Android -- 多线程下载， 断点下载

1. 原理图

2. 示例代码

需要权限

<uses-permission android:name="android.permission.INTERNET"/>
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

activity_main.xml 布局文件

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity" >

    <EditText
        android:id="@+id/et_path"
		android:text="http://192.168.1.100:8080/360.exe"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="请输入下载的路径" >
    </EditText>

    <ProgressBar
        android:id="@+id/pb"
        style="?android:attr/progressBarStyleHorizontal"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="downLoad"
        android:text="下载" />

    <TextView
        android:id="@+id/tv_process"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="下载进度" />
    
</LinearLayout>

MainActivity.java

public class MainActivity extends Activity {
	protected static final int DOWN_LOAD_ERROR = 1;
	protected static final int SERVER_ERROR = 2;
	public static final int DOWN_LAOD_FINSIH = 3;
	public static final int UPDATE_TEXT = 4;
	private EditText et_path;
	private ProgressBar pb; //下载的进度条.
	public static int threadCount = 3;
	public static int runningThread = 3 ;
	
	public int currentProcess = 0; //当前进度.
	
	private TextView tv_process;
	
	
	private Handler handler = new Handler(){
		public void handleMessage(android.os.Message msg) {
			switch (msg.what) {
			case DOWN_LOAD_ERROR:
				Toast.makeText(getApplicationContext(), "下载失败", 0).show();
				break;
			case SERVER_ERROR:
				Toast.makeText(getApplicationContext(), "服务器 错误,下载失败", 0).show();
				break;
			case DOWN_LAOD_FINSIH:
				Toast.makeText(getApplicationContext(), "文件下载完毕", 0).show();
				break;
			case UPDATE_TEXT:
				tv_process.setText("当前进度:"+pb.getProgress()*100/pb.getMax());
				break;
			}
			
			
		};
	};

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		et_path = (EditText) this.findViewById(R.id.et_path);
		pb = (ProgressBar) findViewById(R.id.pb);
		tv_process = (TextView) findViewById(R.id.tv_process);
	}

	public void downLoad(View view) {
		final String path = et_path.getText().toString().trim();
		if (TextUtils.isEmpty(path)) {
			Toast.makeText(this, "下载路径错误", 0).show();
			return;
		}
		currentProcess = 0;
		new Thread() {
			public void run() {
				try {
					//String path = "http://192.168.1.100:8080/360.exe";
					URL url = new URL(path);
					HttpURLConnection conn = (HttpURLConnection) url
							.openConnection();
					conn.setConnectTimeout(5000);
					conn.setRequestMethod("GET");
					int code = conn.getResponseCode();
					if (code == 200) {
						// 服务器返回的数据的长度 实际上就是文件的长度
						int length = conn.getContentLength();
						pb.setMax(length);//设置进度条的最大值.
						System.out.println("文件总长度:" + length);
						// 在客户端本地 创建出来一个大小跟服务器端文件一样大小的临时文件
						RandomAccessFile raf = new RandomAccessFile("/sdcard/setup.exe", 
								"rwd");
						// 指定创建的这个文件的长度
						raf.setLength(length);
						raf.close();

						// 假设是3个线程去下载资源.
						// 平均每一个线程下载的文件的大小.
						int blockSize = length / threadCount;
						for (int threadId = 1; threadId <= threadCount; threadId++) {
							// 第一个线程下载的开始位置
							int startIndex = (threadId - 1) * blockSize;
							int endIndex = threadId * blockSize - 1;
							if (threadId == threadCount) {// 最后一个线程 下载的长度 要稍微长一点
								endIndex = length;
							}
							System.out.println("线程:" + threadId + "下载:---"
									+ startIndex + "--->" + endIndex);
							new DownloadThread(path, threadId, startIndex, endIndex)
									.start();
						}

					} else {
						System.out.println("服务器错误.");
						Message msg = new Message();
						msg.what = SERVER_ERROR;
						handler.sendMessage(msg);
					}
				} catch (Exception e) {
					e.printStackTrace();
					Message msg = new Message();
					msg.what = DOWN_LOAD_ERROR;
					handler.sendMessage(msg);
					
					
				}

			};
		}.start();

