详解Android中的多线程断点下载

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
 xmlns:tools="http://schemas.android.com/tools"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:orientation="vertical"
android:paddingBottom="@dimen/activity_vertical_margin" android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
 tools:context=".MainActivity" >
 <EditText
 android:id="@+id/et_path"
 android:layout_width="match_parent"
 android:layout_height="wrap_content"
 android:hint="请输入要下载的文件资源路径"
android:text="http://192.168.1.104:8080/gg.exe" />
 <Button
 android:layout_width="match_parent"
 android:layout_height="wrap_content"
 android:onClick="download"
 android:text="下载" />
 <ProgressBar
 android:id="@+id/pb0"
 style="?android:attr/progressBarStyleHorizontal"
 android:layout_width="match_parent"
 android:layout_height="wrap_content" />
 <ProgressBar
 android:id="@+id/pb1"
 style="?android:attr/progressBarStyleHorizontal"
 android:layout_width="match_parent"
 android:layout_height="wrap_content" />
 <ProgressBar
 android:id="@+id/pb2"
 style="?android:attr/progressBarStyleHorizontal"
 android:layout_width="match_parent"
 android:layout_height="wrap_content" />
</LinearLayout>

 public class MainActivity extends Activity {
 private EditText et_path;
 private ProgressBar pb0;
 private ProgressBar pb1;
 private ProgressBar pb2;
 /**
 * 开启几个线程从服务器下载数据
 */
 public static int threadCount = 3;
 public static int runningThreadCount;
 private String path;
 @Override
 protected void onCreate(Bundle savedInstanceState) {
 super.onCreate(savedInstanceState);
 setContentView(R.layout.activity_main);
 //初始化控件
 et_path = (EditText) findViewById(R.id.et_path);
 pb0 = (ProgressBar) findViewById(R.id.pb0);
 pb1 = (ProgressBar) findViewById(R.id.pb1);
 pb2 = (ProgressBar) findViewById(R.id.pb2);
 }
 //下载按钮的点击事件
 public void download(View view) {
 path = et_path.getText().toString().trim();
 if (TextUtils.isEmpty(path) || (!path.startsWith("http://"))) {
 Toast.makeText(this, "对不起路径不合法", 0).show();
 return;
 }
 new Thread(){
 public void run() {
 try {
 //1.获取服务器上的目标文件的大小
 URL url = new URL(path);
 HttpURLConnection conn = (HttpURLConnection) url.openConnection();
 conn.setConnectTimeout(5000);
 conn.setRequestMethod("GET");
 int code = conn.getResponseCode();
 if (code == 200) {
 int length = conn.getContentLength();
 System.out.println("服务器文件的长度为:" + length);
 //2.在本地创建一个跟原始文件同样大小的文件
 RandomAccessFile raf = new RandomAccessFile(Environment.getExternalStorageDirectory().getAbsolutePath()+"/"+getFileName(path), "rw");
 raf.setLength(length);
 raf.close();
 //3.计算每个线程下载的起始位置和结束位置
 int blocksize = length / threadCount;
 runningThreadCount = threadCount;
 for (int threadId = 0; threadId < threadCount; threadId++) {
 int startIndex = threadId * blocksize;
 int endIndex = (threadId + 1) * blocksize - 1;
 if (threadId == (threadCount - 1)) {
 endIndex = length - 1;
 }
 //4.开启多个子线程开始下载
 new DownloadThread(threadId, startIndex, endIndex).start();
 }
 }
 } catch (Exception e) {
 e.printStackTrace();
 }
 };
 }.start();
 }
 private class DownloadThread extends Thread {
 /**
 * 线程id
 */
 private int threadId;
 /**
 * 线程下载的理论开始位置
 */
 private int startIndex;
 /**
 * 线程下载的结束位置
 */
 private int endIndex;
 /**
 * 当前线程下载到文件的那一个位置了.
 */
 private int currentPosition;
 public DownloadThread(int threadId, int startIndex, int endIndex) {
 this.threadId = threadId;
 this.startIndex = startIndex;
 this.endIndex = endIndex;
 System.out.println(threadId + "号线程下载的范围为:" + startIndex
 + " ~~ " + endIndex);
 currentPosition = startIndex;
 }
 @Override
 public void run() {
 try {
 URL url = new URL(path);
 HttpURLConnection conn = (HttpURLConnection) url.openConnection();
 //检查当前线程是否已经下载过一部分的数据了 
 File info = new File(Environment.getExternalStorageDirectory().getAbsolutePath()+"/"+threadId+".position");
 RandomAccessFile raf = new RandomAccessFile(Environment.getExternalStorageDirectory().getAbsolutePath()+"/"+getFileName(path), "rw");
 if(info.exists()&&info.length()>0){
 FileInputStream fis = new FileInputStream(info);
 BufferedReader br = new BufferedReader(new InputStreamReader(fis));
 currentPosition = Integer.valueOf(br.readLine());
 conn.setRequestProperty("Range", "bytes="+currentPosition+"-"+endIndex);
 System.out.println("原来有下载进度,从上一次终止的位置继续下载"+"bytes="+currentPosition+"-"+endIndex);
 fis.close();
 raf.seek(currentPosition);//每个线程写文件的开始位置都是不一样的.
 }else{
 //告诉服务器 只想下载资源的一部分
 conn.setRequestProperty("Range", "bytes="+startIndex+"-"+endIndex);
 System.out.println("原来没有有下载进度,新的下载"+ "bytes="+startIndex+"-"+endIndex);
 raf.seek(startIndex);//每个线程写文件的开始位置都是不一样的.
 }
 InputStream is = conn.getInputStream();
 byte[] buffer = new byte[1024];
 int len = -1;
 while((len = is.read(buffer))!=-1){
 //把每个线程下载的数据放在自己的空间里面.
// System.out.println("线程:"+threadId+"正在下载:"+new String(buffer));
 raf.write(buffer,0, len);
 //5.记录下载进度
 currentPosition+=len;
 File file = new File(Environment.getExternalStorageDirectory().getAbsolutePath()+"/"+threadId+".position");
 RandomAccessFile fos = new RandomAccessFile(file,"rwd");
 //System.out.println("线程:"+threadId+"写到了"+currentPosition);
 fos.write(String.valueOf(currentPosition).getBytes());
 fos.close();//fileoutstream数据是不一定被写入到底层设备里面的,有可能是存储在缓存里面.
 //raf 的 rwd模式,数据是立刻被存储到底层硬盘设备里面.
 //更新进度条的显示
 int max = endIndex - startIndex;
 int progress = currentPosition - startIndex;
 if(threadId==0){
 pb0.setMax(max);
 pb0.setProgress(progress);
 }else if(threadId==1){
 pb1.setMax(max);
 pb1.setProgress(progress);
 }else if(threadId==2){
 pb2.setMax(max);
 pb2.setProgress(progress);
 }
 }
 raf.close();
 is.close();
 System.out.println("线程:"+threadId+"下载完毕了...");
 File f = new File(Environment.getExternalStorageDirectory().getAbsolutePath()+"/"+threadId+".position");
 f.renameTo(new File(Environment.getExternalStorageDirectory().getAbsolutePath()+"/"+threadId+".position.finish"));
 synchronized (MainActivity.class) {
 runningThreadCount--;
 //6.删除临时文件
 if(runningThreadCount<=0){
 for(int i=0;i<threadCount;i++){
 File ft = new File(Environment.getExternalStorageDirectory().getAbsolutePath()+"/"+i+".position.finish");
 ft.delete();
 }
 }
 }
 } catch (Exception e) {
 e.printStackTrace();
 }
 }
 }
 /**
 * 获取一个文件名称
 * @param path
 * @return
 */
 public String getFileName(String path){
 int start = path.lastIndexOf("/")+1;
 return path.substring(start);
 }
}

