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HTTP短連接（非持久連接）是指，客戶端和服務(wù)端進(jìn)行一次HTTP請求/響應(yīng)之后，就關(guān)閉連接。所以，下一次的HTTP請求/響應(yīng)操作就需要重新建立連接。

HTTP長連接（持久連接）是指，客戶端和服務(wù)端建立一次連接之后，可以在這條連接上進(jìn)行多次請求/響應(yīng)操作。持久連接可以設(shè)置過期時(shí)間，也可以不設(shè)置。

我為什么沒有說HTTP/1.0 默認(rèn)短連接，HTTP/1.1起，默認(rèn)長連接呢？因?yàn)槲业谝淮慰催@個(gè)說法的時(shí)候，以為自己懂了，其實(shí)并沒有懂。長短連接操作上有什么區(qū)別，有的地方出現(xiàn)的持久連接又是怎么回事？

使用設(shè)置

這里的設(shè)置，我們都以HTTP1.1協(xié)議為例子。

設(shè)置HTTP短連接

在首部字段中設(shè)置Connection:close，則在一次請求/響應(yīng)之后，就會關(guān)閉連接。

設(shè)置HTTP長連接，有過期時(shí)間

在首部字段中設(shè)置Connection:keep-alive 和Keep-Alive: timeout=60，表明連接建立之后，空閑時(shí)間超過60秒之后，就會失效。如果在空閑第58秒時(shí)，再次使用此連接，則連接仍然有效，使用完之后，重新計(jì)數(shù)，空閑60秒之后過期。

設(shè)置HTTP長連接，無過期時(shí)間

在首部字段中只設(shè)置Connection:keep-alive，表明連接永久有效。

實(shí)現(xiàn)原理

了解怎么設(shè)置之后，就開始用起來。然而，問題來了。在請求頭中設(shè)置Connection:keep-alive，為什么連接空閑一段時(shí)間之后，還是斷開了呢？這是因?yàn)閏onnection字段只有服務(wù)端設(shè)置才有效。

HTTP操作是請求/響應(yīng)成對出現(xiàn)的，即先有客戶端發(fā)出請求，后有服務(wù)端處理請求。所以，一次HTTP操作的終點(diǎn)操作在服務(wù)端上，關(guān)閉也是由服務(wù)端發(fā)起的。

接下來我們做做測試，以及show code。下面的測試都是使用Spring RestTemplate，封裝apache http client進(jìn)行的。為方便講解代碼，先說明長連接的情況，最后再對其他形式做測試總結(jié)。

客戶端連接失效時(shí)間大于服務(wù)端失效時(shí)間

如下，為請求日志。客戶端設(shè)置Connection: Keep-Alive和Keep-Alive: timeout=60，服務(wù)端設(shè)置Connection: Keep-Alive和Keep-Alive: timeout=5。

## 客戶端設(shè)置有效期為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"
##服務(wù)端設(shè)置有效期為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"

客戶端設(shè)置的有效期大于服務(wù)端的，那么實(shí)際連接的有效期呢？三分鐘之后再次請求，從連接池中l(wèi)ease連接的時(shí)候，提示Connection expired @ Wed Apr 26 14:08:05，即在上一次請求之后的5s失效，說明是服務(wù)端的設(shè)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

源碼分析

通過源代碼了解一下連接失效時(shí)間的設(shè)置過程。

//org.apache.http.impl.execchain.MainClientExec#execute
......
//從連接池中l(wèi)ease connection
final HttpClientConnectionmanagedConn = connRequest.get(timeout > 0 ? timeout : 0, TimeUnit.MILLISECONDS);
......
//將conenction封裝在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.
//如果返回值消息頭中connection設(shè)置為close，則返回false
if (reuseStrategy.keepAlive(response, context)) {
  // Set the idle duration of this connection
  //取出response消息頭中，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);
  }
  //設(shè)置失效時(shí)間
  connHolder.setValidFor(duration, TimeUnit.MILLISECONDS);
  connHolder.markReusable();
} else {
  connHolder.markNonReusable();
}

待讀取響應(yīng)之后，釋放連接，即：connHolder.releaseConnection()。調(diào)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);
          //設(shè)置失效時(shí)間
          entry.updateExpiry(keepalive, effectiveUnit);
        }
      } finally {
      。。。。。。
        }
      }
    }
  }

然后再下一次HTTP操作，從連接池中獲取連接時(shí)

//org.apache.http.impl.conn.PoolingHttpClientConnectionManager#requestConnection調(diào)用org.apache.http.pool.AbstractConnPool#lease，
//調(diào)用getPoolEntryBlocking,調(diào)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()) {
  //日志中看到的內(nèi)容
    this.log.debug("Connection " + this + " expired @ " + new Date(getExpiry()));
  }
  return expired;
}

