android多线程下载详解

android多线程下载详解

[转载：]  

http://www.ideasandroid.com/?p=328

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) {
 
}
 
}
}
}

程序中做了详细的注释，这里不再阐述。
有问题请留言！