Android中级篇之多线程下载

Android中级篇之多线程下载

2011-06-05 15:50:49

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；