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6.4 Scheduling and Priority

Java makes certain guarantees as to how its threads are scheduled. Every thread has a priority value. If, at any time, a thread of a higher priority than the current thread becomes runnable, it preempts the lower priority thread and begins executing. By default, threads at the same priority are scheduled round robin, which means once a thread starts to run, it continues until it does one of the following:


Calls Thread.sleep() or wait()

Waits for lock

Waits for a lock in order to run a synchronized method

Blocks on I/O

Blocks, for example, in a xread() or an accept() call

Explicitly yields control

Calls yield()


Completes its target method or is terminated by a stop() call

This situation looks something like what's shown in Figure 6.4.

Figure 6.4: Priority preemptive, round robin scheduling

[Graphic: Figure 6-4]

Java leaves certain aspects of scheduling up to the implementation.[2] The main point here is that some, but not all, implementations of Java use time slicing on threads of the same priority.[3] In a time-sliced system, thread processing is chopped up, so that each thread runs for a short period of time before the context is switched to the next thread, as shown in Figure 6.5.

[2] This implementation-dependent aspect of Java isn't a big deal, since it doesn't hurt for an implementation to add time slicing on top of the default round-robin scheduling. It's actually not hard to create a time-slicing effect by simply having a high-priority thread sleeping for a specified time interval. Every time it wakes up, it interrupts a lower-priority thread and causes processing to shift round robin to the next thread.

[3] As of Java Release 1.0, Sun's Java Interpreter for the Windows 95 and Windows NT platforms uses time slicing, as does the Netscape Navigator Java environment. Sun's Java 1.0 for the Solaris UNIX platforms doesn't.

Higher priority threads still preempt lower priority threads in this scheme. The addition of time slicing mixes up the processing among threads of the same priority; on a multiprocessor machine, threads may even be run simultaneously. Unfortunately, this feature can lead to differences in your application's behavior.

Figure 6.5: Priority preemptive, time-sliced scheduling

[Graphic: Figure 6-5]

Since Java doesn't guarantee time slicing, you shouldn't write code that relies on this type of scheduling; any software you write needs to function under the default round-robin scheduling. But if you're wondering what your particular flavor of Java does, try the following experiment:

class Thready { 
    public static void main( String args [] ) { 
        new MyThread("Foo").start(); 
        new MyThread("Bar").start(); 
class MyThread extends Thread { 
    String message; 
    MyThread ( String message ) { 
        this.message = message; 
    public void run() { 
        while ( true )  
            System.out.println( message ); 

The Thready class starts up two MyThread objects. Thready is a thread that goes into a hard loop (very bad form) and prints its message. Since we don't specify a priority for either thread, they both inherit the priority of their creator, so they have the same priority. When you run this example, you will see how your Java implementation does it scheduling. Under a round-robin scheme, only "Foo" should be printed; "Bar" never appears. In a time-slicing implementation, you should occasionally see the "Foo" and "Bar" messages alternate.


Now let's change the priority of the second thread:

class Thready { 
    public static void main( String args [] ) { 
        new MyThread("Foo").start(); 
        Thread bar = new MyThread("Bar"); 
        bar.setPriority( Thread.NORM_PRIORITY + 1 ); 

As you might expect, this changes how our example behaves. Now you may see a few "Foo" messages, but "Bar" should quickly take over and not relinquish control, regardless of the scheduling policy.

Here we have used the setPriority() method of the Thread class to adjust our thread's priority. The Thread class defines three standard priority values, as shown in Table 6.1.

Table 6.1: Thread Priority Values
Value Definition
MIN_PRIORITY Minimum priority
NORM_PRIORITY Normal priority
MAX_PRIORITY Maximum priority

If you need to change the priority of a thread, you should use one of these values or a close relative value. But let me warn you against using MAX_PRIORITY or a close relative value; if you elevate many threads to this priority level, priority will quickly become meaningless. A slight increase in priority should be enough for most needs. For example, specifying NORM_PRIORITY + 1 in our example is enough to beat out our other thread.


As I said earlier, whenever a thread sleeps, waits, or blocks on I/O, it gives up its time slot, and another thread is scheduled. So as long as you don't write methods that use hard loops, all threads should get their due. However, a Thread can also give up its time voluntarily with the yield() call. We can change our previous example to include a yield() on each iteration:

class MyThread extends Thread { 
    public void run() { 
        while ( true ) { 
            System.out.println( message ); 

Now you should see "Foo" and "Bar" messages alternating one for one. If you have threads that perform very intensive calculations, or otherwise eat a lot of CPU time, you might want to find an appropriate place for them to yield control occasionally. Alternatively, you might want to drop the priority of your intensive thread, so that more important processing can proceed around it.

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