Android 多线程下载

       多线程下载网上有很多的例子，其中需要注意的就是：

每个线程该分配多大的算法；

通过请求从返回的 getContentLength() 方法获取需要下载的文件大小。

使用 RandomAccessFile 类来创建文件夹。因为该类可以从文件的任何位置开始读写操作，有 seek() 方法。

本例是用 Tomcat 充当后台来下载的。

下载成功后的示意图：

具体代码如下：

MultiThreadDownload.java :

package com.crazy.multidownload;import android.os.Handler;import android.os.Message;import android.util.Log;import java.io.File;import java.io.IOException;import java.io.InputStream;import java.io.RandomAccessFile;import java.net.HttpURLConnection;import java.net.URL;public class MultiThreadDownload {     /* 需要下载资源的地址*/    private String urlStr;     /* 下载的文件*/    private File localFile;     /* 需要下载文件的存放的本地文件夹路径*/    private String dirStr;     /* 存储到本地的文件名*/    private String filename;     /* 开启的线程数量*/    private int threadCount;     /* 下载文件的大小*/    private long fileSize;    private Handler handler;    public MultiThreadDownload(String urlStr, String dirStr,                               String filename, int threadCount, Handler handler) {        this.urlStr = urlStr;        this.dirStr = dirStr;        this.filename = filename;        this.threadCount = threadCount;        this.handler = handler;    }    public void download() throws IOException {        createFileByUrl();         /* 计算每个线程需要下载的数据长度*/        long block = fileSize % threadCount == 0 ? fileSize / threadCount                : fileSize / threadCount + 1;        for (int i = 0; i < threadCount; i++) {            long start = i * block;            long end = start + block >= fileSize ? fileSize : start + block - 1;            new DownloadThread(new URL(urlStr), localFile, start, end).start();        }    }    /**     * 根据资源的URL获取资源的大小，以及在本地创建文件     */    public void createFileByUrl() throws IOException {        URL url = new URL(urlStr);        HttpURLConnection conn = (HttpURLConnection) url.openConnection();        conn.setConnectTimeout(15 * 1000);        conn.setRequestMethod("GET");        conn.setRequestProperty(                "Accept",                "image/gif, image/jpeg, image/pjpeg, image/pjpeg, application/x-shockwave-flash, " +                        "application/xaml+xml, application/vnd.ms-xpsdocument, " +                        "application/x-ms-xbap, application/x-ms-application, " +                        "application/vnd.ms-excel, application/vnd.ms-powerpoint, " +                        "application/msword, */*");        conn.setRequestProperty("Accept-Language", "zh-CN");        conn.setRequestProperty("Referer", urlStr);        conn.setRequestProperty("Charset", "UTF-8");        conn.setRequestProperty(                "User-Agent",                "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.2; Trident/4.0; " +                        ".NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30;" +                        " .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)");        conn.setRequestProperty("Connection", "Keep-Alive");        conn.connect();        if (conn.getResponseCode() == 200) {            // 根据响应获取文件大小            this.fileSize = conn.getContentLength();            if (fileSize <= 0)                throw new RuntimeException(                        "下载文件大小出错 ... ");            File dir = new File(dirStr);            if (!dir.exists())                dir.mkdirs();            this.localFile = new File(dir, filename);            RandomAccessFile raf = new RandomAccessFile(this.localFile, "rw");            raf.setLength(fileSize);            raf.close();            Log.e("tag", "需要下载的文件大小为 :" + this.fileSize + " , 存储位置为： "                    + dirStr + "/" + filename);        } else {            throw new RuntimeException("连接错误 ...");        }    }    private class DownloadThread extends Thread {        /* 下载文件的URI*/        private URL url;         /* 存的本地路径*/        private File localFile;         /* 是否结束*/        private boolean isFinish;         /* 开始的位置*/        private Long startPos;         /* 结束位置*/        private Long endPos;        public DownloadThread(URL url, File savefile, Long startPos, Long endPos) {            this.url = url;            this.localFile = savefile;            this.startPos = startPos;            this.endPos = endPos;        }        @Override        public void run() {            Log.d("tag", Thread.currentThread().getName() + "开始下载...");            try {                HttpURLConnection conn = (HttpURLConnection) url                        .openConnection();                conn.setConnectTimeout(15 * 1000);                conn.setRequestMethod("GET");                conn.setRequestProperty(                        "Accept",                        "image/gif, image/jpeg, image/pjpeg, image/pjpeg, " +                                "application/x-shockwave-flash, application/xaml+xml, " +                                "application/vnd.ms-xpsdocument, application/x-ms-xbap," +                                " application/x-ms-application, application/vnd.ms-excel, " +                                "application/vnd.ms-powerpoint, application/msword, */*");                conn.setRequestProperty("Accept-Language", "zh-CN");                conn.setRequestProperty("Referer", url.toString());                conn.setRequestProperty("Charset", "UTF-8");                // 设置获取实体数据的范围                conn.setRequestProperty("Range", "bytes=" + startPos + "-"                        + endPos);                conn.setRequestProperty(                        "User-Agent",                        "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.2; Trident/4.0; " +                                ".NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30; " +                                ".NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)");                conn.setRequestProperty("Connection", "Keep-Alive");                conn.connect();                /**                 * 代表服务器已经成功处理了部分GET请求                 */                if (conn.getResponseCode() == 206) {                    InputStream is = conn.getInputStream();                    int len = 0;                    byte[] buf = new byte[1024];                    RandomAccessFile raf = new RandomAccessFile(localFile,                            "rwd");                    raf.seek(startPos);                    while ((len = is.read(buf)) != -1) {                        raf.write(buf, 0, len);                    }                    raf.close();                    is.close();                    Log.e("tag", Thread.currentThread().getName()                            + "完成下载  ： " + startPos + " -- " + endPos);                    setMessageForUI();                    this.isFinish = true;                } else {                    throw new RuntimeException(                            "url 连接错误 ...");                }            } catch (IOException e) {                e.printStackTrace();            }        }    }    private void setMessageForUI(){        new Thread(new Runnable() {            @Override            public void run() {                Message msg = Message.obtain();                msg.what = 0x123;                msg.obj = threadCount;                handler.sendMessage(msg);            }        }).start();    }}

