本文介绍了一种基于多线程技术实现文件下载的方法。通过将文件分割成多个部分，并利用RandomAccessFile进行读写操作，进而实现并发下载。文章详细展示了如何在Android环境中创建多线程下载应用。
多线程下载大概思路就是通过Range 属性实现文件分段，然后用RandomAccessFile 来读写文件，最终合并为一个文件
首先看下效果图
创建工程 ThreadDemo
首先布局文件 threaddemo.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="下载地址"
/>
<TextView
android:id="@+id/downloadurl"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:lines="5"
/>
<TextView
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="线程数"
/>
<EditText
android:id="@+id/downloadnum"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
/>
<ProgressBar
android:id="@+id/downloadProgressBar"
android:layout_width="fill_parent"
style="?android:attr/progressBarStyleHorizontal"
android:layout_height="wrap_content"
/>
<TextView
android:id="@+id/downloadinfo"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="下载进度 0"
/>
<Button
android:id="@+id/downloadbutton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="开始下载"
/>
</LinearLayout> 主界面 Acitivity
public class ThreadDownloadDemo extends Activity {
private TextView downloadurl;
private EditText downloadnum;
private Button downloadbutton;
private ProgressBar downloadProgressBar;
private TextView downloadinfo;
private int downloadedSize = 0;
private int fileSize = 0;
private long downloadtime;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.threaddemo);
downloadurl = (TextView) findViewById(R.id.downloadurl);
downloadurl.setText("http://file16.top100.cn/201105110911/AA5CC27CBE34DEB50A194581D1300881/Special_323149/%E8%8D%B7%E5%A1%98%E6%9C%88%E8%89%B2.mp3");
downloadnum = (EditText) findViewById(R.id.downloadnum);
downloadinfo = (TextView) findViewById(R.id.downloadinfo);
downloadbutton = (Button) findViewById(R.id.downloadbutton);
downloadProgressBar = (ProgressBar) findViewById(R.id.downloadProgressBar);
downloadProgressBar.setVisibility(View.VISIBLE);
downloadProgressBar.setMax(100);
downloadProgressBar.setProgress(0);
downloadbutton.setOnClickListener(new OnClickListener() {
public void onClick(View v) {
download();
downloadtime = SystemClock.currentThreadTimeMillis();
}
});
}
private void download() {
// 获取SD卡目录
String dowloadDir = Environment.getExternalStorageDirectory()
+ "/threaddemodownload/";
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(downloadnum.getText()
.toString()) ? "1" : downloadnum.getText().toString());
String fileName = "hetang.mp3";
//开始下载前把下载按钮设置为不可用
downloadbutton.setClickable(false);
//进度条设为0
downloadProgressBar.setProgress(0);
//启动文件下载线程
new downloadTask("http://file16.top100.cn/201105110911/AA5CC27CBE34DEB50A194581D1300881/Special_323149/%E8%8D%B7%E5%A1%98%E6%9C%88%E8%89%B2.mp3", 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) {
downloadbutton.setClickable(true);
downloadinfo.setText("下载完成！");
Dialog mdialog = new AlertDialog.Builder(ThreadDownloadDemo.this)
.setTitle("提示信息")
.setMessage("下载完成，总用时为:"+(SystemClock.currentThreadTimeMillis()-downloadtime)+"毫秒")
.setNegativeButton("确定", new DialogInterface.OnClickListener(){
@Override
public void onClick(DialogInterface dialog, int which) {
dialog.dismiss();
}
})
.create();
mdialog.show();
} else {
downloadinfo.setText("当前进度:" + progress + "%");
}
downloadProgressBar.setProgress(progress);
}
};
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();
//防止返回-1
InputStream in = conn.getInputStream();
//获取下载文件的总大小
fileSize = conn.getContentLength();
Log.i("bb", "======================fileSize:"+fileSize);
//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++) {
Log.i("bb", "======================i:"+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;
}
}
handler.sendEmptyMessage(0);
//线程暂停一秒
sleep(1000);
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
} 这里启动线程将文件分割为几个部分，每一个部分再启动一个线程去下载数据
下载文件的线程
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;
//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止点
con.setRequestProperty("Range", "bytes=" + startPosition + "-" + endPosition);
Log.i("bb", Thread.currentThread().getName()+" 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");
//设置写文件的起始位置
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) {
e.printStackTrace();
}
}
public boolean isFinished(){
return finished;
}
public int getDownloadSize() {
return downloadSize;
}
}
这里通过RandomAccessFile 的seek方法定位到相应的位置 并实时记录下载量
当然这里需要联网和访问SD卡 所以要加上相应的权限
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"></uses-permission> 本文地址 ： http://www.fengfly.com/plus/view-203373-1.html 挑错 推荐
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