HTTP长链接、短链接使用及测试
概念
HTTP短链接（非持久链接）是指，客户端和服务端进行一次HTTP请求/响应以后，就关闭链接。因此，下一次的HTTP请求/响应操做就须要从新创建链接。
HTTP长链接（持久链接）是指，客户端和服务端创建一次链接以后，能够在这条链接上进行屡次请求/响应操做。持久链接能够设置过时时间，也能够不设置。javascript
我为何没有说HTTP/1.0 默认短链接，HTTP/1.1起，默认长链接呢？由于我第一次看这个说法的时候，觉得本身懂了，其实并无懂。长短链接操做上有什么区别，有的地方出现的持久链接又是怎么回事？html
使用设置
这里的设置，咱们都以HTTP1.1协议为例子。java
设置HTTP短链接
在首部字段中设置Connection:close，则在一次请求/响应以后，就会关闭链接。apache
设置HTTP长链接，有过时时间
在首部字段中设置Connection:keep-alive 和Keep-Alive: timeout=60，代表链接创建以后，空闲时间超过60秒以后，就会失效。若是在空闲第58秒时，再次使用此链接，则链接仍然有效，使用完以后，从新计数，空闲60秒以后过时。json
设置HTTP长链接，无过时时间
在首部字段中只设置Connection:keep-alive，代表链接永久有效。c#
实现原理
了解怎么设置以后，就开始用起来。然而，问题来了。在请求头中设置Connection:keep-alive，为何链接空闲一段时间以后，仍是断开了呢？这是由于connection字段只有服务端设置才有效。api
HTTP操做是请求/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。因此，一次HTTP操做的终点操做在服务端上，关闭也是由服务端发起的。
接下来咱们作作测试，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，先说明长链接的状况，最后再对其余形式作测试总结。浏览器
客户端链接失效时间大于服务端失效时间
以下，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。服务器
## 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[\r][\n]" [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[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "Content-Type: application/json;charset=UTF-8[\r][\n]" [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[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "Accept-Encoding: gzip,deflate[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "Accept-Language: zh-CN[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "Connection: keep-alive[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "Keep-Alive: timeout=60[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "Content-Length: 396[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "Host: bizdomain[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "[\r][\n]" [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[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Date: Wed, 26 Apr 2017 06:07:58 GMT[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Server: Apache-Coyote/1.1[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Content-Type: text/html;charset=utf-8[\r][\n]"[2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Keep-Alive: timeout=5, max=100[\r][\n]"[2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Connection: Keep-Alive[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Transfer-Encoding: chunked[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "[\r][\n]" [2017-04-26 14:08:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "63[\r][\n]" [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，那么实际链接的有效期呢？三分钟以后再次请求，从链接池中lease链接的时候，提示Connection expired @ Wed Apr 26 14:08:05，即在上一次请求以后的5s失效，说明是服务端的设置生效了。网络
[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
经过源代码了解一下链接失效时间的设置过程。
//org.apache.http.impl.execchain.MainClientExec#execute......//从链接池中lease connectionfinal 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.//若是返回值消息头中connection设置为close，则返回falseif (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();
}
待读取响应以后，释放链接，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 {
。。。。。。
}
}
}
}
而后再下一次HTTP操做，从链接池中获取链接时
//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@Overridepublic 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，链接的实际有效时间，是根据response的设置来决定的。
其余状况测试
客户端设置
Connection: Close
##connection:close请求，kept alive的链接为0[2017-04-26 13:57:00 DEBUG] (org.apache.http.impl.conn.PoolingHttpClientConnectionManager:?) - Connection request: [route: {}->http://bizdomain:80][total kept alive: 0; route allocated: 0 of 32; total allocated: 0 of 200]
[2017-04-26 13:57:00 DEBUG] (org.apache.http.impl.conn.PoolingHttpClientConnectionManager:?) - Connection leased: [id: 0][route: {}->http://bizdomain:80][total kept alive: 0; route allocated: 1 of 32; total allocated: 1 of 200]
[2017-04-26 13:57:00 DEBUG] (org.apache.http.impl.execchain.MainClientExec:?) - Opening connection {}->http://bizdomain:80
[2017-04-26 13:57:00 DEBUG] (org.apache.http.impl.conn.DefaultHttpClientConnectionOperator:?) - Connecting to bizdomain/127.0.0.195:80## 创建新链接[2017-04-26 13:57:00 DEBUG] (org.apache.http.impl.conn.DefaultHttpClientConnectionOperator:?) - Connection established 127.0.0.191:49239<->127.0.0.195:80## 客户端设置短链接[2017-04-26 13:57:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "Connection: Close[\r][\n]"## 服务端返回的也是短链接[2017-04-26 13:57:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Connection: close[\r][\n]"
