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.
*
* @param timeout Timeout in milliseconds.
*/
public void setConnectionTimeout(int timeout) {
setIntParameter(CONNECTION_TIMEOUT, timeout);
} /**
* Sets the default socket timeout (<tt>SO_TIMEOUT</tt>) 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
* {@link HttpMethodParams HTTP method parameters}.
*
* @param timeout Timeout in milliseconds
*/
public void setSoTimeout(int timeout) {
setIntParameter(SO_TIMEOUT, timeout);
}
如下示例代码，可以模拟ConnectTimeoutException和SocketTimeoutException
import org.apache.commons.httpclient.HttpClient;
import org.apache.commons.httpclient.HttpException;
import org.apache.commons.httpclient.HttpMethod;
import org.apache.commons.httpclient.methods.GetMethod; import java.io.IOException; public class HttpclientTimeoutTest { public static void main(String[] args) {
HttpClient client = new HttpClient(); 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 = new String(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
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
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