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Java countDownLatch如何實(shí)現(xiàn)多線程任務(wù)阻塞等待

更新時(shí)間：2020年10月15日 08:34:57 投稿：yaominghui

這篇文章主要介紹了Java countDownLatch如何實(shí)現(xiàn)多線程任務(wù)阻塞等待,文中通過(guò)示例代碼介紹的非常詳細(xì)，對(duì)大家的學(xué)習(xí)或者工作具有一定的參考學(xué)習(xí)價(jià)值,需要的朋友可以參考下

我這里需要通過(guò)多線程去處理數(shù)據(jù)，然后在所有數(shù)據(jù)都處理完成后再往下執(zhí)行。這里就用到了CountDownLatch。把countdownlatch作為參數(shù)傳入到每個(gè)線程類里，在線程中處理完數(shù)據(jù)后執(zhí)行countdown方法。在所有countdownlatch歸零后，其await方法結(jié)束阻塞狀態(tài)而往下執(zhí)行。

具體代碼如下：

將多線程任務(wù)提交線程池

   @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;
	}
	//通過(guò)定時(shí)任務(wù)調(diào)用的fetch方法，為了避免定時(shí)任務(wù)在多次執(zhí)行中失效，通異步指定線程池的方式進(jìn)行調(diào)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;
				}
        //作為參數(shù)將CountDownLatch傳入
				InnerRunner innerRunner = new InnerRunner(downLatch, tagNew);
				executor.execute(innerRunner);
			}
			try {
        //等待線程執(zhí)行完畢,如果十分鐘后還沒(méi)結(jié)束也會(huì)停止阻塞狀態(tài)
				downLatch.await(10,TimeUnit.MINUTES);
				fetchFlag.getAndSet(true);
			} catch (Throwable e) {
				log.error("fetch()方法發(fā)生錯(cuò)誤:{}", e);
				fetchFlag.getAndSet(true);
				//e.printStackTrace();
			} finally {
				fetchFlag.getAndSet(true);
			}
		} else {
			log.info("=======上次抓取尚未結(jié)束=========");
		}
	}

InnerRunner為要執(zhí)行具體任務(wù)的線程類

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() {
      //將countDown方法移入到具體方法中的finally塊中，以保證即使在拋出異常的情況下也算執(zhí)行了此次任務(wù)，countdown會(huì)被執(zhí)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 {
      //肯定會(huì)被執(zhí)行
			downLatch.countDown();
		}
	}

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