##请求完以后，关闭链接
[2017-04-26 13:57:00 DEBUG] (org.apache.http.impl.conn.DefaultManagedHttpClientConnection:?) - http-outgoing-0: Close connection[2017-04-26 13:57:00 DEBUG] (org.apache.http.impl.execchain.MainClientExec:?) - Connection discarded[2017-04-26 13:57:00 DEBUG] (org.apache.http.impl.conn.PoolingHttpClientConnectionManager:?) - Connection released: [id: 0][route: {}->http://bizdomain:80][total kept alive: 0; route allocated: 0 of 32; total allocated: 0 of 200]
如上，当服务端返回Connection: Close时，客户端接收完响应，便会关闭链接。
客户端设置60s超时，服务端设置5s超时
##Keep-Alive: timeout=60 第一次请求,与connection:close无差异[2017-04-26 10:57:00 DEBUG] (org.apache.http.impl.conn.PoolingHttpClientConnectionManager:?) - Connection request: [route: {}->http://bizdomain:80][total kept alive: 0; route allocated: 0 of 32; total allocated: 0 of 200]
[2017-04-26 10:57:00 DEBUG] (org.apache.http.impl.conn.PoolingHttpClientConnectionManager:?) - Connection leased: [id: 0][route: {}->http://bizdomain:80][total kept alive: 0; route allocated: 1 of 32; total allocated: 1 of 200]
[2017-04-26 10:57:00 DEBUG] (org.apache.http.impl.execchain.MainClientExec:?) - Opening connection {}->http://bizdomain:80## 客户端设置超时时间60s[2017-04-26 10:57:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "Connection: keep-alive[\r][\n]"
[2017-04-26 10:57:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "Keep-Alive: timeout=60[\r][\n]"## 服务端设置超时时间5s[2017-04-26 10:57:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Keep-Alive: timeout=5, max=100[\r][\n]"[2017-04-26 10:57:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Connection: Keep-Alive[\r][\n]"
## 服务端设置生效，链接能够保持5s[2017-04-26 10:57:00 DEBUG] (org.apache.http.impl.execchain.MainClientExec:?) - Connection can be kept alive for 5000 MILLISECONDS[2017-04-26 10:57:00 DEBUG] (org.apache.http.impl.conn.PoolingHttpClientConnectionManager:?) - Connection [id: 0][route: {}->http://bizdomain:80] can be kept alive for 5.0 seconds
[2017-04-26 10:57:00 DEBUG] (org.apache.http.impl.conn.PoolingHttpClientConnectionManager:?) - Connection released: [id: 0][route: {}->http://bizdomain:80][total kept alive: 1; route allocated: 1 of 32; total allocated: 1 of 200]##Keep-Alive: timeout=60 非第一次请求[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]## 链接在上一次请求结束后5s失效[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 14:10:05 GMT+08:00 2017
客户端设置失效时间，服务端设置不失效
## 客户端设置30s超时[2017-04-26 17:45:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "Connection: keep-alive[\r][\n]" [2017-04-26 17:45:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "Keep-Alive: timeout=30[\r][\n]"## 服务端设置永久链接[2017-04-26 17:45:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Connection: keep-alive[\r][\n]" ## 链接将一直保持 [2017-04-26 17:45:00 DEBUG] (org.apache.http.impl.execchain.MainClientExec:?) - Connection can be kept alive indefinitely
综上，http链接保持时间是由服务端的消息头connection字段和keep-alive字段定的。
在上面前两种状况，请求的是同一个服务端，那么为何一个返回的是短链接，一个返回的是长链接呢？这里转一下 这篇文章的解释：
不论request仍是response的header中包含了值为close的connection，都代表当前正在使用的tcp连接在请求处理完毕后会被断掉。之后client再进行新的请求时就必须建立新的tcp连接了。 HTTP Connection的 close设置容许客户端或服务器中任何一方关闭底层的链接，双方都会要求在处理请求后关闭它们的TCP链接。
补充
TCP长短链接
在网上搜资料的时候，看到不少“HTTP协议的长链接和短链接，实质上是TCP协议的长链接和短链接”。 HTTP和TCP是不一样两层的东西，它们怎么会是同样的呢？HTTP是请求/响应模式的，就是说咱们发一个请求必定要有一个回应。最直观的就是，浏览器上发请求，得不到响应就会一直转圈圈。 而TCP并非必定要有响应。你们之前使用socket模拟一个IM聊天，A跟B打完招呼，彻底能够不用等待B的回应，就本身关掉链接的。TCP keep-alive
另外还有HTTP协议的keep-alive和TCP的keep-alive含义是有差异的。HTTP的keep-alive是为了维持链接，以便复用链接。经过使用keep-alive机制，能够减小tcp链接创建次数，也意味着能够减小TIME_WAIT状态链接，以此提升性能和提升httpd服务器的吞吐率(更少的tcp链接意味着更少的系统内核调用,socket的accept()和close()调用)。可是，长时间的tcp链接容易致使系统资源无效占用。配置不当的keep-alive，有时比重复利用链接带来的损失还更大。
而tcp keep-alive是TCP的一种检测TCP链接情况的机制，涉及到三个参数tcp_keepalive_time, tcp_keepalive_intvl, tcp_keepalive_probes。当网络两端创建了TCP链接以后，闲置（双方没有任何数据流往来）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链接情况(有可能客户端崩溃、强制关闭了应用、主机不可达等等)。若是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，直到收到对方的ack。若是一直没有收到对方的ack,一共会尝试 tcp_keepalive_probes次。若是尝试tcp_keepalive_probes,依然没有收到对方的ack包，则会丢弃该TCP链接。TCP链接默认闲置时间是2小时，通常设置为30分钟足够了。
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