android多线程下载详解

12-05-10 来源：[db:作者]

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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：

Java代码
1. package com.ideasandroid.demo;
2.
3. import java.io.BufferedInputStream;
4. import java.io.File;
5. import java.io.IOException;
6. import java.io.RandomAccessFile;
7. import java.net.URL;
8. import java.net.URLConnection;
9.
10. import android.util.Log;
11. /**
12. * Copyright (C) 2010 ideasandroid
13. * 演示android多线程下载
14. * 欢迎访问http://www.2cto.com
15. * 让程序开发不再那么神秘
16. *
17. * 单个下载线程
18. */
19. public class FileDownloadThread extends Thread{
20.     private static final int BUFFER_SIZE=1024;
21.     private URL url;
22.     private File file;
23.     private int startPosition;
24.     private int endPosition;
25.     private int curPosition;
26.     //用于标识当前线程是否下载完成
27.     private boolean finished=false;
28.     private int downloadSize=0;
29.     public FileDownloadThread(URL url,File file,int startPosition,int endPosition){
30.         this.url=url;
31.         this.file=file;
32.         this.startPosition=startPosition;
33.         this.curPosition=startPosition;
34.         this.endPosition=endPosition;
35.     }
36.     @Override
37.     public void run() {
38.         BufferedInputStream bis = null;
39.         RandomAccessFile fos = null;
40.         byte[] buf = new byte[BUFFER_SIZE];
41.         URLConnection con = null;
42.         try {
43.             con = url.openConnection();
44.             con.setAllowUserInteraction(true);
45.             //设置当前线程下载的起点，终点
46.             con.setRequestProperty("Range", "bytes=" + startPosition + "-" + endPosition);
47.             //使用java中的RandomAccessFile 对文件进行随机读写操作
48.             fos = new RandomAccessFile(file, "rw");
49.             //设置开始写文件的位置
50.             fos.seek(startPosition);
51.             bis = new BufferedInputStream(con.getInputStream());
52.             //开始循环以流的形式读写文件
53.             while (curPosition < endPosition) {
54.                 int len = bis.read(buf, 0, BUFFER_SIZE);
55.                 if (len == -1) {
56.                     break;
57.                 }
58.                 fos.write(buf, 0, len);
59.                 curPosition = curPosition + len;
60.                 if (curPosition > endPosition) {
61.                     downloadSize+=len - (curPosition - endPosition) + 1;
62.                 } else {
63.                     downloadSize+=len;
64.                 }
65.             }
66.             //下载完成设为true
67.             this.finished = true;
68.             bis.close();
69.             fos.close();
70.         } catch (IOException e) {
71.           Log.d(getName() +" Error:", e.getMessage());
72.         }
73.     }
74.
75.     public boolean isFinished(){
76.         return finished;
77.     }
78.
79.     public int getDownloadSize() {
80.         return downloadSize;
81.     }
82. }

