http://www.ideasandroid.com/archives/328#more-328
本文將介紹在android平臺下如何實現多線程下載，大家都知道，android平臺使用java做為開發語言，所以java中支持的多線程下載方式在android平臺下都支持，其中主要有兩種方式可以實現多線程下載。

一種方式是使用很多個線程分別下載文件的不同部分，最后把所有下載完的文件合并成一個文件。另一種方式是使用java為我們提供的RandomAccessFile類實現多線程的下載。

從性能上分析，第二種方式的存取速度會慢一些，但開發起來較為容易，不需要進行合并文件等操作。本文將使用第二種方式來實現多線程下載，最終效果如下圖所示：



使用圖形界面來獲取需要下載的內容，并實時更新下載進度條，代碼如下所示：

    
import java.io.File;
import java.net.URL;
import java.net.URLConnection;
import android.app.Activity;
import android.os.Bundle;
import android.os.Environment;
import android.os.Handler;
import android.os.Message;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ProgressBar;
import android.widget.TextView;
/**
 *  Copyright (C) 2010 ideasandroid
 *  演示android多線程下載
 *  歡迎訪問http://www.ideasandroid.com
 *  讓程序開發不再那么神秘
 */
public class FileDownloadDemo extends Activity {
 
    private EditText downloadUrl;
    private EditText downloadFileName;
    private EditText downloadThreadNum;
    private Button downloadBt;
    private ProgressBar downloadProgressBar;
    private TextView progressMessage;
    private int downloadedSize = 0;
    private int fileSize = 0;
 
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
 
        downloadUrl = (EditText) findViewById(R.id.downloadUrl);
        downloadFileName = (EditText) findViewById(R.id.downloadFileName);
        downloadThreadNum = (EditText) findViewById(R.id.downloadThreadNum);
        progressMessage = (TextView) findViewById(R.id.progressMessage);
        downloadBt = (Button) findViewById(R.id.downloadBt);
        downloadProgressBar = (ProgressBar) findViewById(R.id.downloadProgressBar);
        downloadProgressBar.setVisibility(View.VISIBLE);
        downloadProgressBar.setMax(100);
        downloadProgressBar.setProgress(0);
        downloadBt.setOnClickListener(new OnClickListener() {
            public void onClick(View v) {
                download();
            }
        });
    }
 
    private void download() {
        // 獲取SD卡目錄
        String dowloadDir = Environment.getExternalStorageDirectory()
                + "/ideasdownload/";
        File file = new File(dowloadDir);
        //創建下載目錄
        if (!file.exists()) {
            file.mkdirs();
        }
 
        //讀取下載線程數，如果為空，則單線程下載
        int downloadTN = Integer.valueOf("".equals(downloadThreadNum.getText()
                .toString()) ? "1" : downloadThreadNum.getText().toString());
        //如果下載文件名為空則獲取Url尾為文件名
        int fileNameStart = downloadUrl.getText().toString().lastIndexOf("/");
        String fileName = "".equals(downloadFileName.getText().toString()) ? downloadUrl
                .getText().toString().substring(fileNameStart)
                : downloadFileName.getText().toString();
        //開始下載前把下載按鈕設置為不可用
        downloadBt.setClickable(false);
        //進度條設為0
        downloadProgressBar.setProgress(0);
        //啟動文件下載線程
        new downloadTask(downloadUrl.getText().toString(), Integer
                .valueOf(downloadTN), dowloadDir + fileName).start();
    }
 
    Handler handler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            //當收到更新視圖消息時，計算已完成下載百分比，同時更新進度條信息
            int progress = (Double.valueOf((downloadedSize * 1.0 / fileSize * 100))).intValue();
            if (progress == 100) {
                downloadBt.setClickable(true);
                progressMessage.setText("下載完成！");
            } else {
                progressMessage.setText("當前進度:" + progress + "%");
            }
            downloadProgressBar.setProgress(progress);
        }
 
    };
 
    /**
     * @author ideasandroid
     * 主下載線程
     */
    public class downloadTask extends Thread {
        private int blockSize, downloadSizeMore;
        private int threadNum = 5;
        String urlStr, threadNo, fileName;
 
        public downloadTask(String urlStr, int threadNum, String fileName) {
            this.urlStr = urlStr;
            this.threadNum = threadNum;
            this.fileName = fileName;
        }
 
