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Java 101: Understanding Java threads, Part 3: Thread scheduling and wait/notify

Learn about the mechanisms that help you set and manage thread priority

By Jeff Friesen, JavaWorld.com, 07/05/02

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Page 3 of 7

Note: A runnable thread with the highest priority will not always run. Here's the Java Language Specification's take on priority:

Every thread has a priority. When there is competition for processing resources, threads with higher priority are generally executed in preference to threads with lower priority. Such preference is not, however, a guarantee that the highest priority thread will always be running, and thread priorities cannot be used to reliably implement mutual exclusion.

That admission says much about the implementation of green thread JVMs. Those JVMs cannot afford to let threads block because that would tie up the JVM's sole thread of execution. Therefore, when a thread must block, such as when that thread is reading data slow to arrive from a file, the JVM might stop the thread's execution and use a polling mechanism to determine when data arrives. While the thread remains stopped, the JVM's thread scheduler might schedule a lower-priority thread to run. Suppose data arrives while the lower-priority thread is running. Although the higher-priority thread should run as soon as data arrives, that doesn't happen until the JVM next polls the operating system and discovers the arrival. Hence, the lower-priority thread runs even though the higher-priority thread should run. ou need to worry about this situation only when you need real-time behavior from Java. But then Java is not a real-time operating system, so why worry?

To understand which runnable green thread becomes the currently running green thread, consider the following. Suppose your application consists of three threads: the main thread that runs the main() method, a calculation thread, and a thread that reads keyboard input. When there is no keyboard input, the reading thread blocks. Assume the reading thread has the highest priority and the calculation thread has the lowest priority. (For simplicity's sake, also assume that no other internal JVM threads are available.) Figure 1 illustrates the execution of these three threads.

Figure 1. Thread-scheduling diagram for different priority threads

At time T0, the main thread starts running. At time T1, the main thread starts the calculation thread. Because the calculation thread has a lower priority than the main thread, the calculation thread waits for the processor. At time T2, the main thread starts the reading thread. Because the reading thread has a higher priority than the main thread, the main thread waits for the processor while the reading thread runs. At time T3, the reading thread blocks and the main thread runs. At time T4, the reading thread unblocks and runs; the main thread waits. Finally, at time T5, the reading thread blocks and the main thread runs. This alternation in execution between the reading and main threads continues as long as the program runs. The calculation thread never runs because it has the lowest priority and thus starves for processor attention, a situation known as processor starvation.

We can alter this scenario by giving the calculation thread the same priority as the main thread. Figure 2 shows the result, beginning with time T2. (Prior to T2, Figure 2 is identical to Figure 1.)

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