Learning Java

Scheduling and Priority

Java makes few guarantees about how it schedules threads. Almost all of Java’s thread scheduling is left up to the Java implementation and, to some degree, the application. Although it might have made sense (and would certainly have made many developers happier) if Java’s developers had specified a scheduling algorithm, a single scheduling algorithm isn’t necessarily suitable for all of the roles that Java can play. Instead, Sun decided to put the burden on you to write robust code that works whatever the scheduling algorithm, and let the implementation tune the algorithm for whatever is best.

Therefore, the priority rules that we’ll describe next are carefully worded in the Java language specification to be a general guideline for thread scheduling. You should be able to rely on this behavior overall (statistically), but it is not a good idea to write code that relies on very specific features of the scheduler to work properly. You should instead use the control and synchronization tools that we have described in this chapter to coordinate your threads.^[28]

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:

• Sleeps, by calling Thread.sleep() or wait( )

• Waits for a lock, ...