有时我们用浏览器打开一张图片时，IE默认是下载图片，QQ浏览器、360浏览器、google浏览器等都是在线预览，然而当我们需要把这张图片下载下来观看时该怎么办呢？今天就学习一下浏览器访问资源文件下载文件和下载资源文件。

示例图片

图片地址
http://7xi8d6.com1.z0.glb.clouddn.com/20171227115959_lmlLZ3_Screenshot.jpeg

预览文件

在浏览器中输入图片的地址即可使用浏览器在线进行预览，还有一种方式就是经过servlet进行预览

servlet代码如下:

package com.hyf.servlet;

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class DownLoadNetServlet
 */
@WebServlet("/DownLoadNet")
public class DownLoadNetServlet extends HttpServlet {
	private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public DownLoadNetServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		String fileName = "d6ca7bcb0a46f21f46612acbfd246b600d33aed5.jpg";
		// TODO Auto-generated method stub
//		response.setContentType(getServletContext().getMimeType(fileName));
//		response.setHeader("Content-Disposition", "attachment;filename=" + fileName);
		
		System.out.println("文件类型："+getServletContext().getMimeType(fileName));

		OutputStream outputStream = response.getOutputStream();
		
		URL url = new URL("http://7xi8d6.com1.z0.glb.clouddn.com/20171227115959_lmlLZ3_Screenshot.jpeg");
		HttpURLConnection connection = (HttpURLConnection) url.openConnection();
		connection.connect();
		if (connection.getResponseCode() == 200) {
			InputStream is = connection.getInputStream();
			byte[] bytes = new byte[1024];
			int len = 0;
			while ((len = is.read(bytes)) != -1) {
				outputStream.write(bytes, 0, len);
			}
			outputStream.flush();
			is.close();
			outputStream.close();
		}
	}

	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		doGet(request, response);
	}

}

下载文件

如果不需要预览文件，直接进行下载图片，servlet代码如下：

package com.hyf.servlet;

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class DownLoadNetServlet
 */
@WebServlet("/DownLoadNet")
public class DownLoadNetServlet extends HttpServlet {
	private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public DownLoadNetServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		String fileName = "d6ca7bcb0a46f21f46612acbfd246b600d33aed5.jpeg";
		// TODO Auto-generated method stub
		response.setContentType(getServletContext().getMimeType(fileName));
		response.setHeader("Content-Disposition", "attachment;filename=" + fileName);
		
		System.out.println("文件类型："+getServletContext().getMimeType(fileName));

		OutputStream outputStream = response.getOutputStream();
		
		URL url = new URL("http://7xi8d6.com1.z0.glb.clouddn.com/20171227115959_lmlLZ3_Screenshot.jpeg");
		HttpURLConnection connection = (HttpURLConnection) url.openConnection();
		connection.connect();
		if (connection.getResponseCode() == 200) {
			InputStream is = connection.getInputStream();
			byte[] bytes = new byte[1024];
			int len = 0;
			while ((len = is.read(bytes)) != -1) {
				outputStream.write(bytes, 0, len);
			}
			outputStream.flush();
			is.close();
			outputStream.close();
		}
	}

	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		doGet(request, response);
	}

}

总结

从上述代码中可以看出，预览图片和下载图片都进行了使用文件流的形式进行传输，如果所预览的图片为远程的图片，那么就使用HttpUrlConnection进行获取图片的流，同时将该流传输到前台浏览器，达到中转效果。
而下载，则是依靠一下两行代码

response.setContentType(getServletContext().getMimeType(fileName));
response.setHeader("Content-Disposition", "attachment;filename=" + fileName);

其中第一句是判别文件的MIME类型的，第二句则是关键，在servlet中添加这两句就会使浏览器进行下载文件。

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