Java Threads, Second Edition

Scheduling with Thread Priorities

Let’s delve into the programming that affects thread scheduling; we’ll start by examining how to manipulate the priority level of Java threads. This is the most useful mechanism available to a Java programmer that affects scheduling behavior of threads; often, a few simple adjustments of thread priorities is all that’s required to make a program behave as desired.

Priority-Related Calls in the Java API

In the Java Thread class, there are three static final variables that define the allowable range of thread priorities:

Thread.MIN_PRIORITY

The minimum priority a thread can have

Thread.MAX_PRIORITY

The maximum priority a thread can have

Thread.NORM_PRIORITY

The default priority for threads in the Java interpreter

Every thread has a priority value that lies somewhere in the range between MIN_PRIORITY (which is 1) and MAX_PRIORITY (which is 10). However, not all threads can have a value anywhere within this range: each thread belongs to a thread group, and the thread group has a maximum priority (lower than or equal to MAX_PRIORITY) that its individual threads cannot exceed. We’ll discuss this further in Chapter 10, but for now, you should be aware that the maximum thread priority for a thread within an applet is typically NORM_PRIORITY + 1. In addition, the virtual machine is allowed to create internal threads at a priority of 0, so that there are in effect 11 different priority levels for threads within the virtual machine.

Symbolic Thread Priority ...