Inner Classes Specification
Further Example: Multi-threaded task partitioning
Note: This Inner Classes Specification is available for download as part of the JDK1.1 End Of Life (EOL) section of the Sun website. It has been included here because most of the specification is still relevant to the current Java classfile definition. However, the information has not been transferred into the latest Java Virtual Machine Specification or made available elsewhere in Sun's online Java resources.
It is sometimes useful to parallelize a task with independent sub-tasks, by
assigning each sub-task to a thread. This can make the whole task finish
sooner, if multiple processors are involved. (This will be the case, if the sub-
tasks perform network traffic.) In interactive Java programs, multithreading is
also used to enable partial results from sub-tasks to be pushed through to the
end user, while slower sub-tasks continue to grind away.
The code below (based on an example from Doug Lea) shows a very simple
way to control the rendering of several pictures in such a way that each picture
is delivered to a displayer as soon as it's ready, but the original requester
blocks until all rendering is finished. Each sub-task is coded by an anonymous
implementation of Runnable which is at the heart of each thread.
public class GroupPictureRenderer {
private PictureRenderer renderer;
private PictureDisplayer displayer;
...
public Picture[] render(final byte[][] rawPictures)
throws InterruptedException {
Thread workers[] = new Thread[rawPictures.length];
final Picture results[] = new Picture[rawPictures.length];
// start one thread per rendering sub-task
for (int ii = 0; ii < rawPictures.length; ii++) {
final int i = ii; // capture ii for each new thread
Runnable work = new Runnable() {
public void run() {
results[i] = renderer.render(rawPictures[i]);
displayer.display(results[i]);
}
};
workers[i] = new Thread(work, "Renderer");
workers[i].start();
}
// all threads are running; now wait for them all to finish
for (int i = 0; i < workers.length; i++)
workers[i].join();
// give all the finished pictures to the caller, too:
return results;
}
}
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