        @Override
        public void run() {
            FileDownloadThread[] fds = new FileDownloadThread[threadNum];
            try {
                URL url = new URL(urlStr);
                URLConnection conn = url.openConnection();
                //獲取下載文件的總大小
                fileSize = conn.getContentLength();
                //計算每個線程要下載的數據量
                blockSize = fileSize / threadNum;
                // 解決整除后百分比計算誤差
                downloadSizeMore = (fileSize % threadNum);
                File file = new File(fileName);
                for (int i = 0; i < threadNum; i++) {
                    //啟動線程，分別下載自己需要下載的部分
                    FileDownloadThread fdt = new FileDownloadThread(url, file,
                            i * blockSize, (i + 1) * blockSize - 1);
                    fdt.setName("Thread" + i);
                    fdt.start();
                    fds[i] = fdt;
                }
                boolean finished = false;
                while (!finished) {
                    // 先把整除的余數搞定
                    downloadedSize = downloadSizeMore;
                    finished = true;
                    for (int i = 0; i < fds.length; i++) {
                        downloadedSize += fds[i].getDownloadSize();
                        if (!fds[i].isFinished()) {
                            finished = false;
                        }
                    }
                    //通知handler去更新視圖組件
                    handler.sendEmptyMessage(0);
                    //休息1秒后再讀取下載進度
                    sleep(1000);
                }
            } catch (Exception e) {
 
            }
 
        }
    }
}

  

    
import java.io.BufferedInputStream;
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.net.URL;
import java.net.URLConnection;
 
import android.util.Log;
/**
 *  Copyright (C) 2010 ideasandroid
 *  演示android多線程下載
 *  歡迎訪問http://www.ideasandroid.com
 *  讓程序開發不再那么神秘
 *  
 *  單個下載線程
 */
public class FileDownloadThread extends Thread{
    private static final int BUFFER_SIZE=1024;
    private URL url;
    private File file;
    private int startPosition;
    private int endPosition;
    private int curPosition;
    //用于標識當前線程是否下載完成
    private boolean finished=false;
    private int downloadSize=0;
    public FileDownloadThread(URL url,File file,int startPosition,int endPosition){
        this.url=url;
        this.file=file;
        this.startPosition=startPosition;
        this.curPosition=startPosition;
        this.endPosition=endPosition;
    }
    @Override
    public void run() {
        BufferedInputStream bis = null;
        RandomAccessFile fos = null;                                               
        byte[] buf = new byte[BUFFER_SIZE];
        URLConnection con = null;
        try {
            con = url.openConnection();
            con.setAllowUserInteraction(true);
            //設置當前線程下載的起點，終點
            con.setRequestProperty("Range", "bytes=" + startPosition + "-" + endPosition);
            //使用java中的RandomAccessFile 對文件進行隨機讀寫操作
            fos = new RandomAccessFile(file, "rw");
            //設置開始寫文件的位置
            fos.seek(startPosition);
            bis = new BufferedInputStream(con.getInputStream());  
            //開始循環以流的形式讀寫文件
            while (curPosition < endPosition) {
                int len = bis.read(buf, 0, BUFFER_SIZE);                
                if (len == -1) {
                    break;
                }
                fos.write(buf, 0, len);
                curPosition = curPosition + len;
                if (curPosition > endPosition) {
                    downloadSize+=len - (curPosition - endPosition) + 1;
                } else {
                    downloadSize+=len;
                }
            }
            //下載完成設為true
            this.finished = true;
            bis.close();
            fos.close();
        } catch (IOException e) {
          Log.d(getName() +" Error:", e.getMessage());
        }
    }
 
    public boolean isFinished(){
        return finished;
    }
 
    public int getDownloadSize() {
        return downloadSize;
    }
}

  

android多線程斷點下載
http://llb988.blog.51cto.com/1940549/510035

Andorid多線程斷點續傳下載器
http://www.jcodecraeer.com/a/opensource/2015/0602/2978.html

Android 多線程下載 仿下載助手
http://blog.csdn.net/u011733020/article/details/47016715

Android文件下載（實現斷點續傳）