httpclient4.4简单初始化httpclient的方式：

HttpClient httpClient = HttpClientBuilder.create().build();        ClientHttpRequestFactory requestFactory = new HttpComponentsClientHttpRequestFactory(httpClient);        restTemplate = new RestTemplate(requestFactory);

使用连接池初始化httpclient的方式：

PoolingHttpClientConnectionManager connManager = new PoolingHttpClientConnectionManager();        connManager.setMaxTotal(100);        connManager.setDefaultMaxPerRoute(10);        HttpClientBuilder httpClientBuilder = HttpClientBuilder.create();        httpClientBuilder.setConnectionManager(connManager);        httpClientBuilder.setRetryHandler(new DefaultHttpRequestRetryHandler());        HttpClient httpClient = HttpClientBuilder.create().build();        ClientHttpRequestFactory requestFactory = new HttpComponentsClientHttpRequestFactory(httpClient);        restTemplate = new RestTemplate(requestFactory);

但在测试过程中发现，有时候出现如下的异常，但重试一次就没有问题：

Caused by: org.apache.http.NoHttpResponseException: 10.0.0.25:8080 failed to respond    at org.apache.http.impl.conn.DefaultHttpResponseParser.parseHead(DefaultHttpResponseParser.java:143)    at org.apache.http.impl.conn.DefaultHttpResponseParser.parseHead(DefaultHttpResponseParser.java:57)

网上查了相关的报错信息，找到官网上的描述

org.apache.commons.httpclient.NoHttpResponseException java.io.IOException +- org.apache.commons.httpclient.NoHttpResponseExceptionIn some circumstances, usually when under heavy load, the web server may be able to receive requests but unable to process them. A lack of sufficient resources like worker threads is a good example. This may cause the server to drop the connection to the client without giving any response. HttpClient throws NoHttpResponseException when it encounters such a condition. In most cases it is safe to retry a method that failed with NoHttpResponseException.

根据描述主要就是因为服务端断开连接，但并没有通知客户端，导致下次请求该服务时httpclient继续使用该连接导致报错。

查看了tomcat7的文档，发现在Connector中keepAliveTimeout可以配置连接空闲多久就关闭该连接，它的值默认是和connectionTimeout一样，而在server.xml中connectionTimeout值默认为20s。

通过验证，确实是在第一次请求20秒后，再请求就会报出NoHttpResponseException的异常。

通过查看debug日志，以上代码生成的connection默认是keep alive的，通过如下代码可以将httpclient生成的connection的alive时间可配置。

ConnectionKeepAliveStrategy connectionKeepAliveStrategy = new ConnectionKeepAliveStrategy() {            @Override            public long getKeepAliveDuration(HttpResponse httpResponse, HttpContext httpContext) {                return 20 * 1000; // tomcat默认keepAliveTimeout为20s            }        };        PoolingHttpClientConnectionManager connManager = new PoolingHttpClientConnectionManager(20, TimeUnit.SECONDS);        connManager.setMaxTotal(200);        connManager.setDefaultMaxPerRoute(200);        RequestConfig requestConfig = RequestConfig.custom()                .setConnectTimeout(10 * 1000)                .setSocketTimeout(10 * 1000)                .setConnectionRequestTimeout(10 * 1000)                .build();        HttpClientBuilder httpClientBuilder = HttpClientBuilder.create();        httpClientBuilder.setConnectionManager(connManager);        httpClientBuilder.setDefaultRequestConfig(requestConfig);        httpClientBuilder.setRetryHandler(new DefaultHttpRequestRetryHandler());        httpClientBuilder.setKeepAliveStrategy(connectionKeepAliveStrategy);        HttpClient httpClient = httpClientBuilder.build();        ClientHttpRequestFactory requestFactory = new HttpComponentsClientHttpRequestFactory(httpClient);        restTemplate = new RestTemplate(requestFactory);

主要是增加keepalive的策略，但这又带来一个问题，所有的连接只有20秒，无法使用长连接的性能优势，在查看API时发现PoolingHttpClientConnectionManager提供有

@Override    public void closeIdleConnections(final long idleTimeout, final TimeUnit tunit) {        if (this.log.isDebugEnabled()) {            this.log.debug("Closing connections idle longer than " + idleTimeout + " " + tunit);        }        this.pool.closeIdle(idleTimeout, tunit);    }

的方法，后续研究能否使用该API解决短链接的问题。