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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
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
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
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. }