	}
	
	
	/**
	 * 下载文件的子线程 每一个线程 下载对应位置的文件
	 * 
	 * @author Administrator
	 * 
	 */
	public  class DownloadThread extends Thread {
		private int threadId;
		private int startIndex;
		private int endIndex;
		private String path;

		/**
		 * @param path
		 *            下载文件在服务器上的路径
		 * @param threadId
		 *            线程id
		 * @param startIndex
		 *            线程下载的开始位置
		 * @param endIndex
		 *            线程下载的结束位置.
		 */
		public DownloadThread(String path, int threadId, int startIndex,
				int endIndex) {
			this.threadId = threadId;
			this.startIndex = startIndex;
			this.endIndex = endIndex;
			this.path = path;
		}

		@Override
		public void run() {
			try {

				// 检查是否存在 记录下载长度的文件 ,如果存在读取这个文件的数据.
				//------------------替换成存数据库-------------------------
				File tempFile = new File("/sdcard/"+threadId + ".txt");
				if (tempFile.exists() && tempFile.length() > 0) {
					FileInputStream fis = new FileInputStream(tempFile);
					byte[] temp = new byte[1024];
					int leng = fis.read(temp);
					String downloadLen = new String(temp, 0, leng);
					int downloadlenInt = Integer.parseInt(downloadLen);
					
					int alreadyDownlodint = downloadlenInt - startIndex ;
					currentProcess+=alreadyDownlodint; //计算上次断点 已经下载的文件的长度.
					
					startIndex = downloadlenInt;//修改下载的真实的开始位置.
					fis.close();
				}
				//--------------------------------------------
				URL url = new URL(path);
				HttpURLConnection conn = (HttpURLConnection) url
						.openConnection();
				conn.setRequestMethod("GET");
				// 重要: 请求服务器下载部分的文件 指定文件的位置.
				conn.setRequestProperty("Range", "bytes=" + startIndex + "-"
						+ endIndex);
				System.out.println("线程真实下载:" + threadId + "下载:---" + startIndex
						+ "--->" + endIndex);
				conn.setConnectTimeout(5000);
				int code = conn.getResponseCode(); // 从服务器请求全部资源 200 ok
													// 如果从服务器请求部分资源 206 ok
				if (code == 206) {
					InputStream is = conn.getInputStream();// 已经设置了 请求的位置
					RandomAccessFile raf = new RandomAccessFile("/sdcard/setup.exe",
							"rwd");
					// 随机写文件的时候 从哪个位置开始写
					raf.seek(startIndex);// 定位文件
					int len = 0;
					byte[] buffer = new byte[1024];
					int total = 0;// 已经下载的数据长度
					while ((len = is.read(buffer)) != -1) {
						RandomAccessFile file = new RandomAccessFile("/sdcard/"+threadId
								+ ".txt", "rwd");// 作用: 记录当前线程下载的数据长度
						raf.write(buffer, 0, len);
						total += len;
						//System.out.println("线程:" + threadId + "total:" + total); 
						file.write(( total+startIndex+"").getBytes());//记录的是 下载位置.
						file.close();
						
						//更新进度条
						synchronized (MainActivity.this) {
							currentProcess+=len;//获取所有线程下载的总进度.
							pb.setProgress(currentProcess);//更改界面上progressbar 进度条的进度
							//特殊情况 progressbar progressdialog 进度条对话框 可以直接在子线程里面更新ui  内部代码 特殊处理
							Message msg = Message.obtain();//复用旧的消息 避免创建新的消息
							msg.what = UPDATE_TEXT;
							handler.sendMessage(msg);
						}
						
					}
					is.close();
					raf.close();
					System.out.println("线程:" + threadId + "下载完毕了...");
				} else {
					System.out.println("线程:" + threadId + "下载失败...");
				}
								
				//如何去判断应用程序已经下载完毕.
				
			} catch (Exception e) {
				e.printStackTrace();
			}finally{
				threadFinish();
			}

		}

		private synchronized void threadFinish() {
			runningThread --;
			if(runningThread==0){//所有的线程 已经执行完毕了.
				for(int i= 1;i<=3;i++){
					File file = new File("/sdcard/"+i+".txt");
					file.delete();
				}
				System.out.println("文件下载完毕 ,删除所有的下载记录.");
				Message msg = new Message();
				msg.what = DOWN_LAOD_FINSIH;
				handler.sendMessage(msg);
			}
		}
	}
}