CountDownLatch的介绍和使用

1、类介绍

java.util.concurrent
类 CountDownLatch
java.lang.Object
java.util.concurrent.CountDownLatch

2、CountDownLatch作用

一个同步辅助类，在完成一组正在其他线程中执行的操作之前，它允许一个或多个线程一直等待。意思就是说，有一个工厂老板，可以安排

CountDownLatch去监视所有工人工作，等到所有工人都做完了，工厂老板才会可以离开

2、CountDownLatch使用代码

 /**
     * Announce all seeds from configured source to SeedListeners 
     * (including nonseed lines mixed in). 
     * @see org.archive.modules.seeds.SeedModule#announceSeeds()
     */
    public void announceSeeds() {
        if(getBlockAwaitingSeedLines()>-1) { //如果不是需要全部导入种子
            final CountDownLatch latch = new CountDownLatch(getBlockAwaitingSeedLines());
            new Thread(){
                @Override
                public void run() {
                    announceSeeds(latch); 
                    while(latch.getCount()>0) {
                        latch.countDown();
                    }
                }
            }.start();
            try {
                latch.await();
            } catch (InterruptedException e) {
                // do nothing
            } 
        } else {
            announceSeeds(null); 
        }
    }

上面代码的意思是说：

 /**
     * Announce all seeds (and nonseed possible-directive lines) from
     * the given Reader
     * @param reader source of seed/directive lines
     * @param latchOrNull if non-null, sent countDown after each line, allowing 
     * another thread to proceed after a configurable number of lines processed
     */
    protected void announceSeedsFromReader(BufferedReader reader, CountDownLatch latchOrNull) {
        String s;
        /**
         * 获取迭代器，什么的迭代器呢？文本文件一行一行读取的迭代器，这个迭代器考虑了注释以#号开始
         * 还考虑了空白行，使用正则表达式去处理的,^\\s*[\\S]+\\s*(#.*)?$
         * 注释行：\\s*(#.*)?
         * */
        Iterator<String> iter = 
            new RegexLineIterator(
                    new LineReadingIterator(reader),
                    RegexLineIterator.COMMENT_LINE,
                    RegexLineIterator.NONWHITESPACE_ENTRY_TRAILING_COMMENT,
                    RegexLineIterator.ENTRY);

        int count = 0; 
        while (iter.hasNext()) {
            s = (String) iter.next();
            if(Character.isLetterOrDigit(s.charAt(0))) {
                // consider a likely URI
                seedLine(s);
                count++;
                if(count%20000==0) {
                    System.runFinalization();
                }
            } else {
                // report just in case it's a useful directive
                nonseedLine(s);
            }
            if(latchOrNull!=null) {
                latchOrNull.countDown(); 
            }
        }
        publishConcludedSeedBatch(); 
    }