MainAcivity.java :

package com.crazy.multidownload;import android.os.AsyncTask;import android.os.Environment;import android.os.Handler;import android.os.Message;import android.support.v7.app.AppCompatActivity;import android.os.Bundle;import android.widget.Toast;import java.io.IOException;public class MainActivity extends AppCompatActivity {    private Handler handler;    private int threadCount;    private int count = 0;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        threadCount = 2;        initHandler();        downLoad();    }    private void initHandler(){        handler = new Handler(){            @Override            public void handleMessage(Message msg) {                super.handleMessage(msg);                if (msg.what == 0x123) {                    count++;                    if (count == threadCount)                        Toast.makeText(MainActivity.this, "下载完成", Toast.LENGTH_SHORT).show();                }            }        };    }    private void downLoad() {        new AsyncTask<Void, Void, Void>() {            @Override            protected Void doInBackground(Void... params) {                try {                    new MultiThreadDownload(                            "http://192.168.248.2/zip/multidownload.zip",                            Environment.getExternalStorageDirectory() + "/multidownload",                            "multidownload.zip",                            threadCount,                            handler).download();                } catch (IOException e) {                    e.printStackTrace();                }                return null;            }        }.execute();    }}

开始下载输出的内容见下图：

从上图中可以看到线程的数量和我们所设定的线程数量是相等的。

完成下载后的输出：

从图中可以看出，下载完成的的输出和线程数量相同都为2，所以为了让其真正完成下载（线程数量和输出数量相等时）时在进行提升，才有了 count 计数器，记录接收到的次数。

最后可以打开 ADM 查看下载的路径：

从上面的几张图片中可以看到，文件的总大小是相同的。

最后不要忘了添加相应的权限：

 <uses-permission android:name="android.permission.INTERNET"/> <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/> <uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS"/>