步骤:
1.在本地创建一个与服务器端一样大小的空白文件;
确定服务器端文件的大小:发送一个请求,得到content-length;
创建空白文件,设置文件的大小:RandomAccessFile
2.在设置线程的个数;
创建int类型的变量;
3.计算子线程下载数据的范围;
blockSize = length / threadCount;
4.计算子线程下载数据的开始位置和结束位置;
startIndex = threadIdblockSize;
endIndex = (threadId+1)blockSize-1
最后一个子线程的结束位置:length - 1;
setRequestProperty("Range","bytes=startIndex-endIndex");
5.创建子线程下载数据,设置从哪个位置写数据;
seek();
6.确定文件在什么时间下载完成;
设置一个变量为正在运行的线程的个数,每个子线程下载完成后,都把这个变量-1,当变量为0时表示三个子线程都下载结束了,也表示文件下载完成了;
7.断点续传:实时记录下载到的当前位置,下次下载时,接着上一次下载的位置继续下载;
package com.dominichan.multhreaddownload;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;
public class MultiThreadDownload {
private static String path = "";
private static int threadCount = 3;
public static void main(String[] args) {
//在本地创建一个与服务器一样大小的空白文件
try {
URL url = new URL(path);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestMethod("GET");
conn.setConnectTimeout(3000);
int code = conn.getResponseCode();
if (code == 200) {
//得到服务端文件的大小
int length = conn.getContentLength();
//在本地创建空白文件并设置大小
RandomAccessFile raf = new RandomAccessFile("temp.exe","rw");
//3.计算自线程下载数据的范围
for (int threadId = 0; threadId < threadCount; threadId++) {
int blockSize = length / threadCount;
//4计算子线程下载数据的开始位置和结束位置
int startIndex = threadId * blockSize;
int endIndex = (threadId+1) * blockSize-1;
//判断是否是最后一个子线程，确定下载数据的结束位置
if (threadCount == threadCount - 1) {
endIndex = length - 1;
}
new ChildThreadDownload(path, startIndex, endIndex, threadId, threadCount).start();
}
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
package com.dominichan.multhreaddownload;
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 ChildThreadDownload extends Thread{
private String path;
private int startIndex;
private int endIndex;
private int threadId;
private int threadCount;
private static int runningThreadCount;
public ChildThreadDownload(String path, int startIndex, int endIndex, int threadId, int threadCount) {
this.path = path;
this.startIndex = startIndex;
this.endIndex = endIndex;
this.threadId = threadId;
this.threadCount = threadCount;
this.runningThreadCount = threadCount;
}
@Override
public void run() {
try {
URL url = new URL(path);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestMethod("GET");
conn.setConnectTimeout(3000);
//读取记录下载位置的文件
File file = new File(threadId+".txt");
if (file.exists() && file.length() > 0) {
FileInputStream fis = new FileInputStream(file);
InputStreamReader isr = new InputStreamReader(fis);
BufferedReader br = new BufferedReader(isr);
String currentPosition = br.readLine();
//让子线程从此位置开始下载
startIndex = Integer.parseInt(currentPosition);
}
//告诉服务器只请求这一部分数据
conn.setRequestProperty("Range", "bytes="+startIndex+"-"+endIndex);
int code = conn.getResponseCode();
System.out.print("线程" + threadId + "开始下载数据");
if (code == 206) { //206请求部分资源成功返回206
InputStream is = conn.getInputStream();
int len = -1;
byte[] buffer = new byte[1024 * 1024];
RandomAccessFile raf = new RandomAccessFile("temp.exe","rw");
//从制定的位置写数据
raf.seek(startIndex);
int total = 0;
while ((len = is.read(buffer)) != -1) {
raf.write(buffer, 0, len);
//计算本次下载的数据大小
total += len;
//计算当前下载到的位置
int currentPosition = startIndex + total;
//记录当前下载到的位置
RandomAccessFile r = new RandomAccessFile(threadId+".txt","rwd");
r.write((currentPosition+"").getBytes());
r.close();
}
raf.close();
is.close();
System.out.print("线程" + threadId + "下载数据完毕...");
synchronized (ChildThreadDownload.this) {
runningThreadCount--;
if (runningThreadCount == 0) {
System.out.print("文件下载完毕...");
}
}
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
只是说明了基本原理，抛砖引玉，欢迎各路大神拍砖~
