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,就关闭连接。所以,下一次的 HTTP 请求 / 响应操作就需要重新建立连接。 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?
这里的设置,我们都以 HTTP1.1 协议为例子。
在首部字段中设置
,则在一次请求 / 响应之后,就会关闭连接。
- Connection:close
在首部字段中设置
和
- Connection:keep-alive
,表明连接建立之后,空闲时间超过 60 秒之后,就会失效。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。
- Keep-Alive: timeout=60
在首部字段中只设置
,表明连接永久有效。
- Connection:keep-alive
了解怎么设置之后,就开始用起来。然而,问题来了。在请求头中设置
,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。
- Connection:keep-alive
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,最后再对其他形式做测试总结。
如下,为请求日志。客户端设置
和
- Connection: Keep-Alive
, 服务端设置
- Keep-Alive: timeout=60
和
- Connection: Keep-Alive
。
- 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[\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
,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。
- Connection expired @ Wed Apr 26 14:08:05
- [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
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:
。调用 org.apache.http.impl.conn.PoolingHttpClientConnectionManager#releaseConnection 方法。
- connHolder.releaseConnection()
- @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。
- Connection: Close
- ##connection:close请求,kept alive的连接为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
- ## 建立新连接
- [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
- ## 客户端设置短连接
- [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连接
- [2017-04-26 13:57:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Connection: close[\r][\n]"
- ##请求完之后,关闭连接
- [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
时,客户端接收完响应,便会关闭连接。
- Connection: Close
- ##Keep-Alive: timeout=60 第一次请求,与connection:close无差别
- [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,连接可以保持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]
- ## 连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]"
- ## 服务端设置永久连接
- [2017-04-26 17:45:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 << "Connection: keep-alive[\r][\n]"
- ## 连接将一直保持
- [2017-04-26 17:45:00 DEBUG] (org.apache.http.impl.execchain.MainClientExec:?) - Connection can be kept alive indefinitely
综上,http 连接保持时间是由服务端的消息头 connection 字段和 keep-alive 字段定的。
在上面前两种情况,请求的是同一个服务端,那么为什么一个返回的是短连接,一个返回的是长连接呢?这里转一下 这篇文章的解释:
不论 request 还是 response 的 header 中包含了值为 close 的 connection,都表明当前正在使用的 tcp 链接在请求处理完毕后会被断掉。以后 client 再进行新的请求时就必须创建新的 tcp 链接了。 HTTP Connection 的 close 设置允许客户端或服务器中任何一方关闭底层的连接,双方都会要求在处理请求后关闭它们的 TCP 连接。
而 tcp keep-alive 是 TCP 的一种检测 TCP 连接状况的机制,涉及到三个参数 tcp_keepalive_time, tcp_keepalive_intvl, tcp_keepalive_probes。 当网络两端建立了 TCP 连接之后,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 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,直到收到对方的 ack。如果一直没有收到对方的 ack, 一共会尝试 tcp_keepalive_probes 次。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。参考这里。
来源: http://www.cnblogs.com/shoren/p/http-connection.html