Android 中实现多线程下载和断点续传的原理和代码

在上一章,用Java代码实现了 多线程下载和断点续传, 现在我们将它移植到 Android 上面.

下面这个类能帮助我们实现多线程下载和断点续传功能.

介绍一下这个类的使用方法.

快速下载

1. 该类的构造方法需要提供三个参数.

2.

public MuchThreadDown(String path, String targetFilePath, int threadCount)

- 参数一 : 要下载的资源网络路径.

- 参数二 : 资源保存在手机上的路径

- 参数三 : 开启的线程个数.默认为 3.

3. 创建类的实例

MuchThreadDown threadDown = new MuchThreadDown(url_str, path, progress);

4. 调用类的 download() 方法.

5. 文件会被下载到指定的目录.

下载的过程中更新进度条

如果在下载的时候需要在页面展示进度条.

为每一个线程创建一个进度条控件,封装到

Map<Integer, ProgressBar>

map中的key代表线程的id,比如有当前开了三个线程, 线程的id就是0, 1, 2 进度条就需要我们动态的去创建了,将创建的进度条封装到map中.

threadDown.setProgressBar_map(progressBar_map);

设置给 MuchThreadDown 的对象.

获取线程的下载结果

1. 将handler对象设置给MuchThreadDown 的对象.

threadDown.setHandler(handler);

当一个线程下载完成的时候,返回 1 .

2. Message中封装的数据如下

3. `Message msg = Message.obtain();

msg.what = 66;
msg.obj = 1;
handler.sendMessage(msg);`

package com.yb.muchthreaddown.util;

import java.io.File;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.Map;

import android.os.Handler;
import android.os.Message;
import android.widget.ProgressBar;

/**
* 多线程下载 和 断点续传 for Android
* @author 杨斌.
*
*/
public class MuchThreadDown {

//  private String path = "http://mpge.5nd.com/2016/2016-11-15/74847/1.mp3";    //下载路径
private String path = "http://117.169.69.238/mp3.9ku.com/m4a/186947.m4a";
private String targetFilePath="/";  //下载文件存放目录
private int threadCount = 3;    //线程数量
public Map<Integer, ProgressBar> progressBar_map;//键 线程id, 值 进度条控件
private Handler handler;
/**
* 构造方法
* @param path 要下载文件的网络路径
* @param targetFilePath 保存下载文件的目录
* @param threadCount 开启的线程数量,默认为 3
*/
public MuchThreadDown(String path, String targetFilePath, int threadCount) {
this.path = path;
this.targetFilePath = targetFilePath;
this.threadCount = threadCount;
}

public Map<Integer, ProgressBar> getProgressBar_map() {
return progressBar_map;
}

public Handler getHandler() {
return handler;
}

public void setHandler(Handler handler) {
this.handler = handler;
}

public void setProgressBar_map(Map<Integer, ProgressBar> progressBar_map) {
this.progressBar_map = progressBar_map;
}

/**
* 下载文件
*/
public void download() throws Exception{
//连接资源
URL url = new URL(path);

HttpURLConnection connection = (HttpURLConnection) url.openConnection();
connection.setRequestMethod("GET");
connection.setConnectTimeout(10000);

int code = connection.getResponseCode();
if(code == 200){
//获取资源大小
int connectionLength = connection.getContentLength();
System.out.println(connectionLength);
//在本地创建一个与资源同样大小的文件来占位
RandomAccessFile randomAccessFile = new RandomAccessFile(new File(targetFilePath,getFileName(url)), "rw");
randomAccessFile.setLength(connectionLength);
/*
* 将下载任务分配给每个线程
*/
int blockSize = connectionLength/threadCount;//计算每个线程理论上下载的数量.
for(int threadId = 0; threadId < threadCount; threadId++){//为每个线程分配任务
int startIndex = threadId * blockSize; //线程开始下载的位置
int endIndex = (threadId+1) * blockSize -1; //线程结束下载的位置
if(threadId == (threadCount - 1)){  //如果是最后一个线程,将剩下的文件全部交给这个线程完成
endIndex = connectionLength - 1;
}

if(progressBar_map != null){
new DownloadThread(threadId, startIndex, endIndex, progressBar_map.get(threadId),handler).start();//开启线程下载
}else{
new DownloadThread(threadId, startIndex, endIndex, null,handler).start();//开启线程下载
}

