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Java Thread Priorities



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Thread priorities are used by the thread scheduler to decide when each thread should be allowed to run.

In theory, over a given period of time, the higher-priority threads gets more CPU time than the lower-priority threads.

In practice, the amount of CPU time that a thread gets often depends on several factors besides its priority. For example, how an operating system implements the multitasking can affect the relative availability of CPU time.

A higher-priority thread can also preempt a lower-priority one. For instance, when a lower-priority thread is running and a higher-priority thread resumes (from sleeping or waiting on I/O). For example, it will preempt the lower-priority thread.

In theory, threads of equal priority should get equal access to CPU. But you need to be careful. Remember, Java is designed to work in a wide range of environments. Some of those environments implement multitasking fundamentally differently than others. For safety, threads that share the same priority should yield control once in a while. This ensures that all the threads have a chance to run under a nonpreemptive operating system.

In practice, even in nonpreemptive environments, most threads still get a chance to run, because most threads inevitably encounter some blocking situation, such as waiting for I/O. When this happens, blocked thread is suspended and other threads can run. But, if you want smooth multithreaded execution, you are better off not relying on this. Also, some types of tasks of CPU-intensive. Such threads dominate the CPU. For these types of threads, you want to yield control occasionally so that the other threads can run.

To set a thread's priority, use setPriority() method, which is a member of Thread. This is its general form :

final void setPriority(int level)

Here, level specifies the new priority setting for the calling thread. The value of level must be within the range MIN_PRIORITY and MAX_PRIORITY. Currently, these values are 1 and 10, respectively.

To return a thread a default priority, specify NORM_PRIORITY, which is currently 5. These priorities are defined as static final variables within the Thread.

You can obtain the current priority setting by calling getPriority() method of Thread, shown below :

final int getPriority()

Implementations of Java may have radically different behaviour when it comes to scheduling. Most of the inconsistencies arise when you have threads that are relying on preemptive behaviour, instead of cooperatively giving up CPU time. The safest way to obtain predictable, cross-platform behaviour with Java is to use the threads that voluntarily give up the control of the CPU.


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