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。
[java] view plain copy 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。
[java] view plain copy 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。
[java] view plain copy 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：
[java] view plain copy 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。
转载于:https://www.cnblogs.com/jinhong-lu/p/4559469.html
