android中级篇之多线程下载

要是先多线程下载，则必须对同一个文件可任意位置的写入 ，java中提供这样一个类可任意写入RandomAccessFile 。通过多线程，可将文件分割成多个子断，每一个线程只需下载一段文件即可。实现效果如图：

 

下面看代码部分：

1.布局文件 main.xml

 

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >
<TextView
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="下载路径"
    />
<EditText
    android:id="@+id/mEditText"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:singleLine="true"
    android:text="http://android.yesky.com/uploads/attachments/2010-04/27/a5964152.jpg"/>
<ProgressBar
    android:id="@+id/mBar"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:visibility="invisible"
    style="?android:attr/progressBarStyleHorizontal"/>
<Button
    android:id="@+id/mButton"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="下载"/>
</LinearLayout>

 

 

2.下载工具类 Download.java

 

package com.yin.downloader.utils;
import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import android.content.Context;
import android.util.Log;
public class Download {
    private String SDPath = null;
    private static final String TAG = "com.yin.download";
    private RandomAccessFile randomFile = null;
    private URL  url;
    private Context context;
    private String urlStr;
    private String fileName;
    private int fileSize;
    private int totalReadSize;
    public Download(String urlStr,String fileName,String SDPath,Context context){
        this.context = context;
        this.fileName = fileName;
        this.urlStr = urlStr;
        this.SDPath = SDPath;
    }

    public void downloadFile(){
        try {
            File file = new File(SDPath+fileName);
            url = new URL(urlStr);
            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            conn.setRequestMethod("GET");
            //获得下载文件的大小
            fileSize = conn.getContentLength();
            //设置线程的大小，考虑手机的性能，并不是越大越好
            int threadSize = 3;
            //每个线程下载文件+的部分大小        +1 避免文件下载不完全
            int block = fileSize / threadSize +1;
            for(int i=0;i<3;i++){
                int startPosition = i * block;
                //创建可任意位置读取的文件
                randomFile = new RandomAccessFile(file, "rw");
                randomFile.seek(startPosition);
                new DownloadThread(i+1, startPosition, block, randomFile).start();
            }

        } catch (MalformedURLException e) {
            Log.e(TAG, "MalformedURLException");
            e.printStackTrace();
        } catch (IOException e) {
            Log.e(TAG, "IOException");
            e.printStackTrace();
        }
    }

    //下载文件的线程
    private class DownloadThread extends Thread{

        private int threadID;
        private int startPosition;
        private int block;
        private RandomAccessFile randomFile;
        public DownloadThread(int threadID, int startPosition, int block,
                RandomAccessFile randomFile) {
            super();
            this.threadID = threadID;
            this.startPosition = startPosition;
            this.block = block;
            this.randomFile = randomFile;
        }

        public void run() {
            try {
                HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                conn.setRequestMethod("GET");
                //文件下载位置   规定的格式 “byte=xxxx-”
                String start = "bytes="+startPosition + "-";
                //设置文件开始的下载位置
                conn.setRequestProperty("Range", start);
                InputStream is = conn.getInputStream();
                byte[] buffer = new byte[4*1024];
                int len = -1;
                int readFileSize = 0;
                while((readFileSize < block) && ((len = is.read(buffer)) != -1)){
                    randomFile.write(buffer,0,len);
                    readFileSize += len ;
                    totalReadSize += readFileSize;
                }
                System.out.println("线程 ："+threadID+" 下载完成");
                is.close();
                randomFile.close();
                conn.disconnect();

            } catch (IOException e) {
                Log.e(TAG+":child", "IOException");
                e.printStackTrace();
            }
        }
    }
    public int getFileSize() {
        return fileSize;
    }
    public void setFileSize(int fileSize) {
        this.fileSize = fileSize;
    }
    public int getTotalReadSize() {
        return totalReadSize;
    }
    public void setTotalReadSize(int totalReadSize) {
        this.totalReadSize = totalReadSize;
    }
}

 

3.DownloadActivity.java

 

package com.yin.downloader;
import java.util.Timer;
import java.util.TimerTask;
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.Toast;
import com.yin.downloader.utils.Download;
public class DownloadActivity extends Activity {
    public EditText mEditText;
    public ProgressBar mBar;
    public Button mButton;
    public String urlStr;
    public String fileName;
    public String SDPath = "/sdcard/";
    public int fileSize;
    public int totalReadSize;
    Download download;

    public Handler handler = new Handler(){
        @Override
        public void handleMessage(Message msg) {

            mBar.setProgress(download.getTotalReadSize());
            mBar.invalidate();
        }

    };

    public Timer mTimer = new Timer();
    public TimerTask mTask = new TimerTask() {

        @Override
        public void run() {
            Message msg = new Message();
            handler.sendMessage(msg);
        }
    };

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        mEditText = (EditText) findViewById(R.id.mEditText);
        mBar = (ProgressBar) findViewById(R.id.mBar);
        mButton = (Button) findViewById(R.id.mButton);
        mButton.setOnClickListener(new ClickEven());

        fileName = "my.jpg";
        //判断SDcard是否存在。
        if(Environment.getExternalStorageState() == Environment.MEDIA_MOUNTED){
            SDPath ="/sdcard/";
        }else{
            Toast.makeText(this, "SDcard不存在", Toast.LENGTH_LONG).show();
        }

    }

    private class ClickEven implements OnClickListener{

        public void onClick(View v) {
            urlStr = mEditText.getText().toString();
            mBar.setVisibility(View.VISIBLE);
            download = new Download(urlStr,fileName,SDPath,DownloadActivity.this);
            download.downloadFile();
            //将progressbar的大小设置为下载文件大小
            mBar.setMax(download.getFileSize());
            //定时刷新
            mTimer.schedule(mTask, 0, 200);
        }

    }
}

此例实现比较简单，还可以加入断点续传的功能，把程序停止时当前线程现在的位置存入数据库中，再次下载时从数据库中取出即可；