概念
  HTTP短连接(非持久连接)是指,客户端和服务端进行一次HTTP请求/响应之后,关闭连接。所以,下一次的HTTP请求/响应操作需要重新建立连接。
  HTTP长连接(持久连接)是指,客户端和服务端建立一次连接之后,可以在这条连接上进行多次请求/响应操作。持久连接可以设置过期时间,也可以不设置。
  我为什么没有说HTTP/1.0 默认短连接,HTTP/1.1起,默认长连接呢?因为我第一次看这个说法的时候,以为自己懂了,其实并没有懂。长短连接操作上有什么区别,有的地方出现的持久连接又是怎么回事?
  使用设置
  这里的设置,我们都以HTTP1.1协议为例子。
  设置HTTP短连接
  在首部字段中设置Connection:close,则在一次请求/响应之后,会关闭连接。
  设置HTTP长连接,有过期时间
  在首部字段中设置Connection:keep-alive 和Keep-Alive: timeout=60,表明连接建立之后,空闲时间超过60秒之后,会失效。如果在空闲第58秒时,再次使用此连接,则连接仍然有效,使用完之后,重新计数,空闲60秒之后过期。
  设置HTTP长连接,无过期时间
  在首部字段中只设置Connection:keep-alive,表明连接有效。
  实现原理
  了解怎么设置之后,开始用起来。然而,问题来了。在请求头中设置Connection:keep-alive,为什么连接空闲一段时间之后,还是断开了呢?这是因为connection字段只有服务端设置才有效。
  HTTP操作是请求/响应成对出现的,即先有客户端发出请求,后有服务端处理请求。所以,一次HTTP操作的终点操作在服务端上,关闭也是由服务端发起的。
  接下来我们做做测试,以及show code。下面的测试都是使用Spring RestTemplate,封装apache http client进行的。为方便讲解代码,先说明长连接的情况,后再对其他形式做测试总结。
  客户端连接失效时间大于服务端失效时间
  如下,为请求日志。客户端设置Connection: Keep-Alive和Keep-Alive: timeout=60, 服务端设置Connection: Keep-Alive和Keep-Alive: timeout=5。
  ## 客户端设置有效期为60s
  [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "POST /adx-api/api/creative/upload HTTP/1.1[ ][ ]"
  [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "Accept: application/json, application/*+json, text/html, application/json, text/javascript[ ][ ]"
  [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "Content-Type: application/json;charset=UTF-8[ ][ ]"
  [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "User-Agent: Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/31.0.1650.16 Safari/537.36[ ][ ]"
  [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "Accept-Encoding: gzip,deflate[ ][ ]"
  [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "Accept-Language: zh-CN[ ][ ]"
  [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "Connection: keep-alive[ ][ ]"
  [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "Keep-Alive: timeout=60[ ][ ]"
  [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "Content-Length: 396[ ][ ]"
  [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "Host: bizdomain[ ][ ]"
  [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "[ ][ ]"
  [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 >> "request data"
  ##服务端设置有效期为5s
  [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 << "HTTP/1.1 200 OK[ ][ ]"
  [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 << "Date: Wed, 26 Apr 2017 06:07:58 GMT[ ][ ]"
  [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 << "Server: Apache-Coyote/1.1[ ][ ]"
  [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 << "Content-Type: text/html;charset=utf-8[ ][ ]"
  [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 << "Keep-Alive: timeout=5, max=100[ ][ ]"
  [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 << "Connection: Keep-Alive[ ][ ]"
  [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 << "Transfer-Encoding: chunked[ ][ ]"
  [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 << "[ ][ ]"
  [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 << "63[ ][ ]"
  [2017-04-26 14:08:00 DEBUG]  (org.apache.http.wire:?) - http-outgoing-0 << "response data"
  客户端设置的有效期大于服务端的,那么实际连接的有效期呢?三分钟之后再次请求,从连接池中lease连接的时候,提示Connection expired @ Wed Apr 26 14:08:05,即在上一次请求之后的5s失效,说明是服务端的设置生效了。
  [2017-04-26 14:11:00 DEBUG] (org.apache.http.impl.conn.PoolingHttpClientConnectionManager:?) - Connection request: [route: {}->http://bizdomain:80][total kept alive: 1; route allocated: 1 of 32; total allocated: 1 of 200]
  [2017-04-26 14:11:00 DEBUG] (org.apache.http.impl.conn.CPool:?) - Connection [id:2][route:{}->http://bizdomain:80][state:null] expired @ Wed Apr 26 14:08:05 GMT+08:00 2017
  源码分析
  通过源代码了解一下连接失效时间的设置过程。
//org.apache.http.impl.execchain.MainClientExec#execute
......
//从连接池中lease connection
final HttpClientConnectionmanagedConn = connRequest.get(timeout > 0 ? timeout : 0, TimeUnit.MILLISECONDS);
......
//将conenction封装在ConnectionHolder中
final ConnectionHolder connHolder = new ConnectionHolder(this.log, this.connManager, managedConn);
......
// The connection is in or can be brought to a re-usable state.
//如果返回值消息头中connection设置为close,则返回false
if (reuseStrategy.keepAlive(response, context)) {
// Set the idle duration of this connection
//取出response消息头中,keep-alive的timeout值
final long duration = keepAliveStrategy.getKeepAliveDuration(response, context);
if (this.log.isDebugEnabled()) {
final String s;
if (duration > 0) {
s = "for " + duration + " " + TimeUnit.MILLISECONDS;
} else {
s = "indefinitely";
}
this.log.debug("Connection can be kept alive " + s);
}
//设置失效时间
connHolder.setValidFor(duration, TimeUnit.MILLISECONDS);
connHolder.markReusable();
} else {
connHolder.markNonReusable();
}
  待读取响应之后,释放连接,即:connHolder.releaseConnection()。调用org.apache.http.impl.conn.PoolingHttpClientConnectionManager#releaseConnection方法。
@Override
public void releaseConnection(final HttpClientConnection managedConn,
final Object state,final long keepalive, final TimeUnit tunit) {
Args.notNull(managedConn, "Managed connection");
synchronized (managedConn) {
final CPoolEntry entry = CPoolProxy.detach(managedConn);
if (entry == null) {
return;
}
final ManagedHttpClientConnection conn = entry.getConnection();
try {
if (conn.isOpen()) {
final TimeUnit effectiveUnit = tunit != null ? tunit : TimeUnit.MILLISECONDS;
entry.setState(state);
//设置失效时间
entry.updateExpiry(keepalive, effectiveUnit);
}
} finally {
。。。。。。
}
}
}
}
  然后再下一次HTTP操作,从连接池中获取连接时
//org.apache.http.impl.conn.PoolingHttpClientConnectionManager#requestConnection调用org.apache.http.pool.AbstractConnPool#lease,
//调用getPoolEntryBlocking,调用org.apache.http.impl.conn.CPoolEntry#isExpired
@Override
public boolean isExpired(final long now) {
final boolean expired = super.isExpired(now);
if (expired && this.log.isDebugEnabled()) {
//日志中看到的内容
this.log.debug("Connection " + this + " expired @ " + new Date(getExpiry()));
}
return expired;
}
  综上,连接的实际有效时间,是根据response的设置来决定的。