Apache commons 系列的HttpClient 相信大家都用過，選擇它而非JDK 的java.net.HttpURLConnection ，是為了使用HttpClient 封裝的幾個實用的功能。

目前使用最多的版本還是httpclient-3.x ，在官網(wǎng)http://hc./httpclient-3.x/tutorial.html 有這么一段示例代碼：

import org.apache.commons.httpclient.*;
import org.apache.commons.httpclient.methods.*;
import org.apache.commons.httpclient.params.HttpMethodParams;

import java.io.*;

public class HttpClientTutorial {
  
  private static String url = "http://www./";

  public static void main(String[] args) {
    // Create an instance of HttpClient.
    HttpClient client = new HttpClient();

    // Create a method instance.
    GetMethod method = new GetMethod(url);
    
    // Provide custom retry handler is necessary
    method.getParams().setParameter(HttpMethodParams.RETRY_HANDLER, 
    		new DefaultHttpMethodRetryHandler(3, false));

    try {
      // Execute the method.
      int statusCode = client.executeMethod(method);

      if (statusCode != HttpStatus.SC_OK) {
        System.err.println("Method failed: " + method.getStatusLine());
      }

      // Read the response body.
      byte[] responseBody = method.getResponseBody();

      // Deal with the response.
      // Use caution: ensure correct character encoding and is not binary data
      System.out.println(new String(responseBody));

    } catch (HttpException e) {
      System.err.println("Fatal protocol violation: " + e.getMessage());
      e.printStackTrace();
    } catch (IOException e) {
      System.err.println("Fatal transport error: " + e.getMessage());
      e.printStackTrace();
    } finally {
      // Release the connection.
      method.releaseConnection();
    }  
  }
}

This is a crucial step to keep things flowing. 

We must tell HttpClient that we are done with 

the connection and that it can now be reused. 

Without doing this HttpClient will wait 

indefinitely for a connection to free up so 

that it can be reused.

method.releaseConnection();

這個在串行的處理中或許很有用，但是我所遇到的情況是多線程并發(fā)下，不能共享同一個HttpClient 實例，按照官方示例寫好代碼后，程序跑起來似乎沒什么問題，但是隨著時間的累計，有一天突然發(fā)現(xiàn)這個模塊不工作了，查看了一下當前的網(wǎng)絡連接，這個java 程序同一個地址保持著200多個CLOSE_WAIT 的連接，好吧，連接沒有釋放。

為什么沒有釋放？查看doc，有這樣的說明：

Releases the connection being used by this HTTP method. 

In particular the connection is used to read the response

(if there is one) and will be held until the response has 

been read. If the connection can be reused by other HTTP 

methods it is NOT closed at this point.

查看源碼，找到HttpClient 的構(gòu)造方法，有一個可以指定HttpConnectionManager ，然后這個HttpConnectionManager 又有一個實現(xiàn)的構(gòu)造：

public SimpleHttpConnectionManager(boolean alwaysClose)
The connection manager created with this constructor will 

try to keep the connection open (alive) between consecutive 

requests if the alwaysClose parameter is set to false. 

Otherwise the connection manager will always close 

connections upon release.
Parameters:
alwaysClose - if set true, the connection manager will 

always close connections upon release.

所以，必須

HttpClient client = new HttpClient(new HttpClientParams(),

new SimpleHttpConnectionManager(true));

Java代碼  

1. HttpClient client = new HttpClient();  
2. HttpMethod method = new GetMethod("http://www.");  
3. try {  
4.   client.executeMethod(method);  
5.   byte[] responseBody = null;  
6.     
7.   responseBody = method.getResponseBody();  
8.     
9. } catch (HttpException e) {  
10.   // TODO Auto-generated catch block  
11.   e.printStackTrace();  
12. } catch (IOException e) {  
13.   // TODO Auto-generated catch block  
14.   e.printStackTrace();  
15. }finally{  
16.   method.releaseConnection();  
17.     
18. }  

Java代碼  

1. Timeout 30  
2. KeepAlive On   #表示服務器端不會主動關閉鏈接  
3. MaxKeepAliveRequests 100  
4. KeepAliveTimeout 180   

Java代碼  

1. /** 
2.  * The connection manager created with this constructor will try to keep the  
3.  * connection open (alive) between consecutive requests if the alwaysClose  
4.  * parameter is set to <tt>false</tt>. Otherwise the connection manager will  
5.  * always close connections upon release. 
6.  *  
7.  * @param alwaysClose if set <tt>true</tt>, the connection manager will always 
8.  *    close connections upon release. 
9.  */  
10. public SimpleHttpConnectionManager(boolean alwaysClose) {  
11.     super();  
12.     this.alwaysClose = alwaysClose;  
13. }  

Java代碼  

1. this.httpConnectionManager = new SimpleHttpConnectionManager();  

Java代碼  

1. HttpClient client = new HttpClient(new HttpClientParams(),new SimpleHttpConnectionManager(true) );  

Java代碼  

1. ((SimpleHttpConnectionManager)client.getHttpConnectionManager()).shutdown();  

Java代碼  

1. public void shutdown() {  
2.     httpConnection.close();  
3. }  

Java代碼  

1. public void closeIdleConnections(long idleTimeout) {  
2.     long maxIdleTime = System.currentTimeMillis() - idleTimeout;  
3.     if (idleStartTime <= maxIdleTime) {  
4.         httpConnection.close();  
5.     }  
6. }  

Java代碼  

1. method.setRequestHeader("Connection", "close");