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();
    }
    
}