Java实现的断点续传功能的示例代码

importjava.io.File;

importjava.io.FileNotFoundException;

importjava.io.IOException;

importjava.io.InputStream;

importjava.io.RandomAccessFile;

importjava.net.HttpURLConnection;

importjava.net.MalformedURLException;

importjava.net.URL;

importjava.util.concurrent.CountDownLatch;

importjava.util.concurrent.ExecutorService;

importjava.util.concurrent.Executors;

/*

* Encode:UTF-8

*

* Author:zhiming.xu

*

* 多线程的断点下载程序，根据输入的url和指定线程数，来完成断点续传功能。

*

* 每个线程支负责某一小段的数据下载；再通过RandomAccessFile完成数据的整合。

*/

public class MultiTheradDownLoad {

private String filepath = null;

private String filename = null;

private String tmpfilename = null;

private int threadNum = 0;

private CountDownLatch latch = null;//设置一个计数器，代码内主要用来完成对缓存文件的删除

private long fileLength = 0l;

private long threadLength = 0l;

private long[] startPos;//保留每个线程下载数据的起始位置。

private long[] endPos;//保留每个线程下载数据的截止位置。

private boolean bool = false;

private URL url = null;

//有参构造函数，先构造需要的数据

public MultiTheradDownLoad(String filepath, int threadNum) {

this.filepath = filepath;

this.threadNum = threadNum;

startPos = new long[this.threadNum];

endPos = new long[this.threadNum];

latch = new CountDownLatch(this.threadNum);

}

/*

* 组织断点续传功能的方法

*/

public void downloadPart() {

File file = null;

File tmpfile = null;

HttpURLConnection httpcon = null;

//在请求url内获取文件资源的名称；此处没考虑文件名为空的情况，此种情况可能需使用UUID来生成一个唯一数来代表文件名。

filename = filepath.substring(filepath.lastIndexOf('/') + 1, filepath

.contains("?") ? filepath.lastIndexOf('?') : filepath.length());

tmpfilename = filename + "_tmp";

try {

url = new URL(filepath);

httpcon = (HttpURLConnection) url.openConnection();

setHeader(httpcon);

fileLength = httpcon.getContentLengthLong();//获取请求资源的总长度。

file = new File(filename);

tmpfile = new File(tmpfilename);

threadLength = fileLength / threadNum;//每个线程需下载的资源大小。

System.out.println("fileName: " + filename + " ," + "fileLength= "

+ fileLength + " the threadLength= " + threadLength);

if (file.exists() && file.length() == fileLength) {

System.out

.println("the file you want to download has exited!!");

return;

} else {

setBreakPoint(startPos, endPos, tmpfile);

ExecutorService exec = Executors.newCachedThreadPool();

for (int i = 0; i

exec.execute(new DownLoadThread(startPos[i], endPos[i],

this, i, tmpfile, latch));

}

latch.await();//当你的计数器减为0之前，会在此处一直阻塞。

exec.shutdown();

}

} catch (MalformedURLException e) {

e.printStackTrace();

} catch (IOException e) {

e.printStackTrace();

} catch (InterruptedException e) {

e.printStackTrace();

}

if (file.length() == fileLength) {

if (tmpfile.exists()) {

System.out.println("delect the temp file!!");

tmpfile.delete();

}

}

}

/*

* 断点设置方法，当有临时文件时，直接在临时文件中读取上次下载中断时的断点位置。没有临时文件，即第一次下载时，重新设置断点。

*

* rantmpfile.seek()跳转到一个位置的目的是为了让各个断点存储的位置尽量分开。

*

* 这是实现断点续传的重要基础。

*/

private void setBreakPoint(long[] startPos, long[] endPos, File tmpfile) {

RandomAccessFile rantmpfile = null;

try {

if (tmpfile.exists()) {

System.out.println("the download has continued!!");

rantmpfile = new RandomAccessFile(tmpfile, "rw");

for (int i = 0; i

rantmpfile.seek(8 * i + 8);

startPos[i] = rantmpfile.readLong();

rantmpfile.seek(8 * (i + 1000) + 16);

endPos[i] = rantmpfile.readLong();

System.out.println("the Array content in the exit file: ");

System.out.println("thre thread" + (i + 1) + " startPos:"

+ startPos[i] + ", endPos: " + endPos[i]);

}

} else {

System.out.println("the tmpfile is not available!!");

rantmpfile = new RandomAccessFile(tmpfile, "rw");

//最后一个线程的截止位置大小为请求资源的大小

for (int i = 0; i

startPos[i] = threadLength * i;

if (i == threadNum - 1) {

endPos[i] = fileLength;

} else {

endPos[i] = threadLength * (i + 1) - 1;

}

rantmpfile.seek(8 * i + 8);

rantmpfile.writeLong(startPos[i]);

rantmpfile.seek(8 * (i + 1000) + 16);

rantmpfile.writeLong(endPos[i]);