}
//          randomAccessFile.close();
}

}

//下载的线程
private class DownloadThread extends Thread{

private int threadId;
private int startIndex;
private int endIndex;
private ProgressBar progressBar;
private int currentThreadTotal;//当前线程下载文件的总大小
private Handler handler;

public DownloadThread(int threadId, int startIndex, int endIndex, ProgressBar progressBar,Handler handler) {
this.threadId = threadId;
this.startIndex = startIndex;
this.endIndex = endIndex;
this.currentThreadTotal = endIndex - startIndex + 1;
this.progressBar = progressBar;
this.handler = handler;
}

@Override
public void run() {
System.out.println("线程"+ threadId + "开始下载");
try {
//分段请求网络连接,分段将文件保存到本地.
URL url = new URL(path);

//加载下载位置的文件
File downThreadFile = new File(targetFilePath, getFileName(url) +"_downThread_" + threadId+".dt");
RandomAccessFile downThreadStream = null;
if(downThreadFile.exists()){//如果文件存在
downThreadStream = new RandomAccessFile(downThreadFile,"rwd");
String startIndex_str = downThreadStream.readLine();
this.startIndex = Integer.parseInt(startIndex_str);//设置下载起点

}else{
downThreadStream = new RandomAccessFile(downThreadFile,"rwd");
}

HttpURLConnection connection = (HttpURLConnection) url.openConnection();
connection.setRequestMethod("GET");
connection.setConnectTimeout(10000);

//设置分段下载的头信息。  Range:做分段数据请求用的。格式: Range bytes=0-1024  或者 bytes:0-1024
connection.setRequestProperty("Range", "bytes="+ startIndex + "-" + endIndex);

System.out.println("线程_"+threadId + "的下载起点是 " + startIndex + "  下载终点是: " + endIndex);

if(connection.getResponseCode() == 206){//200：请求全部资源成功， 206代表部分资源请求成功
InputStream inputStream = connection.getInputStream();//获取流
RandomAccessFile randomAccessFile = new RandomAccessFile(
new File(targetFilePath,getFileName(url)), "rw");//获取前面已创建的文件.
randomAccessFile.seek(startIndex);//文件写入的开始位置.

/*
* 将网络流中的文件写入本地
*/
byte[] buffer = new byte[1024*100];
int length = -1;
int total = 0;//记录本次下载文件的大小
boolean flag = false;
if(progressBar != null){
flag = true;
progressBar.setMax(currentThreadTotal);
}
while((length = inputStream.read(buffer)) > 0){
randomAccessFile.write(buffer, 0, length);
total += length;
/*
* 将当前现在到的位置保存到文件中
*/
int currentThreadPosition = startIndex + total; //当前文件下载位置.
downThreadStream.seek(0);
downThreadStream.write((currentThreadPosition + "").getBytes("UTF-8"));

//设置进度条数据.
if(flag){
progressBar.setProgress(currentThreadTotal - (endIndex - currentThreadPosition));
}
}
if(handler != null){
Message msg = Message.obtain();
msg.what = 66;
msg.obj = 1;
handler.sendMessage(msg);
}
downThreadStream.close();
inputStream.close();
randomAccessFile.close();
cleanTemp(downThreadFile);//删除临时文件
System.out.println("线程"+ threadId + "下载完毕");
}else{
System.out.println("响应码是" +connection.getResponseCode() + ". 服务器不支持多线程下载");
}
} catch (Exception e) {
e.printStackTrace();
}

}
}

//删除线程产生的临时文件
private synchronized void cleanTemp(File file){
file.delete();
}

//获取下载文件的名称
private String getFileName(URL url){
String filename = url.getFile();
return filename.substring(filename.lastIndexOf("/")+1);
}

public static void main(String[] args) {
try {
new MuchThreadDown(null, null, 3).download();

} catch (Exception e) {
e.printStackTrace();
}
}

}