<uses-permission android:name="android.permission.INTERNET"/>
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

HttpUtils http = new HttpUtils();
 /**
 * 参数1：原文件网络地址
 * 参数2：本地保存的地址
 * 参数3：是否支持断点续传,true：支持，false:不支持
 * 参数4：回调接口，该接口中的方法都是在主线程中被调用的，
 * 也就是该接口中的方法都可以修改UI
 */
 http.download(path, "/mnt/sdcard/xxx.exe", true, new RequestCallBack<File>() {
 //下载成功后调用一次
 @Override
 public void onSuccess(ResponseInfo<File> arg0) {
 Toast.makeText(MainActivity.this, "下载成功", 0).show();
 }
 /**
 * 每下载一部分就被调用一次，通过该方法可以知道当前下载进度
 * 参数1：原文件总字节数
 * 参数2：当前已经下载好的字节数
 * 参数3：是否在上传，对于下载，该值为false
 */
 @Override
 public void onLoading(long total, long current, boolean isUploading) {
 pb0.setMax((int) total);
 pb0.setProgress((int) current);
 super.onLoading(total, current, isUploading);
 }
 //失败后调用一次
 @Override
 public void onFailure(HttpException arg0, String arg1) {
 Toast.makeText(MainActivity.this, "下载失败"+arg1, 0).show();
 }
 });