System.out.println("the Array content: ");

System.out.println("thre thread" + (i + 1) + " startPos:"

+ startPos[i] + ", endPos: " + endPos[i]);

}

}

} catch (FileNotFoundException e) {

e.printStackTrace();

} catch (IOException e) {

e.printStackTrace();

} finally {

try {

if (rantmpfile != null) {

rantmpfile.close();

}

} catch (IOException e) {

e.printStackTrace();

}

}

}

/*

* 实现下载功能的内部类，通过读取断点来设置向服务器请求的数据区间。

*/

class DownLoadThread implements Runnable {

private long startPos;

private long endPos;

private MultiTheradDownLoad task = null;

private RandomAccessFile downloadfile = null;

private int id;

private File tmpfile = null;

private RandomAccessFile rantmpfile = null;

private CountDownLatch latch = null;

public DownLoadThread(long startPos, long endPos,

MultiTheradDownLoad task, int id, File tmpfile,

CountDownLatch latch) {

this.startPos = startPos;

this.endPos = endPos;

this.task = task;

this.tmpfile = tmpfile;

try {

this.downloadfile = new RandomAccessFile(this.task.filename,

"rw");

this.rantmpfile = new RandomAccessFile(this.tmpfile, "rw");

} catch (FileNotFoundException e) {

e.printStackTrace();

}

this.id = id;

this.latch = latch;

}

@Override

public void run() {

HttpURLConnection httpcon = null;

InputStream is = null;

int length = 0;

System.out.println("the thread " + id + " has started!!");

while (true) {

try {

httpcon = (HttpURLConnection) task.url.openConnection();

setHeader(httpcon);

//防止网络阻塞，设置指定的超时时间；单位都是ms。超过指定时间，就会抛出异常

httpcon.setReadTimeout(20000);//读取数据的超时设置

httpcon.setConnectTimeout(20000);//连接的超时设置

if (startPos

//向服务器请求指定区间段的数据，这是实现断点续传的根本。

httpcon.setRequestProperty("Range", "bytes=" + startPos

+ "-" + endPos);

System.out

.println("Thread " + id

+ " the total size:---- "

+ (endPos - startPos));

downloadfile.seek(startPos);

if (httpcon.getResponseCode() != HttpURLConnection.HTTP_OK

&& httpcon.getResponseCode() != HttpURLConnection.HTTP_PARTIAL) {

this.task.bool = true;

httpcon.disconnect();

downloadfile.close();

System.out.println("the thread ---" + id

+ " has done!!");

latch.countDown();//计数器自减

break;

}

is = httpcon.getInputStream();//获取服务器返回的资源流

long count = 0l;

byte[] buf = new byte[1024];

while (!this.task.bool && (length = is.read(buf)) != -1) {

count += length;

downloadfile.write(buf, 0, length);

//不断更新每个线程下载资源的起始位置，并写入临时文件；为断点续传做准备

startPos += length;

rantmpfile.seek(8 * id + 8);

rantmpfile.writeLong(startPos);

}

System.out.println("the thread " + id

+ " total load count: " + count);

//关闭流

is.close();

httpcon.disconnect();

downloadfile.close();

rantmpfile.close();

}

latch.countDown();//计数器自减

System.out.println("the thread " + id + " has done!!");

break;

} catch (IOException e) {

e.printStackTrace();

} finally {

try {

if (is != null) {

is.close();

}

} catch (IOException e) {

e.printStackTrace();

}

}

}

}

}

/*

* 为一个HttpURLConnection模拟请求头，伪装成一个浏览器发出的请求

*/

privatevoidsetHeader(HttpURLConnection con) {

con.setRequestProperty(

"User-Agent",

"Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.0.3) Gecko/2008092510 Ubuntu/8.04 (hardy) Firefox/3.0.3");

con.setRequestProperty("Accept-Language","en-us,en;q=0.7,zh-cn;q=0.3");

con.setRequestProperty("Accept-Encoding","aa");

con.setRequestProperty("Accept-Charset",

"ISO-8859-1,utf-8;q=0.7,*;q=0.7");

con.setRequestProperty("Keep-Alive","300");

con.setRequestProperty("Connection","keep-alive");

con.setRequestProperty("If-Modified-Since",

"Fri, 02 Jan 2009 17:00:05 GMT");

con.setRequestProperty("If-None-Match",""1261d8-4290-df64d224"");

con.setRequestProperty("Cache-Control","max-age=0");

con.setRequestProperty("Referer",

"http://www.*sky*c*n.com/soft/14857.html");

}

}

publicclassDownLoadTest {

/**

* @param args

*/

publicstaticvoidmain(String[] args) {

String filepath ="http://127.*0.**0.1:8080/file/loadfile.mkv";

MultiTheradDownLoad load =newMultiTheradDownLoad(filepath ,4);

load.downloadPart();

}

}