通过AsyncTask执行下载任务，更新进度条，SharedPreferences保存下载进度 ,简单易懂。

package com.xk.myfiledownload.ui;

import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;

import org.apache.http.HttpStatus;

import android.app.Activity;
import android.content.SharedPreferences;
import android.content.SharedPreferences.Editor;
import android.os.AsyncTask;
import android.os.Bundle;
import android.os.Environment;
import android.util.Log;
import android.view.KeyEvent;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ProgressBar;
import android.widget.Toast;

import com.xk.myfiledownload.R;

public class MainActivity extends Activity implements OnClickListener {
    private Button btnStart;
    private Button btnStop;
    private ProgressBar pb;
    private int length;
    private int start;
    private boolean isPause;
    private SharedPreferences mSharedPreferences;
    private Editor mEditor;
    public int mFinished;
    private boolean flag;
    // 下载路径
    private final static String MYURL = "http://idcbgp.cn/mobile/mukewang.apk";
    // 下载地址
    private final static String DIRSTR = Environment
            .getExternalStorageDirectory().getAbsolutePath() + "/";
    // 下载文件名
    private final static String FILENAME = "mukewang.apk";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.mian_layout);
        init();
    }

    /*
     * 重写返回键,模拟返回按下暂停键
     */
    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {
        switch (keyCode) {
        case KeyEvent.KEYCODE_BACK:
            btnStop.performClick();// 模拟按下暂停键
            break;
        default:
            break;
        }
        return super.onKeyDown(keyCode, event);
    }

    /*
     * 初始化控件
     */
    private void init() {
        btnStart = (Button) findViewById(R.id.button1);
        btnStop = (Button) findViewById(R.id.button2);
        pb = (ProgressBar) findViewById(R.id.progressBar1);
        pb.setMax(100);
        btnStart.setOnClickListener(this);
        btnStop.setOnClickListener(this);
    }

    /*
     * 异步处理
     */
    class MyAsyncTask extends AsyncTask<String, Integer, String> {

        @Override
        protected String doInBackground(String... params) {
            // 获取文件长度
            getFileLength(params[0]);
            // 执行下载任务
            return download(params[0]);
        }

        @Override
        protected void onPostExecute(String result) {
            super.onPostExecute(result);
            Log.d("result: ", result);
            // 返回值设置Toast
            Toast.makeText(MainActivity.this, result, Toast.LENGTH_SHORT)
                    .show();
        }

        @Override
        protected void onProgressUpdate(Integer... values) {
            super.onProgressUpdate(values);
            // 更新进度条
            pb.setProgress(values[0]);
        }

        /*
         * 获取文件长度
         */
        private void getFileLength(String string) {
            HttpURLConnection connection = null;
            try {
                connection = (HttpURLConnection) new URL(string)
                        .openConnection();
                connection.setRequestMethod("GET");
                connection.setConnectTimeout(3000);
                if (connection.getResponseCode() != HttpStatus.SC_OK) {
                    return;
                }
                length = connection.getContentLength();
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                if (connection != null) {
                    connection.disconnect();
                }
            }

        }

        /*
         * 下载任务
         */
        private String download(String string) {
            HttpURLConnection connection = null;
            InputStream in = null;
            RandomAccessFile raf = null;
            try {
                // 从SharedPreferences取出DownloadInfo里的下载进度值
                mSharedPreferences = getSharedPreferences("DownloadInfo",
                        MODE_PRIVATE);
                mEditor = mSharedPreferences.edit();
                start = mSharedPreferences.getInt("Finished", 0);
                mFinished = start;
                connection = (HttpURLConnection) new URL(string)
                        .openConnection();
                connection.setRequestMethod("GET");
                connection.setConnectTimeout(3000);
                Log.d("length: ", length + "");
                connection.setRequestProperty("Range", "bytes=" + start + "-"
                        + length);
                if (connection.getResponseCode() != HttpStatus.SC_PARTIAL_CONTENT) {
                    return "206";
                }
                in = connection.getInputStream();
                File file = new File(DIRSTR, FILENAME);
                raf = new RandomAccessFile(file, "rwd");
                Log.d("start:", start + "");
                raf.seek(start);
                byte[] buffer = new byte[1024 * 4];
                int len;
                while ((len = in.read(buffer)) != -1) {
                    raf.write(buffer, 0, len);
                    mFinished += len;
                    // 设置进度条的值
                    publishProgress(mFinished * 100 / length);
                    // 判断是否暂停
                    if (isPause) {
                        Log.d("mFinished:", mFinished + "");
                        // 将进度值存入SharedPreferences中
                        mEditor.putInt("Finished", mFinished);
                        mEditor.commit();
                        if (raf != null) {
                            raf.close();
                        }
                        return "暂停";
                    }
                }
                flag = false;
                // 下载完成,从SharedPreferences移除下载进度
                mEditor.remove("Finished");
                mEditor.commit();
                return "下载完成!";
            } catch (Exception e) {
                e.printStackTrace();
                return e.getMessage();
            } finally {
                try {
                    if (raf != null) {
                        raf.close();
                    }
                    if (in != null) {
                        in.close();
                    }
                    if (connection != null) {
                        connection.disconnect();
                    }
                } catch (IOException e) {
                    e.printStackTrace();
                    return e.getMessage();
                }
            }
        }

    }

    @Override
    public void onClick(View v) {
        switch (v.getId()) {
        case R.id.button1:
            if (flag) {
                return;
            }
            isPause = false;
            // 防止多次点击下载,造成多个下载
            flag = true;
            Log.d("onClick", "开始");
            // 执行下载任务
            new MyAsyncTask().execute(MYURL);
            break;
        case R.id.button2:
            // 通过isPause暂停下载
            isPause = true;
            flag = false;
            break;
        default:
            break;
        }
    }
}