安卓系统下的多线程断点下载实现

近期研究多线程下载，写了个demo。整理下来，或许会对别人有帮助。
多线程下载的话一般开启两到三个线程吧。假设线程太多的话时间会浪费在线程的切换上，倒是浪费了大把的时间。线程多了也不是一件好事。
原理的话看我的还有一篇博文，其实是将代码移植到了安卓系统上。java实现的多线程下载demo

public class MainActivity extends Activity {
    protected static final int DOWNLOAD_ERROR = 1;
    private static final int THREAD_ERROR = 2;
    public static final int DWONLOAD_FINISH = 3;
    private EditText et_path;
    private EditText et_count;
    /**
     * 存放进度条的布局
     */
    private LinearLayout ll_container;

    /**
     * 进度条的集合
     */
    private List<ProgressBar> pbs;

    /**
     * android下的消息处理器。在主线程创建。才干够更新ui
     */
    private Handler handler = new Handler(){
        public void handleMessage(Message msg) {
            switch (msg.what) {
            case DOWNLOAD_ERROR:
                Toast.makeText(getApplicationContext(), "下载失败", 0).show();
                break;
            case THREAD_ERROR:
                Toast.makeText(getApplicationContext(), "下载失败,请重试", 0).show();
                break;
            case DWONLOAD_FINISH:
                Toast.makeText(getApplicationContext(), "完成下载", 0).show();
                break;
            }
        };
    };

    /**
     * 线程的数量
     */
    private int threadCount = 3;

    /**
     * 每一个下载区块的大小
     */
    private long blocksize;

    /**
     * 正在执行的线程的数量
     */
    private  int runningThreadCount;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        et_path = (EditText) findViewById(R.id.et_addr);
        et_count = (EditText) findViewById(R.id.et_num);
        ll_container = (LinearLayout) findViewById(R.id.ll_pb);
    }


    public void download(View view){
        //下载文件的路径
        final String path = et_path.getText().toString().trim();
        if(TextUtils.isEmpty(path)){
            Toast.makeText(this, "对不起下载路径不能为空", 0).show();
            return;
        }
        String count = et_count.getText().toString().trim();
        if(TextUtils.isEmpty(path)){
            Toast.makeText(this, "对不起,线程数量不能为空", 0).show();
            return;
        }
        threadCount = Integer.parseInt(count);
        //清空掉旧的进度条
        ll_container.removeAllViews();
        //在界面里面加入count个进度条
        pbs = new ArrayList<ProgressBar>();
        for(int j=0;j<threadCount;j++){
            ProgressBar pb = (ProgressBar) View.inflate(this, R.layout.pb, null);
            ll_container.addView(pb);
            pbs.add(pb);
        }
        Toast.makeText(this, "開始下载", 0).show();
        new Thread(){
            public void run() {
                try {
                    URL url = new URL(path);
                    HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                    conn.setRequestMethod("GET");
                    conn.setConnectTimeout(5000);
                    int code = conn.getResponseCode();
                    if (code == 200) {
                        long size = conn.getContentLength();// 得到服务端返回的文件的大小
                        System.out.println("server文件的大小：" + size);
                        blocksize = size / threadCount;
                        // 1.首先在本地创建一个大小跟server一模一样的空白文件。
                        File file = new File(Environment.getExternalStorageDirectory(),getFileName(path));
                        RandomAccessFile raf = new RandomAccessFile(file, "rw");
                        raf.setLength(size);
                        // 2.开启若干个子线程分别去下载相应的资源。

                        runningThreadCount = threadCount;
                        for (int i = 1; i <= threadCount; i++) {
                            long startIndex = (i - 1) * blocksize;
                            long endIndex = i * blocksize - 1;
                            if (i == threadCount) {
                                // 最后一个线程
                                endIndex = size - 1;
                            }
                            System.out.println("开启线程：" + i + "下载的位置：" + startIndex + "~"
                                    + endIndex);
                            int threadSize = (int) (endIndex - startIndex);
                            pbs.get(i-1).setMax(threadSize);
                            new DownloadThread(path, i, startIndex, endIndex).start();
                        }
                    }
                    conn.disconnect();
                } catch (Exception e) {
                    e.printStackTrace();
                    Message msg = Message.obtain();
                    msg.what = DOWNLOAD_ERROR;
                    handler.sendMessage(msg);
                }

