远程控制顺畅无阻碍-java来实现

【 tulaoshi.com - Web开发 】

我平时比较喜欢从网上听歌，有些链接下载速度太慢了。如果用HttpURLConnection类的方法打开连接，然后用InputStream类获得输入流，再用BufferedInputStream构造出带缓冲区的输入流，如果网速太慢的话，无论缓冲区设置多大，听起来都是断断续续的，达不到真正缓冲的目的。于是尝试编写代码实现用缓冲方式读取远程文件，以下贴出的代码是我写的MP3解码器的一部分。我是不怎么赞同使用多线程下载的，加之有的链接下载速度本身就比较快，所以在下载速度足够的情况下，就让下载线程退出，直到只剩下一个下载线程。当然，多线程中令人头痛的死锁问题、HttpURLConnection的超时阻塞问题都会使代码看起来异常复杂。

简要介绍一下实现多线程环形缓冲的方法。将缓冲区buf[]分为16块，每块32K，下载线程负责向缓冲区写数据，每次写一块；读线程（BuffRandAcceURL类）每次读小于32K的任意字节。同步描述：写/写互斥等待空闲块；写/写并发填写buf[]；读/写并发使用buf[]。

经过我很长一段时间使用，我认为比较满意地实现了我的目标，同其它MP3播放器对比，我的这种方法能够比较流畅、稳定地下载并播放。我把实现多线程下载缓冲的方法写出来，不足之处恳请批评指正。

一、HttpReader类功能：HTTP协议从指定URL读取数据

/** *//*** author by http://www.bt285.cn http://www.5a520.cn*/package instream; import java.io.IOException;   import java.io.InputStream;   import java.net.HttpURLConnection;   import java.net.URL; public final class HttpReader {   public static final int MAX_RETRY = 10;   private static long content_length;   private URL url;   private HttpURLConnection httpConnection;   private InputStream in_stream;   private long cur_pos;   //用于决定seek方法中是否执行文件定位   private int connect_timeout;   private int read_timeout;public HttpReader(URL u) {   this(u, 5000, 5000);   }public HttpReader(URL u, int connect_timeout, int read_timeout) {   this.connect_timeout = connect_timeout;   this.read_timeout = read_timeout;   url = u;   if (content_length == 0) { int retry = 0; while (retry  HttpReader.MAX_RETRY) try { this.seek(0); content_length = httpConnection.getContentLength(); break; } catch (Exception e) { retry++; }   }   }public static long getContentLength() {   return content_length;   }public int read(byte[] b, int off, int len) throws IOException {   int r = in_stream.read(b, off, len);   cur_pos += r;   return r;   }public int getData(byte[] b, int off, int len) throws IOException {   int r, rema = len;   while (rema  0) { if ((r = in_stream.read(b, off, rema)) == -1) { return -1; } rema -= r; off += r; cur_pos += r;   }   return len;   }public void close() {   if (httpConnection != null) { httpConnection.disconnect(); httpConnection = null;   }   if (in_stream != null) { try { in_stream.close(); } catch (IOException e) {} in_stream = null;   }   url = null;   }/**//*   * 抛出异常通知再试   * 响应码503可能是由某种暂时的原因引起的,例如同一IP频繁的连接请求可能遭服务器拒绝   */  public void seek(long start_pos) throws IOException {   if (start_pos == cur_pos && in_stream != null) return;   if (httpConnection != null) { httpConnection.disconnect(); httpConnection = null;   }   if (in_stream != null) { in_stream.close(); in_stream = null;   }   httpConnection = (HttpURLConnection) url.openConnection();   httpConnection.setConnectTimeout(connect_timeout);   httpConnection.setReadTimeout(read_timeout);   String sProperty = "bytes=" + start_pos + "-";   httpConnection.setRequestProperty("Range", sProperty);   //httpConnection.setRequestProperty("Connection", "Keep-Alive");   int responseCode = httpConnection.getResponseCode();   if (responseCode  200 || responseCode = 300) { try { Thread.sleep(500); } catch (InterruptedException e) { e.printStackTrace(); } throw new IOException("HTTP responseCode="+responseCode);   } in_stream = httpConnection.getInputStream();   cur_pos = start_pos;   } }

二、IWriterCallBack接口功能：实现读/写通信。

package instream; public interface IWriterCallBack {   public boolean tryWriting(Writer w) throws InterruptedException;   public void updateBuffer(int i, int len);   public void updateWriterCount();   public void terminateWriters();   }