http是基于TCP/IP进行通信的，tcp通过3次握手建立连接，并最终以4次挥手终止通信。

知乎上对三次握手和四次挥手有如下解释：

作者：知乎用户

链接：https://www.zhihu.com/question/67772889/answer/256760079

来源：知乎

http是应用层协议，主要依赖于运输层TCP协议(HTTP协议没有规定具体使用哪个运输层协议)。

tcp连接建立和断开方式涉及到客户端和服务器端的端口,缓存等资源的分配与释放问题。

建立连接时，主动连接方(客户端)向服务器请求建立连接(SYN)，服务器端收到后会给客户端响应(ack)，表示它这边准备好连接了，但是他要确保客户端收到了它的响应，于是需要第三个数据包 客户端向服务器端确认(ack)，这样相互确认之后服务器端就可以给这条连接分配必要的端口、缓存等资源。 假设只有两次数据包交换就分配资源，若服务器端响应客户端连接请求的包丢失了，客户端会认为服务器未响应，放弃本次连接请求，而服务器端之前分配的资源就会被闲置浪费。 更多次的报文交换是可以的，但是完全没有必要。 三次交换是建立连接所需最少的数量。

断开连接时， 任务先完成的一方(这里假设是客户端)会向另一方(即服务端)发送断开请求(FIN)，表示自己这一方传输任务已经完成，服务器端收到后会响应。 但是这里 服务器端传送给客户端的数据可能还没有完，他需要维持当前连接来完成剩余的传输任务，即客户端还不能释放当前连接所需的资源。当服务端数据传输任务完成后，他会向客户端发送断开请求(FIN)，客户端响应。 这样双方相互确认数据传输完成，可以断开连接，分别释放当前连接占用的系统资源，而不是提前释放导致传输的数据丢失。

TCP三次握手

TCP四次挥手

httpConnection有两个重要的属性：http.connection.timeout和http.socket.timeout。connection timeout是建立连接的超时时间，socket timeout表示的是等待服务端响应数据的超时时间。

SocketTimeoutException 和 ConnectTimeoutException 均派生自 InterruptedIOException(IO被中断异常、IO被阻断异常)

commons-httpclient 3.1里HttpConnectionParams.java里有如下2个方法：

/*** Sets the timeout until a connection is etablished. A value of zero

* means the timeout is not used. The default value is zero.

*

*@paramtimeout Timeout in milliseconds.*/

public void setConnectionTimeout(inttimeout) {

setIntParameter(CONNECTION_TIMEOUT, timeout);

}/*** Sets the default socket timeout (SO_TIMEOUT) in milliseconds which is the

* timeout for waiting for data. A timeout value of zero is interpreted as an infinite

* timeout. This value is used when no socket timeout is set in the

* {@linkHttpMethodParams HTTP method parameters}.

*

*@paramtimeout Timeout in milliseconds*/

public void setSoTimeout(inttimeout) {

setIntParameter(SO_TIMEOUT, timeout);

}

如下示例代码，可以模拟ConnectTimeoutException和SocketTimeoutException

importorg.apache.commons.httpclient.HttpClient;importorg.apache.commons.httpclient.HttpException;importorg.apache.commons.httpclient.HttpMethod;importorg.apache.commons.httpclient.methods.GetMethod;importjava.io.IOException;public classHttpclientTimeoutTest {public static voidmain(String[] args) {

HttpClient client= newHttpClient();

HttpMethod method= new GetMethod("https://linkedin.com/company/stack-overflow");

client.getHttpConnectionManager().getParams().setConnectionTimeout(10);

client.getHttpConnectionManager().getParams().setSoTimeout(1000);

System.out.println("begin..");long start =System.currentTimeMillis();try{int statusCode =client.executeMethod(method);

System.out.println(statusCode);byte[] responseBody = null;

responseBody=method.getResponseBody();

String result= newString(responseBody);

System.out.println(result);

}catch(HttpException e) {//TODO Auto-generated catch block

e.printStackTrace();

}catch(IOException e) {//TODO Auto-generated catch block

e.printStackTrace();

}finally{

System.out.println("end..Duration MS:" + (System.currentTimeMillis() -start));

}

}

}

这里，将建立连接的超时时间设置为小到10ms。即可复现出来ConnectTimeoutException---connect timed out。

begin..