            };
        }.start();

    }
    private class DownloadThread extends Thread {

        private int threadId;
        private long startIndex;
        private long endIndex;
        private String path;

        public DownloadThread(String path, int threadId, long startIndex,
                long endIndex) {
            this.path = path;
            this.threadId = threadId;
            this.startIndex = startIndex;
            this.endIndex = endIndex;
        }

        @Override
        public void run() {
            try {
                // 当前线程下载的总大小
                int total = 0;
                File positionFile = new File(Environment.getExternalStorageDirectory(),getFileName(path)+threadId + ".txt");
                URL url = new URL(path);
                HttpURLConnection conn = (HttpURLConnection) url
                        .openConnection();
                conn.setRequestMethod("GET");
                // 接着从上一次的位置继续下载数据
                if (positionFile.exists() && positionFile.length() > 0) {// 推断是否有记录
                    FileInputStream fis = new FileInputStream(positionFile);
                    BufferedReader br = new BufferedReader(
                            new InputStreamReader(fis));
                    // 获取当前线程上次下载的总大小是多少
                    String lasttotalstr = br.readLine();
                    int lastTotal = Integer.valueOf(lasttotalstr);
                    System.out.println("上次线程" + threadId + "下载的总大小："
                            + lastTotal);
                    startIndex += lastTotal;
                    total += lastTotal;// 加上上次下载的总大小。
                    fis.close();
                    //存数据库。
                    //_id path threadid total
                }

                conn.setRequestProperty("Range", "bytes=" + startIndex + "-"
                        + endIndex);

                conn.setConnectTimeout(5000);
                int code = conn.getResponseCode();
                System.out.println("code=" + code);
                InputStream is = conn.getInputStream();
                File file = new File(Environment.getExternalStorageDirectory(),getFileName(path));
                RandomAccessFile raf = new RandomAccessFile(file, "rw");
                // 指定文件開始写的位置。

                raf.seek(startIndex);
                System.out.println("第" + threadId + "个线程：写文件的開始位置："
                        + String.valueOf(startIndex));
                int len = 0;
                byte[] buffer = new byte[1024];
                while ((len = is.read(buffer)) != -1) {
                    RandomAccessFile rf = new RandomAccessFile(positionFile,
                            "rwd");
                    raf.write(buffer, 0, len);
                    total += len;
                    rf.write(String.valueOf(total).getBytes());
                    rf.close();
                    pbs.get(threadId-1).setProgress(total);
                }
                is.close();
                raf.close();

            } catch (Exception e) {
                e.printStackTrace();
                Message msg = Message.obtain();
                msg.what = THREAD_ERROR;
                handler.sendMessage(msg);
            } finally {
                // 仅仅有全部的线程都完成下载后 才干够删除记录文件。
                synchronized (MainActivity.class) {
                    System.out.println("线程" + threadId + "完成下载了");
                    runningThreadCount--;
                    if (runningThreadCount < 1) {
                        System.out.println("全部的线程都工作完成了。
删除暂时记录的文件");
                        for (int i = 1; i <= threadCount; i++) {
                            File f = new File(Environment.getExternalStorageDirectory(),getFileName(path)+ i + ".txt");
                            System.out.println(f.delete());
                        }
                        Message msg = Message.obtain();
                        msg.what = DWONLOAD_FINISH;
                        handler.sendMessage(msg);
                    }
                }

            }
        }
    }

    private String getFileName(String path){
        int start = path.lastIndexOf("/")+1;
        return path.substring(start);
    }

}

安卓系统下须要加上訪问网络的权限和訪问本地内存卡的权限。例如以下图：

源代码已经上传到了CSDN: http://download.csdn.net/detail/rootusers/8508137