java的斷點續傳是基於之前java檔案下載基礎上的功能拓展

首先設定一個以執行緒ID為名的下載進度檔案，

每一次下載的進度會儲存在這個檔案中，下一次下載的時候，會根據進度檔案裡面的內容來判斷下載的進度。

package com.ldw.multilthreaddownload;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;

public class Multidownload {

	static int ThreadCount = 3;   //執行緒的個數
	static int finishedThread = 0;   //初始化下載完成的執行緒的個數
	static String path = "http://192.168.0.102:8080/QQ.exe";  //確定下載地址
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		
		//傳送get請求，請求這個地址的資源
		try {
			URL url = new URL(path);
			HttpURLConnection conn = (HttpURLConnection) url.openConnection();
			conn.setRequestMethod("GET");
			conn.setConnectTimeout(5000);
			conn.setReadTimeout(5000);
			if(conn.getResponseCode() == 200){
				//獲取到請求資原始檔的長度
				int length = conn.getContentLength();
				File file = new File("QQ.exe");
				//建立隨機儲存檔案
				RandomAccessFile raf = new RandomAccessFile(file, "rwd");
				//設定臨時檔案的大小
				raf.setLength(length);
				//關閉raf
				raf.close();
				//計算出每一個執行緒下載多少位元組
				
				int size = length / Multidownload.ThreadCount;
				
				for(int i = 0; i < Multidownload.ThreadCount; i ++){
					//startIndex,endIndex分別代表執行緒的開始和結束位置
					int startIndex = i * size;
					int endIndex = (i + 1) * size - 1;
					if(i == ThreadCount - 1){
						//如果是最後一個執行緒，那麼結束位置寫死
						endIndex = length -1;
					}
					//System.out.println("執行緒" + i + "的下載區間是" + startIndex + "到" + endIndex);
					new DownLoadThread(startIndex, endIndex, i).start();
				}
			}
		} catch (Exception e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}

	}

}

class DownLoadThread extends Thread{
	int startIndex;
	int endIndex;
	int threadId;
	
	public DownLoadThread(int startIndex, int endIndex, int threadId) {
		super();
		this.startIndex = startIndex;
		this.endIndex = endIndex;
		this.threadId = threadId;
	}

	@Override
	public void run(){
		//使用http請求下載安裝包檔案
		URL url;
		try {
			File fileProgress = new File(threadId + ".txt");
			//判斷儲存下載進度的臨時檔案是否存在，
			if(fileProgress.exists()){
				FileInputStream fis = new FileInputStream(fileProgress);
				BufferedReader br = new BufferedReader(new InputStreamReader(fis));
				//從下載進度的臨時檔案中讀取上一次下載的總進度，然後和原來文字的開始位置相加，得到新的下載位置
				startIndex += Integer.parseInt(br.readLine());
				fis.close();
			}
			System.out.println("執行緒" + threadId + "下載區間是" + startIndex +"====" + endIndex);
			url = new URL(Multidownload.path);
			HttpURLConnection conn = (HttpURLConnection) url.openConnection();
			conn.setRequestMethod("GET");
			conn.setConnectTimeout(5000);
			conn.setReadTimeout(5000);
			//設定請求資料的區間
			conn.setRequestProperty("Range", "bytes=" + startIndex + "-" + endIndex);
			//請求部分資料的響應碼是206
			if(conn.getResponseCode() == 206){
				//獲取一部分資料來讀取
				InputStream is = conn.getInputStream();
				byte[] b = new byte[1024];
				int len = 0;
				int total = 0;
				//拿到臨時檔案的引用
				File file = new File("QQ.exe");
				RandomAccessFile raf = new RandomAccessFile(file, "rwd");
				//更新檔案的寫入位置，startIndex
				raf.seek(startIndex);
				while((len = is.read(b)) != -1 ){
					//每次讀取流裡面的資料，同步吧資料寫入臨時檔案
					raf.write(b, 0, len);
					total += len;
					//System.out.println("執行緒" + threadId + "下載了" + total);
					
					//生成一個專門記錄下載進度的臨時檔案
					//File fileProgress = new File(threadId + ".txt");
					RandomAccessFile fileProgressraf = new RandomAccessFile(fileProgress, "rwd");
					//每一次讀取流裡面的資料以後，把當前執行緒下載的總進度寫入臨時檔案中
					fileProgressraf.write((total + "").getBytes());
					fileProgressraf.close();
					
				}
				System.out.println("執行緒" + threadId + "下載過程結束===========================");
				raf.close();
				//三條執行緒下載完成以後，清理臨時檔案
				Multidownload.finishedThread++;
				//執行緒安全
				synchronized(Multidownload.path){
					if(Multidownload.finishedThread == Multidownload.ThreadCount){
						for(int i = 0; i < Multidownload.ThreadCount; i++){
							File filefinish = new File(i + ".txt");
							filefinish.delete();
							
						}
						Multidownload.finishedThread = 0;
					}
				}
			}
		} catch (Exception e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		
	}
}