一、目标

1.多线程抢占服务器资源下载。

2.断点续传。

二、实现思路。

假设分为三个线程：

1.各个线程分别向服务器请求文件的不同部分。 这个涉及Http协议，可以在Header中使用Range参数设置向服务器请求文件的范围。

2.文件部分的合并。

2.1RandomAccessFile（随机读写文件类）。

2.2分别写三个文件，最后合并。

三、编码实现

这里使用RandomAcessFile来实现，首先实现了多线程下载。

public class MainActivity extends AppCompatActivity
        implements View.OnClickListener {
    int i = 0;
    private int progress = 0;
    android.os.Handler mHandler;

    @Override protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        final ProgressBar bar = (ProgressBar) findViewById(R.id.progressBar);
        Button downBtn = (Button) findViewById(R.id.button);
        final EditText tcEt= (EditText) findViewById(R.id.editText2);
        Button restartBtn= (Button) findViewById(R.id.button2);
        restartBtn.setOnClickListener(this);
        final EditText urlEdit = (EditText) findViewById(R.id.editText);
        downBtn.setOnClickListener(new View.OnClickListener() {
            @Override public void onClick(View v) {
                new Thread() {
                    @Override public void run() {
                        try {
                            URL url = new URL(urlEdit.getText().toString());
                            HttpURLConnection connection
                                    = (HttpURLConnection) url.openConnection();
                            connection.setRequestMethod("GET");
                            connection.setConnectTimeout(5000);
                            int length = connection.getContentLength();
                            Log.i("xiahong","size:"+length);
                            if (length < 0) {
                                return;
                            }
                            bar.setMax(length);
  File file = new File(
                                    Environment.getExternalStorageDirectory()
                                               .toString(), "donw.fiel");
                            RandomAccessFile randomAccessFile
                                    = new RandomAccessFile(file, "rw");
                            randomAccessFile.setLength(length);
     int conut=Integer.valueOf(tcEt.getText().toString());
                            int blockSize = length / conut;
                            for (int i = 0; i < conut; i++) {
                                int begin = i * blockSize;
                                int end = (i + 1) * blockSize;
                                if (i == conut-1) {
                                    end = length;
                                }
                                //Create Thread to Download File
                           Runnable runnable=     new DownloadRunable(begin,
                                   end,file,
                                        url,i) ;

                                new Thread(runnable).start();
                            }
                        } catch (MalformedURLException e) {
                            e.printStackTrace();
                        } catch (IOException e) {
                            e.printStackTrace();
                        }
                    }
                }.start();
            }
        });
        mHandler=new android.os.Handler(){
            @Override public void handleMessage(Message msg) {
                bar.setProgress(progress);
            }
        };
    }

    @Override public void onClick(View v) {
        Intent intent=getBaseContext().getPackageManager()
                                      .getLaunchIntentForPackage(getBaseContext().getPackageName());;
        intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
        startActivity(intent);

    }

　　//文件下载类
    class DownloadRunable implements Runnable {
        private int begin;
        private int end;
        private File mFile;
        private URL mURL;
        private int id;

        public DownloadRunable(int begin, int end, File mFile, URL mURL, int id) {
            this.begin = begin;
            this.end = end;
            this.mFile = mFile;
            this.mURL = mURL;
            this.id = id;
        }

        @Override public void run() {
            try {
                HttpURLConnection connection
                        = (HttpURLConnection) mURL.openConnection();
                connection.setRequestMethod("GET");
                connection.setReadTimeout(5000);
                connection.setRequestProperty("Range",
                        "bytes=" + begin + "-" + end);//向服务器请求文件范围。

                InputStream is = connection.getInputStream();
                byte[] bs = new byte[1024 * 1024];
   RandomAccessFile randomAccessFile =
                        new RandomAccessFile(mFile,  "rw");
                randomAccessFile.seek(begin);

                int length = 0;
                while ((length = is.read(bs)) != -1) {
                    randomAccessFile.write(bs, 0, length);
                    updateProgress(length);
                }
                is.close();
                randomAccessFile.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

      synchronized   void updateProgress(int len) {
            progress += len;
            mHandler.sendEmptyMessage(0);
        }
    }
}

下面实现断点续传。
实现思路就是将线程中耗时的循环操作加上一个Boolean类型的Flag标志，每次下载时将下载的进度保存到一个队列中，
在需要回复的时候重新创建线程，读取已保存的列表进行下载，即可。