httpclient長鏈接與短鏈接

http keep-alive與tcp keep-alive

http keep-alive與tcp keep-alive，不是同一回事，意圖不一樣。http keep-alive是爲了讓tcp活得更久一點，以便在同一個連接上傳送多個http，提高socket的效率。而tcp keep-alive是TCP的一種檢測TCP連接狀況的保鮮機制。tcp keep-alive保鮮定時器，支持三個系統內核配置參數：

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

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

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

keepalive是TCP保鮮定時器，當網絡兩端建立了TCP連接之後，閒置idle（雙方沒有任何數據流發送往來）了tcp_keepalive_time後，服務器內核就會嘗試向客戶端發送偵測包，來判斷TCP連接狀況(有可能客戶端崩潰、強制關閉了應用、主機不可達等等)。如果沒有收到對方的回答(ack包)，則會在 tcp_keepalive_intvl後再次嘗試發送偵測包，直到收到對對方的ack,如果一直沒有收到對方的ack,一共會嘗試 tcp_keepalive_probes次，每次的間隔時間在這裏分別是15s, 30s, 45s, 60s, 75s。如果嘗試tcp_keepalive_probes,依然沒有收到對方的ack包，則會丟棄該TCP連接。TCP連接默認閒置時間是2小時，一般設置爲30分鐘足夠了。

在httpclient中，創建連接並獲得請求結果後，會對改連接設置長連接策略，如在MinimalClientExec.java類中，

            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();
            }

在看看默認的長連接設置策略

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;
    }

}

當然也可以自己來時間長連接策略，比如下面的這個

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();