同步同容器类和工具类2——阻塞队列和生产者-消费者模式

阻塞队列和生产者-消费者模式

    阻塞队列提供可阻塞的put和take方法，以及支持定时的offer和poll方法，如果队列已经满了，那么put方法将阻塞直到有可用空间。

如果队列为空，那么take方法将会阻塞直到有元素可用。队列可以是有界的也可以是无界的。

   BlockingQueue简化了生产者-消费者设计的实现过程。它支持任意数量的生产者和消费者。一种常见的shenchanzhe-消费者模式就是线程池与工作队列的组合。在Executor任务执行框架中就体现了这种模式。

   在类库中包含了Blockingqueue的多种实现。其中linkedBlockingQueue和ArrayBlockingQueue是FIFO队列。二者分别与linkedList和ArrayList类似。PriorityBlockingQueue是一种按优先级排序的队列，当你希望按某种顺序而不是FIFO处理元素时，这个队列非常有用。

   最后一个Blockingqueue实现是SynchronousQueue，它没有存储功能，因此take和put会一直阻塞，直到另外一个线程已经准备好参与交付过程中。

public class ProducerConsumer {

    static class FileCrawler implements Runnable {

        private final BlockingQueue<File> fileQueue;

        private final FileFilter fileFilter;

        private final File root;

        public FileCrawler(BlockingQueue<File> fileQueue,

                           final FileFilter fileFilter,

                           File root) {

            this.fileQueue = fileQueue;

            this.root = root;

            this.fileFilter = new FileFilter() {

                public boolean accept(File f) {

                    return f.isDirectory() || fileFilter.accept(f);

                }

            };

        }

        private boolean alreadyIndexed(File f) {

            return false;

        }

        public void run() {

            try {

                crawl(root);

            } catch (InterruptedException e) {

                Thread.currentThread().interrupt();

            }

        }

        private void crawl(File root) throws InterruptedException {

            File[] entries = root.listFiles(fileFilter);

            if (entries != null) {

                for (File entry : entries)

                    if (entry.isDirectory())

                        crawl(entry);

                    else if (!alreadyIndexed(entry))

                        fileQueue.put(entry);

            }

        }

    }

    static class Indexer implements Runnable {

        private final BlockingQueue<File> queue;

        public Indexer(BlockingQueue<File> queue) {

            this.queue = queue;

        }

        public void run() {

            try {

                while (true)

                    indexFile(queue.take());

            } catch (InterruptedException e) {

                Thread.currentThread().interrupt();

            }

        }

        public void indexFile(File file) {

            // Index the file...

        };

    }

    private static final int BOUND = 10;

    private static final int N_CONSUMERS = Runtime.getRuntime().availableProcessors();

    public static void startIndexing(File[] roots) {

        BlockingQueue<File> queue = new LinkedBlockingQueue<File>(BOUND);

        FileFilter filter = new FileFilter() {

            public boolean accept(File file) {

                return true;

            }

        };

        for (File root : roots)

            new Thread(new FileCrawler(queue, filter, root)).start();

        for (int i = 0; i < N_CONSUMERS; i++)

            new Thread(new Indexer(queue)).start();

    }

}