HttpClient基础教程

1、HttpClient相关的重要资料

官方网站：http://hc.apache.org/

API：http://hc.apache.org/httpcomponents-client-4.3.x/httpclient/apidocs/index.html

tutorial: http://hc.apache.org/httpcomponents-client-4.3.x/tutorial/html/index.html  【PDF版本】http://hc.apache.org/httpcomponents-client-4.3.x/tutorial/pdf/httpclient-tutorial.pdf

2、HttpClient有2个版本

org.apache.http.impl.client.HttpClients 与 org.apache.commons.httpclient.HttpClient

目前后者已被废弃，apache已不再支持。

一般而言，使用HttpClient均需导入httpclient.jar与httpclient-core.jar2个包。

3、使用HttpClient进行网络处理的基本步骤

（1）通过get的方式获取到Response对象。

 CloseableHttpClient httpClient = HttpClients.createDefault();
 HttpGet httpGet = new HttpGet("http://www.baidu.com/");
 CloseableHttpResponse response = httpClient.execute(httpGet);

注意，必需要加上http://的前缀，否则会报：Target host is null异常。

（2）获取Response对象的Entity。

 HttpEntity entity = response.getEntity();

注：HttpClient将Response的正文及Request的POST/PUT方法中的正文均封装成一个HttpEntity对象。可以通过entity.getContenType()，entity.getContentLength()等方法获取到正文的相关信息。但最重要的方法是通过getContent()获取到InputStream对象。

（3）通过Entity获取到InputStream对象，然后对返回内容进行处理。

 is = entity.getContent();
 sc = new Scanner(is);
 // String filename = path.substring(path.lastIndexOf('/')+1);
 String filename = "2.txt";
 os = new PrintWriter(filename);
 while (sc.hasNext()) {
     os.write(sc.nextLine());
 }

使用HtppClient下载一个网页的完整代码如下：

 package com.ljh.test;

 import java.io.IOException;
 import java.io.InputStream;
 import java.io.PrintWriter;
 import java.io.Writer;
 import java.util.Scanner;

 import org.apache.http.HttpEntity;
 import org.apache.http.HttpStatus;
 import org.apache.http.client.ClientProtocolException;
 import org.apache.http.client.methods.CloseableHttpResponse;
 import org.apache.http.client.methods.HttpGet;
 import org.apache.http.impl.client.CloseableHttpClient;
 import org.apache.http.impl.client.HttpClients;

 public class DownloadWebPage{

     public static void downloadPagebyGetMethod() throws IOException {

         // 1、通过HttpGet获取到response对象
         CloseableHttpClient httpClient = HttpClients.createDefault();
         HttpGet httpGet = new HttpGet("http://www.baidu.com/");
         CloseableHttpResponse response = httpClient.execute(httpGet);

         InputStream is = null;
         Scanner sc = null;
         Writer os = null;
         if (response.getStatusLine().getStatusCode() == HttpStatus.SC_OK) {
             try {
                 // 2、获取response的entity。
                 HttpEntity entity = response.getEntity();

                 // 3、获取到InputStream对象，并对内容进行处理
                 is = entity.getContent();
                 sc = new Scanner(is);
                 // String filename = path.substring(path.lastIndexOf('/')+1);
                 String filename = "2.txt";
                 os = new PrintWriter(filename);
                 while (sc.hasNext()) {
                     os.write(sc.nextLine());
                 }

             } catch (ClientProtocolException e) {
                 e.printStackTrace();
             } finally {
                 if (sc != null) {
                     sc.close();
                 }
                 if (is != null) {
                     is.close();
                 }
                 if (os != null) {
                     os.close();
                 }
                 if (response != null) {
                     response.close();
                 }
             }
         }

     }

     public static void main(String[] args) {
         try {
             downloadPagebyGetMethod();
         } catch (IOException e) {
             e.printStackTrace();
         }
     }

 }

注意：直接将HttpGet改为HttpPost，返回的结果有误，百度返回302状态，即重定向，新浪返回拒绝访问。怀疑大多网站均不允许POST方法直接访问网站。