上一篇文章用了多线程下载技术,那么这篇文章来说一说续传的问题.
其实,续传很简单,只要把上次程序退出的位置保存起来,启动的时候如果存在配置文件,那么久读取,并且把startIndex设置成相应的值就好了.
但是还有一个问题就是传统的文件rw打开模式是先把文件写入缓冲区,然后缓冲区满了之后再写到磁盘文件,那么如果我们关掉程序的时候缓冲区的内容没被写到磁盘里面,那么就会造成文件的部分字节丢失.还是很幸运,
RandomAccessFile类提供了一种实时写到磁盘的打开方式,下面开始编码吧。
package snippet;
import java.io.File;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;
public class MutiDownload {
private static final int THREAD_COUNT = 5;
private static final String DOWNLOAD_URL = "http://s1.music.126.net/download/osx/NeteaseMusic_1.4.3_452_web.dmg";
private static final String fileName = "/Users/August/Desktop/NeteaseMusic_1.4.3_452_web.dmg";
public static void main(String[] args) {
long fileSize;
HttpURLConnection connection = null;
try {
connection = (HttpURLConnection) new URL(DOWNLOAD_URL).openConnection();
connection.setRequestMethod("GET");
connection.setConnectTimeout(8000);
connection.setReadTimeout(8000);
if (connection.getResponseCode() == 200) {
fileSize = connection.getContentLength();
long eachSize = fileSize / THREAD_COUNT;
RandomAccessFile raf = new RandomAccessFile(fileName, "rwd");
raf.setLength(fileSize);
raf.close();
for (int i = 0; i < THREAD_COUNT; i++) {
long startIndex = i * eachSize;
long endIndex = (i + 1) * eachSize - 1;
if (i == THREAD_COUNT - 1) {
endIndex = fileSize;
}
new DownloadThread(DOWNLOAD_URL, fileName, i, startIndex, endIndex).start();
}
}
} catch (Exception e) {
e.printStackTrace();
} finally {
if (connection != null) {
connection.disconnect();
connection = null;
}
}
}
}
class DownloadThread extends Thread {
private String url;
private String fileName;
private int threadID;
private long startIndex;
private long endIndex;
private HttpURLConnection connection;
private RandomAccessFile raf;
private InputStream inputStream;
/**
* 进度文件
*/
private RandomAccessFile progressRaf;
public DownloadThread(String url, String fileName, int threadID, long startIndex, long endIndex) {
super();
this.url = url;
this.fileName = fileName;
this.threadID = threadID;
this.startIndex = startIndex;
this.endIndex = endIndex;
}
@Override
public void run() {
try {
HttpURLConnection connection = (HttpURLConnection) new URL(url + "?ts=" + System.currentTimeMillis())
.openConnection();
connection.setRequestMethod("GET");
connection.setConnectTimeout(8000);
connection.setReadTimeout(8000);
connection.setRequestProperty("RANGE", "bytes=" + startIndex + "-" + endIndex);
File progressFile = new File(String.valueOf(threadID));
if (progressFile.exists()) {
progressRaf = new RandomAccessFile(progressFile, "rwd");
} else {
progressFile.createNewFile();
progressRaf = new RandomAccessFile(progressFile, "rwd");
progressRaf.write(String.valueOf(startIndex).getBytes());
}
/**
* 判断进度文件是否存在,不存在则创建,并且存入startIndex的值
*/
progressRaf.seek(0);
startIndex = Long.valueOf(progressRaf.readLine());
/**
* 这时进度文件一定存在,就读取上次结束为止 若为第一次下载,读取的依旧是startIndex的值
*/
if (connection.getResponseCode() == 206) {
inputStream = connection.getInputStream();
byte[] bs = new byte[1024];
int len;
raf = new RandomAccessFile(fileName, "rwd");
raf.seek(startIndex);
long total = 0;
long position = startIndex + total;
while ((len = inputStream.read(bs)) != -1) {
total += len;
System.out.println("线程" + threadID + ":" + total);
raf.write(bs, 0, len);
progressRaf.seek(0);
progressRaf.write(String.valueOf(position).getBytes());
/**
* 实时同步进度 必须写到raf.write(bs, 0, len);之后.
* 因为如果先写到进度再去写目标文件的话,那么当写完进度后被停掉程序,目标文件那部分是没有数据的
* 下次续传的时候还是从上次结束那里开始的话,数据依旧是丢失的
*
* 考虑若先同步目标文件再去同步进度文件,即使同步文目标文件程序被停掉,
* 那么下次续传顶多就把没有写入进度那部分再重新下载一次.对整个程序影响不大
*/
}
progressFile.delete();
/**
* 下载完毕就把进度文件删除
*/
}
} catch (Exception e) {
e.printStackTrace();
} finally {
try {
if (connection != null) {
connection.disconnect();
connection = null;
}
if (raf != null) {
raf.close();
raf = null;
}
if (inputStream != null) {
inputStream.close();
inputStream = null;
}
if (progressRaf != null) {
progressRaf.close();
progressRaf = null;
}
} catch (Exception e2) {
e2.printStackTrace();
}
}
}
}