java文件的多线程断点续传大致原理，供大家参考，具体内容如下
谈到文件断点续传那么就离不开java.io.RandomAcessFile HttpUrlConnection类
大致思路如下：
1、HttpUrlConnection去请求服务器 获得文件的长度con.getContentLength()
2、创建一个空的RandomAcessFile来接收，并且指定刚刚获取的长度setLength
3、开启N个线程 计算每个线程需要下载的长度
4、获取之前先去看看下载的进度保存文件是否存在 如果存在就从文件里获取已经下载的进度
5、开始文件下载
6、临时文件的删除 资源的关闭
下面贴出完整代码
package demo;
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;
class MultiDownloaFile {
public static final String path = "http://192.168.217.1:8080/androidsimpleserver/HTTP.pdf";
public static final int TOTAL_THREAD_COUNT = 3;
public static int runningThreadCount = 0;
public static void main(String[] args) {
try {
long start = System.currentTimeMillis();
URL url = new URL(path);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestMethod("GET");
int code = conn.getResponseCode();
if (code == 200) {
int length = conn.getContentLength();
System.out.println("file length:" + length);
//create a null file to save its length
RandomAccessFile raf = new RandomAccessFile(getDownloadFileName(path), "rw");
raf.setLength(length);
raf.close();
//parse every thread that they need how much room to download
int blockSize = length / TOTAL_THREAD_COUNT;
System.out.println("every block size:" + blockSize);
runningThreadCount = TOTAL_THREAD_COUNT;
for (int threadId = 0; threadId < TOTAL_THREAD_COUNT; threadId++) {
int startPosition = threadId * blockSize;
int endPosition = (threadId + 1) * blockSize - 1;
if (threadId == (TOTAL_THREAD_COUNT - 1)) {
endPosition = length - 1;
}
System.out.println("thread::" + threadId + " download range:" + startPosition + "~~" + endPosition);
//start thread to download
new DownloadThread(threadId, startPosition, endPosition).start();
}
} else {
System.out.println(" connection error ");
}
} catch (Exception e) {
e.printStackTrace();
}
}
/**
* 从网络路径获取文件名
*
* @param path 网络路径
* @return 文件名
*/
private static String getDownloadFileName(String path) {
return path.substring(path.lastIndexOf("/") + 1);
}
/**
* 下载文件的线程
*/
private static class DownloadThread extends Thread {
/**
* 线程id
*/
private int threadId;
/**
* 当前线程下载的起始位置
*/
private int startPosition;
/**
* 当前线程下载的终止位置
*/
private int endPosition;
public DownloadThread(int threadId, int startPosition, int endPosition) {
this.threadId = threadId;
this.startPosition = startPosition;
this.endPosition = endPosition;
}
@Override
public void run() {
System.out.println("thread:" + threadId + " begin working");
// lest thread download it's self range data
try {
File finfo = new File(TOTAL_THREAD_COUNT + getDownloadFileName(path) + threadId + ".txt");
// 断点续传
if (finfo.exists() && finfo.length() > 0) {
System.out.println(" 断点续传开始");
FileInputStream fis = new FileInputStream(finfo);
BufferedReader br = new BufferedReader(new InputStreamReader(fis));
String lastPosition = br.readLine();
// This thread download data before times;
int intLastPosition = Integer.parseInt(lastPosition);
startPosition = intLastPosition;
fis.close();
}
URL url = new URL(path);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestMethod("GET");
System.out.println("begin and end:" + threadId + " range of download: " + startPosition + "~~" + endPosition);
conn.setRequestProperty("Range", "bytes=" + startPosition + "-" + endPosition);
// Download Resource from server
int code = conn.getResponseCode();
if (code == 206) {
InputStream is = conn.getInputStream();
// RandomAccessFile raf = new RandomAccessFile(getDownloadFileName(path), "rw");
RandomAccessFile raf = new RandomAccessFile(getDownloadFileName(path), "rw");
// vary important, position of begin to write
raf.seek(startPosition);
byte[] buffer = new byte[1024 * 100];
int len = -1;
int total = 0; // downloaded data of current thread in this times;
while ((len = is.read(buffer)) != -1) {
raf.write(buffer, 0, len);
// record position of current thread to downloading
total += len;
RandomAccessFile inforaf = new RandomAccessFile(TOTAL_THREAD_COUNT + getDownloadFileName(path) + threadId + ".txt", "rwd");
// save position of current thread
inforaf.write(String.valueOf(startPosition + total).getBytes());
inforaf.close();
}
is.close();
raf.close();
System.out.println("thread:" + threadId + " download complete...");
} else {
System.out.println("request download failed.");
}
} catch (Exception e) {
e.printStackTrace();
} finally {
synchronized (MultiDownloaFile.class) {
runningThreadCount--;
if (runningThreadCount <= 0) {
System.out.println(" all multi thread download complete. success!!!");
for (int i = 0; i < TOTAL_THREAD_COUNT; i++) {
File finfo = new File(TOTAL_THREAD_COUNT + getDownloadFileName(path) + i + ".txt");
System.out.println(finfo.delete());
}
}
}
}
}
}
}
