It dies a natural death because the run method exits normally.

  • It dies abruptly because an uncaught exception terminates the run method.

In particular, it is possible to kill a thread by invoking its stop method. That method throws a ThreadDeath error object which kills the thread. However, the stop method is deprecated, and you should not call it in your own code. We will explain later in this chapter why the stop method is inherently dangerous.

To find out whether a thread is currently alive (that is, either runnable or blocked), use the isAlive method. This method returns true if the thread is runnable or blocked, false if the thread is still new and not yet runnable or if the thread is dead.


You cannot find out if an alive thread is runnable or blocked, or if a runnable thread is actually running. In addition, you cannot differentiate between a thread that has not yet become runnable and one that has already died.

Daemon Threads

A thread can be turned into a daemon thread by calling


There is nothing demonic about such a thread. A daemon is simply a thread that has no other role in life than to serve others. Examples are timer threads that send regular "timer ticks" to other threads. When only daemon threads remain, then the program exits. There is no point in keeping the program running if all remaining threads are daemons.

Thread Groups

Some programs contain quite a few threads. It then becomes useful to categorize them by functionality. For example, consider an Internet browser. If many threads are trying to acquire images from a server and the user clicks on a "Stop" button to interrupt the loading of the current page, then it is handy to have a way of interrupting all of these threads simultaneously. The Java programming language lets you construct what it calls a thread group so you can simultaneously work with a group of threads.

You construct a thread group with the constructor:

String groupName = . . .;
ThreadGroup g = new ThreadGroup(groupName)

The string argument of the ThreadGroup constructor identifies the group and must be unique. You then add threads to the thread group by specifying the thread group in the thread constructor.

Thread t = new Thread(g, threadName);

To find out whether any threads of a particular group are still runnable, use the activeCount method.

if (g.activeCount() == 0)
  // all threads in the group g have stopped

To interrupt all threads in a thread group, simply call interrupt on the group object.

g.interrupt(); // interrupt all threads in group g

We'll discuss interrupting threads in greater detail in the next section.

Thread groups can have child subgroups. By default, a newly created thread group becomes a child of the current thread group. But you can also explicitly name the parent group in the constructor (see the API notes). Methods such as activeCount and interrupt refer to all threads in their group and all child groups.

One nice feature of thread groups is that you can get a notification if a thread died because of an exception. You need to subclass the ThreadGroup class and override the uncaughtException method. You can then replace the default action (which prints a stack trace to the standard error stream) with something more sophisticated, such as logging the error in a file or displaying a dialog.


  • Thread(ThreadGroup g, String name)
    creates a new Thread that belongs to a given ThreadGroup.



the thread group to which the new thread belongs



the name of the new thread

  • ThreadGroup getThreadGroup()
    returns the thread group of this thread.

  • boolean isAlive()
    returns true if the thread has started and has not yet terminated.

  • void suspend()
    suspends this thread's execution. This method is deprecated.

  • void resume()
    resumes this thread. This method is only valid after suspend() has been invoked. This method is deprecated.

  • void setDaemon(boolean on)
    marks this thread as a daemon thread or a user thread. When there are only daemon threads left running in the system, the program exits. This method must be called before the thread is started.

  • void stop()
    stops the thread. This method is deprecated.


  • ThreadGroup(String name)
    creates a new ThreadGroup. Its parent will be the thread group of the current thread.



the name of the new thread group

  • ThreadGroup(ThreadGroup parent, String name)
    creates a new ThreadGroup.



the parent thread group of the new thread group

  • int activeCount()
    returns an upper bound for the number of active threads in the thread group.

  • int enumerate(Thread[] list)
    gets references to every active thread in this thread group. You can use the activeCount method to get an upper bound for the array; this method returns the number of threads put into the array. If the array is too short (presumably because more threads were spawned after the call to activeCount), then as many threads as fit are inserted.



an array to be filled with the thread references

  • ThreadGroup getParent()
    gets the parent of this thread group.

  • void interrupt()
    interrupts all threads in this thread group and all of its child groups.

  • void uncaughtException(Thread t, Throwable e)
    Override this method to react to exceptions that terminate any threads in this thread group. The default implementation calls this method of the parent thread group if there is a parent, or otherwise prints a stack trace to the standard error stream. (However, if e is a ThreadDeath object, then the stack trace is suppressed. ThreadDeath objects are generated by the deprecated stop method.).

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