httpclient下載文件詳解

一、httpclient是什麼

HttpClient是一個開源的Java庫，它支持HTTP協議。它總共只分為四個模塊：HttpClient是模塊的核心，用於提供httpclient的api， HttpCore提供基本的網路操作，至於協議相關的內容可以去Apache的官網查閱，HttpAsyncClient是非同步的httpclient，HttpMime用於處理網路數據的mime類型。

二、使用httpclient下載文件的步驟

使用httpclient下載文件要經過以下幾個步驟：

1、創建HttpClient對象

2、創建HttpGet或HttpPost請求對象

3、執行請求，獲得響應結果

4、如果響應的狀態碼是200，說明響應成功，獲取響應的輸入流並讀取數據

5、關閉輸入流和HttpClient對象。

三、httpclient下載文件的示例代碼

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、設置httpclient的請求頭

在http請求中，請求頭的內容是非常重要的。對於一些網站，它們可以根據請求頭來判斷請求的來源，並進行相應的處理。在使用httpclient進行文件下載時，也可以設置請求頭，以達到隱藏下載行為的目的。

為了設置請求頭，我們可以使用httpget.setHeader方法。下面是示例代碼：

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、設置httpclient的請求參數

如果您需要帶上一些請求參數，以便進行授權或其它的認證，就可以使用httpclient的HttpParams類。HttpParams類是一個介面，它的實現類是BasicHttpParams。

這裡的請求參數包括Timeout、socketBufferSize、http.socket.timeout、http.connection.timeout、http.connection-manager.timeout。下面是示例代碼：

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();  
    }  
}