近期研究多線程下載，寫了個demo。整理下來，或許會對別人有幫助。
多線程下載的話一般開啟兩到三個線程吧。假設線程太多的話時間會浪費在線程的切換上，倒是浪費了大把的時間。線程多了也不是一件好事。
原理的話看我的還有一篇博文，其實是將代碼移植到了安卓系統上。java實現的多線程下載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;
    /**
     * 存放進度條的布局
     */
    private LinearLayout ll_container;

    /**
     * 進度條的集合
     */
    private List<ProgressBar> pbs;

    /**
     * android下的消息處理器。在主線程創建。才幹夠更新ui
     */
 

    private Handler handler = new Handler(){
        public void handleMessage(Message msg) {
            switch (msg.what) {
            case DOWNLOAD_ERROR:
                Toast.makeText(getApplicationContext(), "下載失敗", 0).show();
                break;
            case THREAD_ERROR:
                Toast.makeText(getApplicationContext(), "下載失敗,請重試"
 
, 0).show();
                break;
            case DWONLOAD_FINISH:
                Toast.makeText(getApplicationContext(), "完成下載", 0).show();
                break;
            }
        };
    };

    /**
     * 線程的數量
     */
    private int threadCount = 3;

    /**
     * 每一個下載區塊的大小
     */
    private long blocksize;

    /**
     * 正在執行的線程的數量
     */
    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){
        //下載文件的路徑
        final String path = et_path.getText().toString().trim();
        if(TextUtils.isEmpty(path)){
            Toast.makeText(this, "對不起下載路徑不能為空", 0).show();
            return;
        }
        String count = et_count.getText().toString().trim();
        if(TextUtils.isEmpty(path)){
            Toast.makeText(this, "對不起,線程數量不能為空", 0).show();
            return;
        }
        threadCount = Integer.parseInt(count);
        //清空掉舊的進度條
        ll_container.removeAllViews();
        //在界面裏面加入count個進度條
        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, "開始下載", 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();// 得到服務端返回的文件的大小
                        System.out.println("server文件的大小：" + size);
                        blocksize = size / threadCount;
                        // 1.首先在本地創建一個大小跟server一模一樣的空白文件。
                        File file = new File(Environment.getExternalStorageDirectory(),getFileName(path));
                        RandomAccessFile raf = new RandomAccessFile(file, "rw");
                        raf.setLength(size);
                        // 2.開啟若幹個子線程分別去下載相應的資源。

                        runningThreadCount = threadCount;
                        for (int i = 1; i <= threadCount; i++) {
                            long startIndex = (i - 1) * blocksize;
                            long endIndex = i * blocksize - 1;
                            if (i == threadCount) {
                                // 最後一個線程
                                endIndex = size - 1;
                            }
                            System.out.println("開啟線程：" + i + "下載的位置：" + 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 {
                // 當前線程下載的總大小
                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");
                // 接著從上一次的位置繼續下載數據
                if (positionFile.exists() && positionFile.length() > 0) {// 推斷是否有記錄
                    FileInputStream fis = new FileInputStream(positionFile);
                    BufferedReader br = new BufferedReader(
                            new InputStreamReader(fis));
                    // 獲取當前線程上次下載的總大小是多少
                    String lasttotalstr = br.readLine();
                    int lastTotal = Integer.valueOf(lasttotalstr);
                    System.out.println("上次線程" + threadId + "下載的總大小："
                            + lastTotal);
                    startIndex += lastTotal;
                    total += lastTotal;// 加上上次下載的總大小。
                    fis.close();
                    //存數據庫。
                    //_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");
                // 指定文件開始寫的位置。

                raf.seek(startIndex);
                System.out.println("第" + threadId + "個線程：寫文件的開始位置："
                        + 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 {
                // 僅僅有全部的線程都完成下載後 才幹夠刪除記錄文件。
                synchronized (MainActivity.class) {
                    System.out.println("線程" + threadId + "完成下載了");
                    runningThreadCount--;
                    if (runningThreadCount < 1) {
                        System.out.println("全部的線程都工作完成了。
刪除暫時記錄的文件");
                        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);
    }

}

安卓系統下須要加上訪問網絡的權限和訪問本地內存卡的權限。例如以下圖：

源代碼已經上傳到了CSDN: http://download.csdn.net/detail/rootusers/8508137

安卓系統下的多線程斷點下載實現