Java countDownLatch Java countDownLatch怎样实现多线程任务阻塞等待

时间:2021-03-15 人气:0

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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<FetchTag> 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(GOOGLE_URL_IN + URLEncoder.encode("\"" + tag + "\"", "utf-8")), 30000);
			Elements elements = document.getElementsByTag("item");
			int rank = 1;
			for (Element element : elements) {
				
				String sourceTitle = element.getElementsByTag("title").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
			downLatch.countDown();
		}
	}

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