文件下載----OutputStream流和PrintWriter流的區別

使用OutputStream流和PrintWriter流下載文件

最近在項目中使用PrintWriter流去實現文件下載的功能，但是總會出現下載失敗的情況，更換爲OutputStream流後該BUG得到解決，藉此文章記錄一下OutputStream流和PrintWriter流實現文件下載時的區別。

廢話不多說直接上代碼

OutputStream流實現文件下載

package controller;

import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.URLEncoder;

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

public class ServletDemo01 extends HttpServlet {
    private static final long serialVersionUID = 1L;
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doPost(request, response);
    }
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //獲取文件的下載絕對路徑
        String downloadPath=this.getServletContext().getRealPath("/download/測試圖片.jpg");
        //獲取文件名
        String fileName=downloadPath.substring(downloadPath.lastIndexOf("\\")+1);
        //設置響應頭，告訴瀏覽器以下載的方式打來文件，設置中文編碼，如果不設置會出現亂碼
         response.setHeader("content-disposition", "attachment;filename="+URLEncoder.encode(fileName, "UTF-8"));
        //獲取文件流
         InputStream is=new FileInputStream(downloadPath);
         int len=0;
         byte[]b=new byte[1024];
         OutputStream os=response.getOutputStream();
         while((len=is.read(b))!=-1){
             //將緩衝區數據輸出到瀏覽器
             os.write(b,0,len);
         }
         is.close();
    }
}

PrintWriter流實現文件下載

package controller;

import java.io.FileInputStream;
import java.io.FileReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.net.URLEncoder;

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

public class ServletDemo03 extends HttpServlet {
    private static final long serialVersionUID = 1L;
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doPost(request, response);
    }
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //獲取文件的下載絕對路徑
        String downloadPath=this.getServletContext().getRealPath("/download/測試圖片.jpg");
        //獲取文件名
        String fileName=downloadPath.substring(downloadPath.lastIndexOf("\\")+1);
        //設置響應頭，告訴瀏覽器以下載的方式打來文件，設置中文編碼，如果不設置會出現亂碼
         response.setHeader("content-disposition", "attachment;filename="+URLEncoder.encode(fileName, "UTF-8"));
        //獲取文件流
         FileReader fr=new FileReader(downloadPath);
         int len=0;
         char[] b=new char[1024];
         PrintWriter os=response.getWriter();
         while((len=fr.read(b))!=-1){
             //將緩衝區數據輸出到瀏覽器
             os.write(b,0,len);
         }
         fr.close();
    }
}

出現下載失敗的原因：

PrintWriter流處理字節數據，會導致數據丟失，因此在編寫下載文件功能時，要使用OutputStream流。
因爲OutputStream流是字節流，可以處理任意類型的數據；
而PrintWriter流是字符流，只能處理字符數據，如果用字符流處理字節數據，會導致數據丟失。