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。