httpclient下载文件详解

一、httpclient是什么

HttpClient是一个开源的Java库，它支持HTTP协议。它总共只分为四个模块：HttpClient是模块的核心，用于提供httpclient的api， HttpCore提供基本的网络操作，至于协议相关的内容可以去Apache的官网查阅，HttpAsyncClient是异步的httpclient，HttpMime用于处理网络数据的mime类型。

二、使用httpclient下载文件的步骤

使用httpclient下载文件要经过以下几个步骤：

1、创建HttpClient对象

2、创建HttpGet或HttpPost请求对象

3、执行请求，获得响应结果

4、如果响应的状态码是200，说明响应成功，获取响应的输入流并读取数据

5、关闭输入流和HttpClient对象。

三、httpclient下载文件的示例代码

import java.io.*;  
import org.apache.http.HttpResponse;  
import org.apache.http.client.HttpClient;  
import org.apache.http.client.methods.HttpGet;  
import org.apache.http.impl.client.DefaultHttpClient;  

public class HttpDownload{
    public static void main(String[] args) throws Exception{  
        String url = "http://www.example.com/file/example.jpg"; 

        HttpClient httpclient = new DefaultHttpClient();  
        HttpGet httpget = new HttpGet(url);  
        HttpResponse response = httpclient.execute(httpget);  

        if(response.getStatusLine().getStatusCode() == 200){  
            InputStream is = response.getEntity().getContent();  
            File file = new File("example.jpg");
            FileOutputStream fos = new FileOutputStream(file);  
            byte[] buffer = new byte[1024];
            int len = 0;
            while((len = is.read(buffer))!=-1){  
                fos.write(buffer, 0, len);  
            }  
            fos.close();  
            is.close();  
        }  

        httpclient.getConnectionManager().shutdown();  
    }  
} 

四、设置httpclient的请求头

在http请求中，请求头的内容是非常重要的。对于一些网站，它们可以根据请求头来判断请求的来源，并进行相应的处理。在使用httpclient进行文件下载时，也可以设置请求头，以达到隐藏下载行为的目的。

为了设置请求头，我们可以使用httpget.setHeader方法。下面是示例代码：

import java.io.*;  
import org.apache.http.HttpResponse;  
import org.apache.http.client.HttpClient;  
import org.apache.http.client.methods.HttpGet;  
import org.apache.http.impl.client.DefaultHttpClient;  

public class HttpDownload{
    public static void main(String[] args) throws Exception{  
        String url = "http://www.example.com/file/example.jpg"; 

        HttpClient httpclient = new DefaultHttpClient();  
        HttpGet httpget = new HttpGet(url);  

        httpget.setHeader("User-Agent", "Mozilla/4.0 (compatible; MSIE 5.0; Windows NT; DigExt)");  

        HttpResponse response = httpclient.execute(httpget);  

        if(response.getStatusLine().getStatusCode() == 200){  
            InputStream is = response.getEntity().getContent();  
            File file = new File("example.jpg");
            FileOutputStream fos = new FileOutputStream(file);  
            byte[] buffer = new byte[1024];
            int len = 0;
            while((len = is.read(buffer))!=-1){  
                fos.write(buffer, 0, len);  
            }  
            fos.close();  
            is.close();  
        }  

        httpclient.getConnectionManager().shutdown();  
    }  
} 

五、设置httpclient的请求参数

如果您需要带上一些请求参数，以便进行授权或其它的认证，就可以使用httpclient的HttpParams类。HttpParams类是一个接口，它的实现类是BasicHttpParams。

这里的请求参数包括Timeout、socketBufferSize、http.socket.timeout、http.connection.timeout、http.connection-manager.timeout。下面是示例代码：

import java.io.*;  
import org.apache.http.HttpResponse;  
import org.apache.http.client.HttpClient;  
import org.apache.http.client.methods.HttpGet;  
import org.apache.http.impl.client.DefaultHttpClient;  
import org.apache.http.impl.conn.PoolingClientConnectionManager;  
import org.apache.http.params.CoreConnectionPNames;  
import org.apache.http.params.HttpParams;  
import org.apache.http.params.BasicHttpParams;  

public class HttpDownload{
    public static void main(String[] args) throws Exception{  
        String url = "http://www.example.com/file/example.jpg"; 

        HttpParams params = new BasicHttpParams();  
        params.setParameter(CoreConnectionPNames.CONNECTION_TIMEOUT, 1000);  
        params.setParameter(CoreConnectionPNames.SO_TIMEOUT, 1000);  
        params.setParameter(CoreConnectionPNames.SOCKET_BUFFER_SIZE, 1024);  
        params.setParameter("http.socket.timeout", new Integer(1000));  
        params.setParameter("http.connection.timeout", new Integer(1000));  
        params.setParameter("http.connection-manager.timeout", new Long(1000));  

        HttpClient httpclient = new DefaultHttpClient(new PoolingClientConnectionManager(), params);  
        HttpGet httpget = new HttpGet(url);  

        httpget.setHeader("User-Agent", "Mozilla/4.0 (compatible; MSIE 5.0; Windows NT; DigExt)");  

        HttpResponse response = httpclient.execute(httpget);  

        if(response.getStatusLine().getStatusCode() == 200){  
            InputStream is = response.getEntity().getContent();  
            File file = new File("example.jpg");
            FileOutputStream fos = new FileOutputStream(file);  
            byte[] buffer = new byte[1024];
            int len = 0;
            while((len = is.read(buffer))!=-1){  
                fos.write(buffer, 0, len);  
            }  
            fos.close();  
            is.close();  
        }  

        httpclient.getConnectionManager().shutdown();  
    }  
}