Android进阶：实现多线程下载文件_android开启多线程下载资源-优快云博客

本文介绍了一种基于多线程的文件下载方法，利用Range属性分段下载，并通过RandomAccessFile实现文件的读写与合并。展示了具体的代码实现过程及效果。

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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>