Java countDownLatch如何实现多线程任务阻塞等待

Java countDownLatch如何实现多线程任务阻塞等待

我这里需要通过多线程去处理数据，然后在所有数据都处理完成后再往下执行。这里就用到了CountDownLatch。把countdownlatch作为参数传入到每个线程类里，在线程中处理完数据后执行countdown方法。在所有countdownlatch归零后，其await方法结束阻塞状态而往下执行。

具体代码如下：

将多线程任务提交线程池

@Bean(name = "ggnews_executor")

public Executor postExecutor() {

ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();

executor.setCorePoolSize(1);

executor.setMaxPoolSize(1);

executor.setQueueCapacity(1);

executor.setKeepAliveSeconds(120);

executor.setThreadNamePrefix("executor-");

executor.setRejectedExecutionHandler(new ThreadPoolExecutor.DiscardPolicy());

return executor;

}

//通过定时任务调用的fetch方法，为了避免定时任务在多次执行中失效，通异步指定线程池的方式进行调用

@Async("ggnews_executor")

public void fetch() {

if(fetchFlag.getAndSet(false)) {

List tags = fetchTagService.selectFetchTagList(fetchTag);

CountDownLatch downLatch = new CountDownLatch(tags.size());

for (FetchTag tag : tags) {

FetchTag tagNew;

try {

tagNew =(FetchTag) tag.clone();

} catch (Throwable e) {

log.error("",e);

continue;

}

//作为参数将CountDownLatch传入

InnerRunner innerRunner = new InnerRunner(downLatch, tagNew);

executor.execute(innerRunner);

}

try {

//等待线程执行完毕,如果十分钟后还没结束也会停止阻塞状态

downLatch.await(10,TimeUnit.MINUTES);

fetchFlag.getAndSet(true);

} catch (Throwable e) {

log.error("fetch()方法发生错误:{}", e);

fetchFlag.getAndSet(true);

//e.printStackTrace();

} finally {

fetchFlag.getAndSet(true);

}

} else {

log.info("=======上次抓取尚未结束=========");

}

}

InnerRunner为要执行具体任务的线程类

private class InnerRunner implements Runnable {

private CountDownLatch downLatch;

private FetchTag tag;

private InnerRunner(CountDownLatch downLatch, FetchTag tag) {

this.downLatch = downLatch;

this.tag = tag;

}

@Override

public void run() {

//将countDown方法移入到具体方法中的finally块中，以保证即使在抛出异常的情况下也算执行了此次任务，countdown会被执行

fetchGG(tag.getTag(), downLatch);

//downLatch.countDown();

this.tag = null;

}

}

private static final String GOOGLE_URL_IN = "https://news.google.com/rss/search?hl=hi&gl=IN&ceid=IN:hi&q=";

public void fetchGG(String tag, CountDownLatch downLatch) {

try {

Document document = jsoup.parse(new URL(GOOhttp://GLE_URL_IN + URLEncoder.encode("\"" + tag + "http://\"", "utf-8")), 30000);

Elements elements = document.getElementsByTag("item");

int rank = 1;

for (Element element : elements) {

String sourceTitle = element.getElementsByTag("tihttp://tle").get(0).text();

log.info("source title:" + sourceTitle);

}

} catch (Throwable e) {

log.info("fetch google url error", e);

} finally {

//肯定会被执行

dowhttp://nLatch.countDown();

}

}

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