1、每個連接任何時刻只能由一個線程在進行操作
2、一般使用PoolingHttpClientConnectionManager 來管理連接池，默認每個route只創建兩個connection，而且最大是20個連接，但是實際情況並不是。
連接池管理器為我們管理了connection的創建和回收，生命周期的管理，connection的分配等。
3、HttpClient是線程安全的
4、HttpClient的定制化程度很高，可以寫各種本地化代碼。
5、connection什麼時候無效？keep-alive應該保持多長時間？
5、無用連接的清除（服務端掛掉之後，客戶端可能無法感知）
此時最好的辦法就是有一個監控線程不斷的去清除連接池裡面的無效連接
If the connection gets closed on the server side, the client side connection is unable to detect the change in the connection state (and react appropriately by closing the socket on its end).
The only feasible solution that does not involve a one thread per socket model for idle connections is a dedicated monitor thread used to evict connections that are considered expired due to a long period of inactivity. The monitor thread can periodically call ClientConnectionManager#closeExpiredConnections() method to close all expired connections and evict closed connections from the pool. It can also optionally call ClientConnectionManager#closeIdleConnections() method to close all connections that have been idle over a given period of time.
public static class IdleConnectionMonitorThread extends Thread {
private final HttpClientConnectionManager connMgr;
private volatile boolean shutdown;
public IdleConnectionMonitorThread(HttpClientConnectionManager connMgr) {
super();
this.connMgr = connMgr;
}
@Override
public void run() {
try {
while (!shutdown) {
synchronized (this) {
wait(5000);
// Close expired connections
connMgr.closeExpiredConnections();
// Optionally, close connections
// that have been idle longer than 30 sec
connMgr.closeIdleConnections(30, TimeUnit.SECONDS);
}
}
} catch (InterruptedException ex) {
// terminate
}
}
public void shutdown() {
shutdown = true;
synchronized (this) {
notifyAll();
}
}
}
6、keep-alive的時間
如果服務端的響應頭有keep-alive頭時，客戶端將響應頭的keep-alive設置為保持連接的時間。如果響應沒有該字段，則默認keep-alive是無限的。
The HTTP specification does not specify how long a persistent connection may be and should be kept alive. Some HTTP servers use a non-standard Keep-Alive header to communicate to the client the period of time in seconds they intend to keep the connection alive on the server side. HttpClient makes use of this information if available. If the Keep-Alive header is not present in the response, HttpClient assumes the connection can be kept alive indefinitely. However, many HTTP servers in general use are configured to drop persistent connections after a certain period of inactivity in order to conserve system resources, quite often without informing the client. In case the default strategy turns out to be too optimistic, one may want to provide a custom keep-alive strategy.
keep-alive時間是可以設置的：
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.naughty-server.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();
7、思考這個問題：connection的keep-alive應該設置多長？怎麼能很好的清除無效的連接？
