httpclient长链接与短链接

http keep-alive与tcp keep-alive

http keep-alive与tcp keep-alive，不是同一回事，意图不一样。http keep-alive是为了让tcp活得更久一点，以便在同一个连接上传送多个http，提高socket的效率。而tcp keep-alive是TCP的一种检测TCP连接状况的保鲜机制。tcp keep-alive保鲜定时器，支持三个系统内核配置参数：

1echo 1800 > /proc/sys/net/ipv4/tcp_keepalive_time

2echo 15 > /proc/sys/net/ipv4/tcp_keepalive_intvl

3echo 5 > /proc/sys/net/ipv4/tcp_keepalive_probes

keepalive是TCP保鲜定时器，当网络两端建立了TCP连接之后，闲置idle（双方没有任何数据流发送往来）了tcp_keepalive_time后，服务器内核就会尝试向客户端发送侦测包，来判断TCP连接状况(有可能客户端崩溃、强制关闭了应用、主机不可达等等)。如果没有收到对方的回答(ack包)，则会在 tcp_keepalive_intvl后再次尝试发送侦测包，直到收到对对方的ack,如果一直没有收到对方的ack,一共会尝试 tcp_keepalive_probes次，每次的间隔时间在这里分别是15s, 30s, 45s, 60s, 75s。如果尝试tcp_keepalive_probes,依然没有收到对方的ack包，则会丢弃该TCP连接。TCP连接默认闲置时间是2小时，一般设置为30分钟足够了。

在httpclient中，创建连接并获得请求结果后，会对改连接设置长连接策略，如在MinimalClientExec.java类中，

            httpProcessor.process(request, context);            final HttpResponse response = requestExecutor.execute(request, managedConn, context);            httpProcessor.process(response, context);            // The connection is in or can be brought to a re-usable state.            if (reuseStrategy.keepAlive(response, context)) {                // Set the idle duration of this connection                final long duration = keepAliveStrategy.getKeepAliveDuration(response, context);                releaseTrigger.setValidFor(duration, TimeUnit.MILLISECONDS);                releaseTrigger.markReusable();            } else {                releaseTrigger.markNonReusable();            }

在看看默认的长连接设置策略

public class DefaultConnectionKeepAliveStrategy implements ConnectionKeepAliveStrategy {    public static final DefaultConnectionKeepAliveStrategy INSTANCE = new DefaultConnectionKeepAliveStrategy();    public long getKeepAliveDuration(final HttpResponse response, final HttpContext context) {        Args.notNull(response, "HTTP response");        final HeaderElementIterator it = new BasicHeaderElementIterator(                response.headerIterator(HTTP.CONN_KEEP_ALIVE));        while (it.hasNext()) {            final HeaderElement he = it.nextElement();            final String param = he.getName();            final String value = he.getValue();            if (value != null && param.equalsIgnoreCase("timeout")) {                try {                    return Long.parseLong(value) * 1000;                } catch(final NumberFormatException ignore) {                }            }        }        return -1;    }}

当然也可以自己来时间长连接策略，比如下面的这个

ConnectionKeepAliveStrategy myStrategy = new ConnectionKeepAliveStrategy() {    public long getKeepAliveDuration(HttpResponse response, HttpContext context) {        // Honor 'keep-alive' header        HeaderElementIterator it = new BasicHeaderElementIterator(                response.headerIterator(HTTP.CONN_KEEP_ALIVE));        while (it.hasNext()) {            HeaderElement he = it.nextElement();            String param = he.getName();            String value = he.getValue();            if (value != null && param.equalsIgnoreCase("timeout")) {                try {                    return Long.parseLong(value) * 1000;                } catch(NumberFormatException ignore) {                }            }        }        HttpHost target = (HttpHost) context.getAttribute(                HttpClientContext.HTTP_TARGET_HOST);        if ("www.baidu.com".equalsIgnoreCase(target.getHostName())) {            // Keep alive for 5 seconds only            return 5 * 1000;        } else {            // otherwise keep alive for 30 seconds            return 30 * 1000;        }    }};CloseableHttpClient client = HttpClients.custom()        .setKeepAliveStrategy(myStrategy)        .build();