Android文件下载（实现断点续传）

本文将介绍在android平台下如何实现多线程下载，大家都知道，android平台使用java做为开发语言，所以java中支持的多线程下载方式在android平台下都支持，其中主要有两种方式可以实现多线程下载。
一种方式是使用很多个线程分别下载文件的不同部分，最后把所有下载完的文件合并成一个文件。另一种方式是使用java为我们提供的RandomAccessFile类实现多线程的下载。

从性能上分析，第二种方式的存取速度会慢一些，但开发起来较为容易，不需要进行合并文件等操作。本文将使用第二种方式来实现多线程下载，最终效果如下图所示：

使用图形界面来获取需要下载的内容，并实时更新下载进度条，代码如下所示：

    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多线程下载
     *  欢迎访问http://www.ideasandroid.com
     *  让程序开发不再那么神秘
     */
    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 = ;
        private int fileSize = ;  

        @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();
            downloadProgressBar.setProgress();
            downloadBt.setOnClickListener(new OnClickListener() {
                public void onClick(View v) {
                    download();
                }
            });
        }  

        private void download() {
            // 获取SD卡目录
            String dowloadDir = Environment.getExternalStorageDirectory()
                    + "/ideasdownload/";
            File file = new File(dowloadDir);
            //创建下载目录
            if (!file.exists()) {
                file.mkdirs();
            }  

            //读取下载线程数，如果为空，则单线程下载
            int downloadTN = Integer.valueOf("".equals(downloadThreadNum.getText()
                    .toString()) ? "" : downloadThreadNum.getText().toString());
            //如果下载文件名为空则获取Url尾为文件名
            int fileNameStart = downloadUrl.getText().toString().lastIndexOf("/");
            String fileName = "".equals(downloadFileName.getText().toString()) ? downloadUrl
                    .getText().toString().substring(fileNameStart)
                    : downloadFileName.getText().toString();
            //开始下载前把下载按钮设置为不可用
            downloadBt.setClickable(false);
            //进度条设为0
            downloadProgressBar.setProgress();
            //启动文件下载线程
            new downloadTask(downloadUrl.getText().toString(), Integer
                    .valueOf(downloadTN), dowloadDir + fileName).start();
        }  

        Handler handler = new Handler() {
            @Override
            public void handleMessage(Message msg) {
                //当收到更新视图消息时，计算已完成下载百分比，同时更新进度条信息
                int progress = (Double.valueOf((downloadedSize * 1.0 / fileSize * ))).intValue();
                if (progress == ) {
                    downloadBt.setClickable(true);
                    progressMessage.setText("下载完成！");
                } else {
                    progressMessage.setText("当前进度:" + progress + "%");
                }
                downloadProgressBar.setProgress(progress);
            }  

        };  

        /**
         * @author ideasandroid
         * 主下载线程
         */
        public class downloadTask extends Thread {
            private int blockSize, downloadSizeMore;
            private int threadNum = ;
            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();
                    //获取下载文件的总大小
                    fileSize = conn.getContentLength();
                    //计算每个线程要下载的数据量
                    blockSize = fileSize / threadNum;
                    // 解决整除后百分比计算误差
                    downloadSizeMore = (fileSize % threadNum);
                    File file = new File(fileName);
                    for (int i = ; i < threadNum; i++) {
                        //启动线程，分别下载自己需要下载的部分
                        FileDownloadThread fdt = new FileDownloadThread(url, file,
                                i * blockSize, (i + ) * blockSize - );
                        fdt.setName("Thread" + i);
                        fdt.start();
                        fds[i] = fdt;
                    }
                    boolean finished = false;
                    while (!finished) {
                        // 先把整除的余数搞定
                        downloadedSize = downloadSizeMore;
                        finished = true;
                        for (int i = ; i < fds.length; i++) {
                            downloadedSize += fds[i].getDownloadSize();
                            if (!fds[i].isFinished()) {
                                finished = false;
                            }
                        }
                        //通知handler去更新视图组件
                        handler.sendEmptyMessage();
                        //休息1秒后再读取下载进度
                        sleep();
                    }
                } 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多线程下载
 *  欢迎访问http://www.ideasandroid.com
 *  让程序开发不再那么神秘
 *
 *  单个下载线程
 */
public class FileDownloadThread extends Thread{
    private static final int BUFFER_SIZE=;
    private URL url;
    private File file;
    private int startPosition;
    private int endPosition;
    private int curPosition;
    //用于标识当前线程是否下载完成
    private boolean finished=false;
    private int downloadSize=;
    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);
            //设置当前线程下载的起点，终点
            con.setRequestProperty("Range", "bytes=" + startPosition + "-" + endPosition);
            //使用java中的RandomAccessFile 对文件进行随机读写操作
            fos = new RandomAccessFile(file, "rw");
            //设置开始写文件的位置
            fos.seek(startPosition);
            bis = new BufferedInputStream(con.getInputStream());
            //开始循环以流的形式读写文件
            while (curPosition < endPosition) {
                int len = bis.read(buf, , BUFFER_SIZE);
                if (len == -) {
                    break;
                }
                fos.write(buf, , len);
                curPosition = curPosition + len;
                if (curPosition > endPosition) {
                    downloadSize+=len - (curPosition - endPosition) + ;
                } else {
                    downloadSize+=len;
                }
            }
            //下载完成设为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;
    }
}

Andorid多线程断点续传下载器:http://www.jcodecraeer.com/a/opensource/2015/0602/2978.html