綜上，連接的實(shí)際有效時(shí)間，是根據(jù)response的設(shè)置來決定的。

其他情況測試

客戶端設(shè)置Connection: Close

##connection:close請求，kept alive的連接為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
## 建立新連接
[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
## 客戶端設(shè)置短連接
[2017-04-26 13:57:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0 >> "Connection: Close[\r][\n]"
## 服務(wù)端返回的也是短連接
[2017-04-26 13:57:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0  "Connection: close[\r][\n]"
##請求完之后，關(guān)閉連接
[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，當(dāng)服務(wù)端返回Connection: Close時(shí)，客戶端接收完響應(yīng)，便會關(guān)閉連接。

客戶端設(shè)置60s超時(shí)，服務(wù)端設(shè)置5s超時(shí)

##Keep-Alive: timeout=60 第一次請求,與connection:close無差別
[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
## 客戶端設(shè)置超時(shí)時(shí)間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]"
## 服務(wù)端設(shè)置超時(shí)時(shí)間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]"
## 服務(wù)端設(shè)置生效，連接可以保持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 非第一次請求
[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]
## 連接在上一次請求結(jié)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

客戶端設(shè)置失效時(shí)間，服務(wù)端設(shè)置不失效

## 客戶端設(shè)置30s超時(shí)
[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]"
## 服務(wù)端設(shè)置永久連接
[2017-04-26 17:45:00 DEBUG] (org.apache.http.wire:?) - http-outgoing-0  "Connection: keep-alive[\r][\n]"
## 連接將一直保持
[2017-04-26 17:45:00 DEBUG] (org.apache.http.impl.execchain.MainClientExec:?) - Connection can be kept alive indefinitely

綜上，http連接保持時(shí)間是由服務(wù)端的消息頭connection字段和keep-alive字段定的。

在上面前兩種情況，請求的是同一個(gè)服務(wù)端，那么為什么一個(gè)返回的是短連接，一個(gè)返回的是長連接呢？這里轉(zhuǎn)一下這篇文章的解釋：

不論request還是response的header中包含了值為close的connection，都表明當(dāng)前正在使用的tcp鏈接在請求處理完畢后會被斷掉。以后client再進(jìn)行新的請求時(shí)就必須創(chuàng)建新的tcp鏈接了。 HTTP Connection的 close設(shè)置允許客戶端或服務(wù)器中任何一方關(guān)閉底層的連接，雙方都會要求在處理請求后關(guān)閉它們的TCP連接。

補(bǔ)充

TCP長短連接

在網(wǎng)上搜資料的時(shí)候，看到很多“HTTP協(xié)議的長連接和短連接，實(shí)質(zhì)上是TCP協(xié)議的長連接和短連接”。 HTTP和TCP是不同兩層的東西，它們怎么會是一樣的呢？HTTP是請求/響應(yīng)模式的，就是說我們發(fā)一個(gè)請求一定要有一個(gè)回應(yīng)。最直觀的就是，瀏覽器上發(fā)請求，得不到響應(yīng)就會一直轉(zhuǎn)圈圈。而TCP并不是一定要有響應(yīng)。大家以前使用socket模擬一個(gè)IM聊天，A跟B打完招呼，完全可以不用等待B的回應(yīng)，就自己關(guān)掉連接的。

TCP keep-alive

另外還有HTTP協(xié)議的keep-alive和TCP的keep-alive含義是有差別的。HTTP的keep-alive是為了維持連接，以便復(fù)用連接。通過使用keep-alive機(jī)制，可以減少tcp連接建立次數(shù)，也意味著可以減少TIME_WAIT狀態(tài)連接，以此提高性能和提高h(yuǎn)ttpd服務(wù)器的吞吐率(更少的tcp連接意味著更少的系統(tǒng)內(nèi)核調(diào)用,socket的accept()和close()調(diào)用)。但是，長時(shí)間的tcp連接容易導(dǎo)致系統(tǒng)資源無效占用。配置不當(dāng)?shù)膋eep-alive，有時(shí)比重復(fù)利用連接帶來的損失還更大。

而tcp keep-alive是TCP的一種檢測TCP連接狀況的機(jī)制，涉及到三個(gè)參數(shù)tcp_keepalive_time, tcp_keepalive_intvl, tcp_keepalive_probes。

當(dāng)網(wǎng)絡(luò)兩端建立了TCP連接之后，閑置（雙方?jīng)]有任何數(shù)據(jù)流往來）了tcp_keepalive_time后，服務(wù)器內(nèi)核就會嘗試向客戶端發(fā)送偵測包，來判斷TCP連接狀況(有可能客戶端崩潰、強(qiáng)制關(guān)閉了應(yīng)用、主機(jī)不可達(dá)等等)。如果沒有收到對方的回答(ack包)，則會在 tcp_keepalive_intvl后再次嘗試發(fā)送偵測包，直到收到對方的ack。如果一直沒有收到對方的ack,一共會嘗試 tcp_keepalive_probes次。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(rèn)閑置時(shí)間是2小時(shí)，一般設(shè)置為30分鐘足夠了。

更多關(guān)于HTTP長連接與短連接使用方法請查看下面的相關(guān)鏈接

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