鸡爪对女人的危害:android多线程下载详解

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：

第一步，我们先写一个线程类，来完成对指定区域的数据进行下载，如下所示：
package com.ideasandroid.demo; 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=1024; 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=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); //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, 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; }}
接下来就是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：
package com.ideasandroid.demo; 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 = 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() { // 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()) ? "1" : 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(0); //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 * 100))).intValue(); if (progress == 100) { 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 = 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(); //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 = 0; 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 + 1) * blockSize - 1); 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 = 0; 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(0); //休息1秒后再读取下载进度 sleep(1000); } } catch (Exception e) {  }  } }}