版权声明：本文为博主原创文章，转载请标明出处。 https://blog.csdn.net/chaoyu168/article/details/50971687
多线程下载文件的过程是：
(1)首先获得下载文件的长度，然后设置本地文件的长度。
HttpURLConnection.getContentLength();//获取下载文件的长度
RandomAccessFile file = new RandomAccessFile("QQSetup.exe","rwd");
file.setLength(filesize);//设置本地文件的长度
(2)根据文件长度和线程数计算每条线程下载的数据长度和下载位置。
如：文件的长度为6M，线程数为3，那么，每条线程下载的数据长度为2M，每条线程开始下载的位置如下图所示。
例如10M大小，使用3个线程来下载，
线程下载的数据长度 (10%3 == 0 ？ 10/3:10/3+1) ,第1，2个线程下载长度是4M，第三个线程下载长度为2M
下载开始位置：线程id*每条线程下载的数据长度 = ?
下载结束位置：（线程id+1）*每条线程下载的数据长度-1=?
(3)使用Http的Range头字段指定每条线程从文件的什么位置开始下载，下载到什么位置为止，
如：指定从文件的2M位置开始下载，下载到位置(4M-1byte)为止
代码如下：HttpURLConnection.setRequestProperty("Range", "bytes=2097152-4194303");
(4)保存文件，使用RandomAccessFile类指定每条线程从本地文件的什么位置开始写入数据。
RandomAccessFile threadfile = new RandomAccessFile("QQWubiSetup.exe ","rwd");
threadfile.seek(2097152);//从文件的什么位置开始写入数据
/**
*多线程下载，UI更新类
*@author young
* */
public class MultiThreadDownload extends Thread{
private static final String TAG = "MultiThreadDownload";
/**每一个线程需要下载的大小 */
private int blockSize;
/*** 线程数量<br> 默认为5个线程下载*/
private int threadNum = 5;
/*** 文件大小 */
private int fileSize;
/** * 已经下载多少 */
private int downloadSize;
/**文件的url,线程编号，文件名称*/
private String UrlStr,ThreadNo,fileName;
/***保存的路径*/
private String savePath;
/**下载的百分比*/
private int downloadPercent = 0;
/**下载的 平均速度*/
private int downloadSpeed = 0;
/**下载用的时间*/
private int usedTime = 0;
/**当前时间*/
private long curTime;
/**是否已经下载完成*/
private boolean completed = false;
private Handler handler ;
/**
* 下载的构造函数
* @param url 请求下载的URL
* @param handler UI更新使用
* @param savePath 保存文件的路径
*/
public MultiThreadDownload(Handler handler,String url,String savePath)
{
this.handler = handler;
this.UrlStr = url;
this.savePath = savePath;
Log.e(TAG, toString());
}
@Override
public void run() {
FileDownloadThread[] fds = new FileDownloadThread[threadNum];//设置线程数量
try {
URL url = new URL(UrlStr);
URLConnection conn = url.openConnection();
fileSize = conn.getContentLength();
this.fileName = FileUtil.getFileName(UrlStr);
//只创建一个文件，saveFile下载内容
File saveFile = new File(savePath+"/"+fileName);
Log.e(TAG, "文件一共："+fileSize+" savePath "+savePath+" fileName "+fileName);
RandomAccessFile accessFile = new RandomAccessFile(saveFile,"rwd");
//设置本地文件的长度和下载文件相同
accessFile.setLength(fileSize);
accessFile.close();
//Handler更新UI，发送消息
sendMsg(FileUtil.startDownloadMeg);
//每块线程下载数据
blockSize = ((fileSize%threadNum)==0)?(fileSize/threadNum):(fileSize/threadNum+1);
Log.e(TAG, "每个线程分别下载 ："+blockSize);
for (int i = 0; i < threadNum; i++) {
int curThreadEndPosition = (i+1)!=threadNum ? ((i+1)*blockSize-1) : fileSize;
FileDownloadThread fdt = new FileDownloadThread(url, saveFile, i*blockSize, curThreadEndPosition);
fdt.setName("thread"+i);
fdt.start();
fds[i]=fdt;
}
/**
* 获取数据，更新UI，直到所有下载线程都下载完成。
*/
boolean finished = false;
//开始时间，放在循环外，求解的usedTime就是总时间
long startTime = System.currentTimeMillis();
while(!finished)
{
downloadSize = 0;
finished = true;
for (int i = 0; i < fds.length; i++) {
downloadSize+= fds[i].getDownloadSize();
if(!fds[i].isFinished())
{
finished = false;
}
}
downloadPercent = (downloadSize*100)/fileSize;
curTime = System.currentTimeMillis();
System.out.println("curTime = "+curTime+" downloadSize = "+downloadSize+" usedTime "+(int) ((curTime-startTime)/1000));
usedTime = (int) ((curTime-startTime)/1000);
if(usedTime==0)usedTime = 1;
downloadSpeed = (downloadSize/usedTime)/1024;
sleep(1000);/*1秒钟刷新一次界面*/
sendMsg(FileUtil.updateDownloadMeg);
}
Log.e(TAG, "下载完成");
completed = true;
sendMsg(FileUtil.endDownloadMeg);
} catch (Exception e) {
Log.e(TAG, "multi file error Exception "+e.getMessage());