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：

Java代码
1. package com.ideasandroid.demo;
2.
3. import java.io.File;
4. import java.net.URL;
5. import java.net.URLConnection;
6. import android.app.Activity;
7. import android.os.Bundle;
8. import android.os.Environment;
9. import android.os.Handler;
10. import android.os.Message;
11. import android.view.View;
12. import android.view.View.OnClickListener;
13. import android.widget.Button;
14. import android.widget.EditText;
15. import android.widget.ProgressBar;
16. import android.widget.TextView;
17. /**
18. * Copyright (C) 2010 ideasandroid
19. * 演示android多线程下载
20. * 欢迎访问http://www.2cto.com
21. * 让程序开发不再那么神秘
22. */
23. public class FileDownloadDemo extends Activity {
24.
25.     private EditText downloadUrl;
26.     private EditText downloadFileName;
27.     private EditText downloadThreadNum;
28.     private Button downloadBt;
29.     private ProgressBar downloadProgressBar;
30.     private TextView progressMessage;
31.     private int downloadedSize = 0;
32.     private int fileSize = 0;
33.
34.     @Override
35.     public void onCreate(Bundle savedInstanceState) {
36.         super.onCreate(savedInstanceState);
37.         setContentView(R.layout.main);
38.
39.         downloadUrl = (EditText) findViewById(R.id.downloadUrl);
40.         downloadFileName = (EditText) findViewById(R.id.downloadFileName);
41.         downloadThreadNum = (EditText) findViewById(R.id.downloadThreadNum);
42.         progressMessage = (TextView) findViewById(R.id.progressMessage);
43.         downloadBt = (Button) findViewById(R.id.downloadBt);
44.         downloadProgressBar = (ProgressBar) findViewById(R.id.downloadProgressBar);
45.         downloadProgressBar.setVisibility(View.VISIBLE);
46.         downloadProgressBar.setMax(100);
47.         downloadProgressBar.setProgress(0);
48.         downloadBt.setOnClickListener(new OnClickListener() {
49.             public void onClick(View v) {
50.                 download();
51.             }
52.         });
53.     }
54.
55.     private void download() {
56.         // 获取SD卡目录
57.         String dowloadDir = Environment.getExternalStorageDirectory()
58.                 + "/ideasdownload/";
59.         File file = new File(dowloadDir);
60.         //创建下载目录
61.         if (!file.exists()) {
62.             file.mkdirs();
63.         }
64.
65.         //读取下载线程数，如果为空，则单线程下载
66.         int downloadTN = Integer.valueOf("".equals(downloadThreadNum.getText()
67.                 .toString()) ? "1" : downloadThreadNum.getText().toString());
68.         //如果下载文件名为空则获取Url尾为文件名
69.         int fileNameStart = downloadUrl.getText().toString().lastIndexOf("/");
70.         String fileName = "".equals(downloadFileName.getText().toString()) ? downloadUrl
71.                 .getText().toString().substring(fileNameStart)
72.                 : downloadFileName.getText().toString();
73.         //开始下载前把下载按钮设置为不可用
74.         downloadBt.setClickable(false);
75.         //进度条设为0
76.         downloadProgressBar.setProgress(0);
77.         //启动文件下载线程
78.         new downloadTask(downloadUrl.getText().toString(), Integer
79.                 .valueOf(downloadTN), dowloadDir + fileName).start();
80.     }
81.
82.     Handler handler = new Handler() {
83.         @Override
84.         public void handleMessage(Message msg) {
85.             //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
86.             int progress = (Double.valueOf((downloadedSize * 1.0 / fileSize * 100))).intValue();
87.             if (progress == 100) {
88.                 downloadBt.setClickable(true);
89.                 progressMessage.setText("下载完成！");
90.             } else {
91.                 progressMessage.setText("当前进度:" + progress + "%");
92.             }
93.             downloadProgressBar.setProgress(progress);
94.         }
95.
96.     };
97.
98.     /**
99.      * @author ideasandroid
100.      * 主下载线程
101.      */
102.     public class downloadTask extends Thread {
103.         private int blockSize, downloadSizeMore;
104.         private int threadNum = 5;
105.         String urlStr, threadNo, fileName;
106.
107.         public downloadTask(String urlStr, int threadNum, String fileName) {
108.             this.urlStr = urlStr;
109.             this.threadNum = threadNum;
110.             this.fileName = fileName;
111.         }
112.
113.         @Override
114.         public void run() {
115.             FileDownloadThread[] fds = new FileDownloadThread[threadNum];
116.             try {
117.                 URL url = new URL(urlStr);
118.                 URLConnection conn = url.openConnection();
119.                 //获取下载文件的总大小
120.                 fileSize = conn.getContentLength();
121.                 //计算每个线程要下载的数据量
122.                 blockSize = fileSize / threadNum;
123.                 // 解决整除后百分比计算误差
124.                 downloadSizeMore = (fileSize % threadNum);
125.                 File file = new File(fileName);
126.                 for (int i = 0; i < threadNum; i++) {
127.                     //启动线程，分别下载自己需要下载的部分
128.                     FileDownloadThread fdt = new FileDownloadThread(url, file,
129.                             i * blockSize, (i + 1) * blockSize - 1);
130.                     fdt.setName("Thread" + i);
131.                     fdt.start();
132.                     fds[i] = fdt;
133.                 }
134.                 boolean finished = false;
135.                 while (!finished) {
136.                     // 先把整除的余数搞定
137.                     downloadedSize = downloadSizeMore;
138.                     finished = true;
139.                     for (int i = 0; i < fds.length; i++) {
140.                         downloadedSize += fds[i].getDownloadSize();
141.                         if (!fds[i].isFinished()) {
142.                             finished = false;
143.                         }
144.                     }
145.                     //通知handler去更新视图组件
146.                     handler.sendEmptyMessage(0);
147.                     //休息1秒后再读取下载进度
148.                     sleep(1000);
149.                 }
150.             } catch (Exception e) {
151.
152.             }
153.
154.         }
155.     }

摘自 whoyuhui

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