CountDownLatch使用解说

一、CountDownLatch是什么

二、CountDownLatch如何工作

//Constructs a CountDownLatch initialized with the given count.
public void CountDownLatch(int count) {...}

三、CountDownLatch使用解说

CountDownLatch是java5中新增的一个并发工具类，其使用非常简单，下面通过伪代码简单看一下使用方式： 这是一个使用CountDownLatch非常简单的例子，创建的时候，需要指定一个初始状态值，本例为2，主线程调用 latch.await时，除非latch状态值为0，否则会一直阻塞休眠。当所有任务执行完后，主线程唤醒，最终执行打印动作。 以上只是一个最简单的例子，接着咱们再来看一个，这回，咱们想要在任务执行完后做更多的事情，如下图所示： 这一次，在线程3和线程4中，分别调用了latch.await()，当latch状态值为0时，这两个线程将会继续执行任务，但是顺序性是无法保证的。 CountDownLatch的方便之处在于，你可以在一个线程中使用，也可以在多个线程上使用，一切只依据状态值，这样便不会受限于任何的场景。

作者：AskHarries 链接：https://juejin.im/post/5aebd052518825673954971f 来源：掘金 著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。

四、CountDownLatch使用例子

package org.java.base.pool; import java.io.IOException; import java.io.InterruptedIOException; import java.net.UnknownHostException; import java.util.concurrent.CountDownLatch; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import javax.net.ssl.SSLException; import javax.net.ssl.SSLHandshakeException; import org.apache.http.HttpEntity; import org.apache.http.HttpEntityEnclosingRequest; import org.apache.http.HttpHost; import org.apache.http.HttpRequest; import org.apache.http.NoHttpResponseException; import org.apache.http.client.HttpRequestRetryHandler; import org.apache.http.client.config.RequestConfig; import org.apache.http.client.methods.CloseableHttpResponse; import org.apache.http.client.methods.HttpGet; import org.apache.http.client.methods.HttpRequestBase; import org.apache.http.client.protocol.HttpClientContext; import org.apache.http.config.Registry; import org.apache.http.config.RegistryBuilder; import org.apache.http.conn.ConnectTimeoutException; import org.apache.http.conn.routing.HttpRoute; import org.apache.http.conn.socket.ConnectionSocketFactory; import org.apache.http.conn.socket.LayeredConnectionSocketFactory; import org.apache.http.conn.socket.PlainConnectionSocketFactory; import org.apache.http.conn.ssl.SSLConnectionSocketFactory; import org.apache.http.impl.client.CloseableHttpClient; import org.apache.http.impl.client.HttpClients; import org.apache.http.impl.conn.PoolingHttpClientConnectionManager; import org.apache.http.protocol.HttpContext; import org.apache.http.util.EntityUtils; public class PoolTest { private static void config(HttpRequestBase httpRequestBase) { httpRequestBase.setHeader("User-Agent", "Mozilla/5.0"); httpRequestBase.setHeader("Accept","text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8"); httpRequestBase.setHeader("Accept-Language", "zh-CN,zh;q=0.8,en-US;q=0.5,en;q=0.3");//"en-US,en;q=0.5"); httpRequestBase.setHeader("Accept-Charset", "ISO-8859-1,utf-8,gbk,gb2312;q=0.7,*;q=0.7"); // 配置请求的超时设置 RequestConfig requestConfig = RequestConfig.custom() .setConnectionRequestTimeout(3000) .setConnectTimeout(3000) .setSocketTimeout(3000) .build(); httpRequestBase.setConfig(requestConfig); } public static void main(String[] args) { ConnectionSocketFactory plainsf = PlainConnectionSocketFactory.getSocketFactory(); LayeredConnectionSocketFactory sslsf = SSLConnectionSocketFactory.getSocketFactory(); Registry<ConnectionSocketFactory> registry = RegistryBuilder.<ConnectionSocketFactory>create() .register("http", plainsf) .register("https", sslsf) .build(); PoolingHttpClientConnectionManager cm = new PoolingHttpClientConnectionManager(registry); // 将最大连接数增加到200 cm.setMaxTotal(200); // 将每个路由基础的连接增加到20 cm.setDefaultMaxPerRoute(20); // 将目标主机的最大连接数增加到50 HttpHost localhost = new HttpHost("http://blog.csdn.net/gaolu",80); cm.setMaxPerRoute(new HttpRoute(localhost), 50); //请求重试处理 HttpRequestRetryHandler httpRequestRetryHandler = new HttpRequestRetryHandler() { public boolean retryRequest(IOException exception,int executionCount, HttpContext context) { if (executionCount >= 5) {// 如果已经重试了5次，就放弃 return false; } if (exception instanceof NoHttpResponseException) {// 如果服务器丢掉了连接，那么就重试 return true; } if (exception instanceof SSLHandshakeException) {// 不要重试SSL握手异常 return false; } if (exception instanceof InterruptedIOException) {// 超时 return false; } if (exception instanceof UnknownHostException) {// 目标服务器不可达 return false; } if (exception instanceof ConnectTimeoutException) {// 连接被拒绝 return false; } if (exception instanceof SSLException) {// ssl握手异常 return false; } HttpClientContext clientContext = HttpClientContext.adapt(context); HttpRequest request = clientContext.getRequest(); // 如果请求是幂等的，就再次尝试 if (!(request instanceof HttpEntityEnclosingRequest)) { return true; } return false; } }; //连接客户端 CloseableHttpClient httpClient = HttpClients.custom() .setConnectionManager(cm) .setRetryHandler(httpRequestRetryHandler) .build(); // URL列表数组 String[] urisToGet = { "http://blog.csdn.net/gaolu/article/details/48466059", "http://blog.csdn.net/gaolu/article/details/48243103", "http://blog.csdn.net/gaolu/article/details/47656987", "http://blog.csdn.net/gaolu/article/details/47055029" }; long start = System.currentTimeMillis(); try { int pagecount = urisToGet.length; ExecutorService executors = Executors.newFixedThreadPool(pagecount); CountDownLatch countDownLatch = new CountDownLatch(pagecount); for(int i = 0; i< pagecount;i++){ HttpGet httpget = new HttpGet(urisToGet[i]); config(httpget); //启动线程抓取 executors.execute(new GetRunnable(httpClient,httpget,countDownLatch)); } countDownLatch.await(); //等到计数器完成 executors.shutdown(); } catch (InterruptedException e) { e.printStackTrace(); } finally { System.out.println("线程" + Thread.currentThread().getName() + "," + System.currentTimeMillis() + ", 所有线程已完成，开始进入下一步！"); } long end = System.currentTimeMillis(); System.out.println("consume -> " + (end - start)); } /** * 内部類 * @author Liuhaihua * */ static class GetRunnable implements Runnable { private CountDownLatch countDownLatch; private final CloseableHttpClient httpClient; private final HttpGet httpget; public GetRunnable(CloseableHttpClient httpClient, HttpGet httpget, CountDownLatch countDownLatch){ this.httpClient = httpClient; this.httpget = httpget; this.countDownLatch = countDownLatch; } @Override public void run() { CloseableHttpResponse response = null; try { response = httpClient.execute(httpget,HttpClientContext.create()); HttpEntity entity = response.getEntity(); //System.out.println(EntityUtils.toString(entity, "utf-8")) ; EntityUtils.consume(entity); } catch (IOException e) { e.printStackTrace(); } finally { countDownLatch.countDown(); try { if(response != null) response.close(); } catch (IOException e) { e.printStackTrace(); } } } } }