Java单线程文件下载，支持断点续传功能

前言：

程序下载文件时，有时会因为各种各样的原因下载中断，对于小文件来说影响不大，可以快速重新下载，但是下载大文件时，就会耗费很长时间，所以断点续传功能对于大文件很有必要。

文件下载的断点续传：

　　1、先下载临时文件，用于记录已下载大小：

    2、http请求时设置Range参数

      3、下载此次请求的数据；

直接上代码：

 package com.test.service;

 import java.io.File;
 import java.io.InputStream;
 import java.io.RandomAccessFile;
 import java.net.HttpURLConnection;
 import java.net.URL;
 import java.text.NumberFormat;

 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 import org.springframework.beans.factory.annotation.Value;
 import org.springframework.stereotype.Component;

 /**
  * <p>
  *     文件下载,可以支持断点续传
  *     暂未使用
  * </p>
  *  @author
  *  @version 1.0
  * */
 @Component
 public class DownloadOnly {

     private static final Logger logger = LoggerFactory.getLogger(DownloadOnly.class);

     @Value("${onair.download.ddxc:true}")
     boolean ddxc = true;

     int startIndex = 0;

     long downloadSize = 0;

     boolean downloadFinish = false;

     int totleSize = 0;

     public boolean download(String url,String file_path,int downloadTimeout){

         //起一个线程 检测下载进度
         new Thread(new Runnable() {

             @Override
             public void run() {
                 try {
                      NumberFormat nt = NumberFormat.getPercentInstance();
                       //设置百分数精确度3即保留三位小数
                      nt.setMinimumFractionDigits(1);
                     while(!downloadFinish){
                         Thread.sleep(30000);
                         logger.debug("已下载大小{},进度{}",getDownloadSize(),nt.format(getDownloadSize()* 1.0 /totleSize));
                     }
                 } catch (InterruptedException e) {
                     e.printStackTrace();
                 }

             }
         }).start();

         logger.info("下载文件：源路径{},目标路径：{}",url,file_path);
         RandomAccessFile raf = null;
         InputStream in = null;

         try {
             URL file_url = new URL(url);
             HttpURLConnection conn = (HttpURLConnection)file_url.openConnection();
             conn.setConnectTimeout(downloadTimeout);
             conn.setRequestMethod("GET");
             File tmpFile = new File(file_path+"_tmp");
             if(ddxc){
                 if(tmpFile.exists() && tmpFile.isFile()){
                     downloadSize = tmpFile.length();
                     startIndex = (int)downloadSize;
                 }
                 conn.setRequestProperty("Range", "bytes=" + startIndex + "-");
             }else{
                 if(tmpFile.exists() && tmpFile.isFile())
                     tmpFile.delete();
             }
             int status = conn.getResponseCode();
             totleSize = (int)downloadSize + conn.getContentLength();
             logger.info("文件总大小{}，下载请求获得的返回状态码：{},需要下载的大小{}",totleSize,status,totleSize-downloadSize);
             if(status== 200 || status == 206 ){
                 raf = new RandomAccessFile(tmpFile, "rwd");
                 raf.seek(startIndex);
                 in = conn.getInputStream();
                 byte[] buffer = new byte[1024];
                 int size = 0;
                 while((size=in.read(buffer)) !=-1 ){
                     raf.write(buffer, 0, size);
                     downloadSize += size;
                 }
                 raf.close();
                 in.close();
                 File dest = new File(file_path);
                 return tmpFile.renameTo(dest);
             }
         } catch (Throwable e) {
             logger.error("文件下载失败：{}",e.getMessage(),e);
         }finally {
             downloadFinish = true; //下载完成或中断
         }
         return false;
     }

     public long getDownloadSize() {
         return downloadSize;
     }

     public static void main(String[] args) {
         DownloadOnly downloadOnly = new DownloadOnly();
     }

 }