e.printStackTrace();
}
super.run();
}
/**
* 得到文件的大小
* @return
*/
public int getFileSize()
{
return this.fileSize;
}
/**
* 得到已经下载的数量
* @return
*/
public int getDownloadSize()
{
return this.downloadSize;
}
/**
* 获取下载百分比
* @return
*/
public int getDownloadPercent(){
return this.downloadPercent;
}
/**
* 获取下载速度
* @return
*/
public int getDownloadSpeed(){
return this.downloadSpeed;
}
/**
* 修改默认线程数
* @param threadNum
*/
public void setThreadNum(int threadNum){
this.threadNum = threadNum;
}
/**
* 分块下载完成的标志
* @return
*/
public boolean isCompleted(){
return this.completed;
}
@Override
public String toString() {
return "MultiThreadDownload [threadNum=" + threadNum + ", fileSize="
+ fileSize + ", UrlStr=" + UrlStr + ", ThreadNo=" + ThreadNo
+ ", savePath=" + savePath + "]";
}
/**
* 发送消息，用户提示
* */
private void sendMsg(int what)
{
Message msg = new Message();
msg.what = what;
handler.sendMessage(msg);
} public class FileDownloadThread extends Thread{
private static final String TAG = "FileDownloadThread";
/**缓冲区 */
private static final int BUFF_SIZE = 1024;
/**需要下载的URL*/
private URL url;
/**缓存的FIle*/
private File file;
/**开始位置*/
private int startPosition;
/**结束位置*/
private int endPosition;
/**当前位置*/
private int curPosition;
/**完成*/
private boolean finished = false;
/**已经下载多少*/
private int downloadSize = 0;
/***
* 分块文件下载，可以创建多线程模式
* @param url 下载的URL
* @param file 下载的文件
* @param startPosition 开始位置
* @param endPosition 结束位置
*/
public FileDownloadThread(URL url, File file, int startPosition,
int endPosition) {
this.url = url;
this.file = file;
this.startPosition = startPosition;
this.curPosition = startPosition;
this.endPosition = endPosition;
Log.e(TAG, toString());
}
@Override
public void run() {
BufferedInputStream bis = null;
RandomAccessFile rAccessFile = null;
byte[] buf = new byte[BUFF_SIZE];
URLConnection conn = null;
try {
conn = url.openConnection();
conn.setConnectTimeout(10000);//设置超时
conn.setReadTimeout(10000);
conn.setAllowUserInteraction(true);
System.out.println(this.getName()+" startPosition "+startPosition+" endPosition "+endPosition);
conn.setRequestProperty("Range", "bytes="+(startPosition)+"-"+endPosition); //取剩余未下载的
rAccessFile = new RandomAccessFile(file,"rwd");//读写
//设置从什么位置开始写入数据
rAccessFile.seek(startPosition);
bis = new BufferedInputStream(conn.getInputStream(), BUFF_SIZE);
while(curPosition<endPosition) //当前位置小于结束位置 继续下载
{
int len = bis.read(buf,0,BUFF_SIZE);
if(len==-1) //下载完成
{
break;
}
rAccessFile.write(buf,0,len);
curPosition = curPosition +len;
if(curPosition > endPosition)
{ //如果下载多了，则减去多余部分
System.out.println(" curPosition > endPosition !!!!");
int extraLen = curPosition-endPosition;
downloadSize += (len-extraLen+1);
}else{
downloadSize+=len;
}
}
this.finished = true; //当前阶段下载完成
Log.e(TAG, "当前"+this.getName()+"下载完成");
} catch (Exception e) {
Log.e(TAG, "download error Exception "+e.getMessage());
e.printStackTrace();
}finally{
//关闭流
FileUtil.closeInputStream(bis);
try {
rAccessFile.close();
} catch (IOException e) {
// TODO Auto-generated catch block
Log.e("AccessFile", "AccessFile IOException "+e.getMessage());
}
}
super.run();
}
/**
* 是否完成当前段下载完成
* @return
*/
public boolean isFinished() {
return finished;
}
/**
* 已经下载多少
* @return
*/
public int getDownloadSize() {
return downloadSize;
}
@Override
public String toString() {
return "FileDownloadThread [url=" + url + ", file=" + file
+ ", startPosition=" + startPosition + ", endPosition="
+ endPosition + ", curPosition=" + curPosition + ", finished="
+ finished + ", downloadSize=" + downloadSize + "]";
}
多线程下载是分段下载，创建保存一个文件，子线程分别通过RandomAccessFile类进行写入操作。
示例源码：demo下载