org.apache.commons.httpclient.ConnectTimeoutException: The host did not accept the connection within timeout of 10 ms

at org.apache.commons.httpclient.protocol.ReflectionSocketFactory.createSocket(ReflectionSocketFactory.java:155)

at org.apache.commons.httpclient.protocol.SSLProtocolSocketFactory.createSocket(SSLProtocolSocketFactory.java:130)

at org.apache.commons.httpclient.HttpConnection.open(HttpConnection.java:707)

at org.apache.commons.httpclient.HttpMethodDirector.executeWithRetry(HttpMethodDirector.java:387)

at org.apache.commons.httpclient.HttpMethodDirector.executeMethod(HttpMethodDirector.java:171)

at org.apache.commons.httpclient.HttpClient.executeMethod(HttpClient.java:397)

at org.apache.commons.httpclient.HttpClient.executeMethod(HttpClient.java:323)

at com.emax.paycenter.HttpclientTimeoutTest.main(HttpclientTimeoutTest.java:24)

Caused by: java.net.SocketTimeoutException: connect timed out

at java.net.DualStackPlainSocketImpl.waitForConnect(Native Method)

at java.net.DualStackPlainSocketImpl.socketConnect(DualStackPlainSocketImpl.java:85)

at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:339)

at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:200)

at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:182)

at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:172)

at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)

at java.net.Socket.connect(Socket.java:579)

at sun.security.ssl.SSLSocketImpl.connect(SSLSocketImpl.java:618)

at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)

at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)

at java.lang.reflect.Method.invoke(Method.java:606)

at org.apache.commons.httpclient.protocol.ReflectionSocketFactory.createSocket(ReflectionSocketFactory.java:140)

... 7 more

end..Duration MS: 1064

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将读取数据的响应超时时间设置为小到10ms，即可复现出来SocketTimeoutException---Read timed out。

begin..

java.net.SocketTimeoutException: Read timed out

at java.net.SocketInputStream.socketRead0(Native Method)

at java.net.SocketInputStream.read(SocketInputStream.java:152)

at java.net.SocketInputStream.read(SocketInputStream.java:122)

at sun.security.ssl.InputRecord.readFully(InputRecord.java:442)

at sun.security.ssl.InputRecord.read(InputRecord.java:480)

at sun.security.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:927)

at sun.security.ssl.SSLSocketImpl.performInitialHandshake(SSLSocketImpl.java:1312)

at sun.security.ssl.SSLSocketImpl.writeRecord(SSLSocketImpl.java:702)

at sun.security.ssl.AppOutputStream.write(AppOutputStream.java:122)

at java.io.BufferedOutputStream.flushBuffer(BufferedOutputStream.java:82)

at java.io.BufferedOutputStream.flush(BufferedOutputStream.java:140)

at org.apache.commons.httpclient.HttpConnection.flushRequestOutputStream(HttpConnection.java:828)

at org.apache.commons.httpclient.HttpMethodBase.writeRequest(HttpMethodBase.java:2116)

at org.apache.commons.httpclient.HttpMethodBase.execute(HttpMethodBase.java:1096)

at org.apache.commons.httpclient.HttpMethodDirector.executeWithRetry(HttpMethodDirector.java:398)

at org.apache.commons.httpclient.HttpMethodDirector.executeMethod(HttpMethodDirector.java:171)

at org.apache.commons.httpclient.HttpClient.executeMethod(HttpClient.java:397)

at org.apache.commons.httpclient.HttpClient.executeMethod(HttpClient.java:323)

at com.emax.paycenter.HttpclientTimeoutTest.main(HttpclientTimeoutTest.java:25)

end..Duration MS: 1516

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具体应用中，需要以实际需要来做超时设置。

references：

文中分析了httpclient3.*的一些漏洞。推荐使用httpclient4.2.3。1. 将connection请求头设置为close，则会关闭连接socket。(如果是keep-alive ，不会关闭)；2、在使用httpclient3.1时(4.2.3没问题)，尽量不要调用 byte[] getResponseBody() ：因为如果Content-Length没设置或者传输的数据大于1M，会有大量冗余日志。

分析了TCP为什么需要3次握手，和为什么需要4次挥手