httpclient4.4 出现NoHttpResponseException的异常解决

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);

使用链接池初始化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);

但在测试过程当中发现，有时候出现以下的异常，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.

根据描述主要就是由于服务端断开链接，但并无通知客户端，致使下次请求该服务时httpclient继续使用该链接致使报错。

查看了tomcat7的文档，发如今Connector中keepAliveTimeout能够配置链接空闲多久就关闭该链接，它的值默认是和connectionTimeout同样，而在server.xml中connectionTimeout值默认为20s。

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

经过查看debug日志，以上代码生成的connection默认是keep alive的，经过以下代码能够将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);

主要是增长keepalive的策略，但这又带来一个问题，全部的链接只有20秒，没法使用长链接的性能优点，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，后续研究可否使用该API解决短连接的问题。