JAVA实现环形缓冲多线程读取远程文件

       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。多线程中的同步、HttpURLConnection的超时阻塞等因素都会使代码看起来异常复杂。

      问题模型 数组buf[]，N个“写”线程从文件顺序读入固定长度数据写入buf[]；一个“读”线程按照文件内容顺序从缓冲区读取，一次读任意长度。一个写线程发生错误或文件读完，这个读线程和所有写线程退出。

      针对上述问题，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；每次读小于32K的任意字节。线程同步：各个写线程互斥等待空闲块，用信号量机制分配空闲块；各写线程并发填写buf[]；读线程和各写线程并发使用buf[]。

      为突出主题略去了一些和本文无关的代码。

 

一、HttpReader类功能：HTTP协议从指定URL读取数据。

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 URL url;
	private HttpURLConnection httpConnection;
	private InputStream in_stream;
	private long cur_pos;			//决定seek方法中是否执行文件读取定位
	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;
	}

	public int read(byte[] b, int off, int len) throws IOException {
		int r = in_stream.read(b, off, len);
		cur_pos += r;
		return r;
	}
			
	/*
	 * 抛出异常通知重试.
	 * 例如响应码503可能是由某种暂时的原因引起的,同一IP频繁的连接请求会遭服务器拒绝.
	 */
	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(200);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
			throw new IOException("HTTP responseCode="+responseCode);
		}

		in_stream = httpConnection.getInputStream();
		cur_pos = start_pos;
	}
}

  

二、IWriterCallBack接口

package instream;

/*
 * 读/写通信接口.类似于C++的回调函数
 * 
 * 例:
 * class BuffRandAcceURL 内实现本接口的方法tryWriting()
 * class BuffRandAcceURL 内new Writer(this, ...)传值到Writer.icb
 * class Writer 内调用icb.tryWriting()
 */
public interface IWriterCallBack {
	public int tryWriting() throws InterruptedException;
	public void updateBuffer(int i, int len);
	public void updateWriterCount();
	public int getWriterCount();
	public void terminateWriters();
}

  

三、Writer类：下载线程，负责向buf[]写数据。

package instream;
import java.net.URL;

public final class Writer implements Runnable {
	private static boolean isalive = true;	// 一个线程超时其它线程也能退出
	private static byte[] buf;
	private static IWriterCallBack icb;
	private HttpReader hr;
	
	public Writer(IWriterCallBack cb, URL u, byte[] b, int i) {
		hr = new HttpReader(u);
		icb = cb;
		buf = b;
		Thread t = new Thread(this,"dt_"+i);
		t.setPriority(Thread.NORM_PRIORITY + 1);
		t.start();
	}
	
	public void run() {
		int wbytes=0, wpos=0, rema = 0, retry = 0;
		int idxmask = BuffRandAcceURL.UNIT_COUNT - 1;
		boolean cont = true;
		int index = 0;		//buf[]内"块"索引号
		int startpos = 0;	//index对应的文件位置(相对于文件首的偏移量)
		long time0 = 0;
		while (cont) {
			try {
				// 1.等待空闲块
				if(retry == 0) {
					if ((startpos = icb.tryWriting()) == -1)
						break;
					index = (startpos >> BuffRandAcceURL.UNIT_LENGTH_BITS) & idxmask;
					wpos = startpos & BuffRandAcceURL.BUF_LENGTH_MASK;
					wbytes = 0;
					rema = BuffRandAcceURL.UNIT_LENGTH;
					time0 = System.currentTimeMillis();
				}
				
				// 2.定位
				hr.seek(startpos);

				// 3.下载"一块"
				int w;
				while (rema > 0 && isalive) {
					w = (rema < 2048) ? rema : 2048; //每次读几K合适?
					if ((w = hr.read(buf, wpos, w)) == -1) {
						cont = false;
						break;
					}
					rema -= w;
					wpos += w;
					startpos += w;	// 能断点续传
					wbytes += w;
				}
				
				// 下载速度足够快就结束本线程
				long t = System.currentTimeMillis() - time0;
				if(icb.getWriterCount() > 1 && t < 500)
					cont = false;
				
				//4.通知"读"线程
				retry = 0;
				icb.updateBuffer(index, wbytes);
			} catch (Exception e) {
				if(++retry == HttpReader.MAX_RETRY) {
					isalive = false;
					icb.terminateWriters();
					break;
				}
			}
		}
		icb.updateWriterCount();
		try {
			hr.close();
		} catch (Exception e) {}
		hr = null;
	}
}

 

四、BuffRandAcceURL类功能：创建下载线程；read方法从buf[]读数据。关键是如何简单有效防止死锁？

package instream;
import java.net.URL;

public final class BuffRandAcceURL implements IWriterCallBack {
	public static final int UNIT_LENGTH_BITS = 15;
	public static final int UNIT_LENGTH = 1 << UNIT_LENGTH_BITS; //2^16=32K
	public static final int BUF_LENGTH = UNIT_LENGTH << 4;
	public static final int UNIT_COUNT = BUF_LENGTH >> UNIT_LENGTH_BITS; //16块
	public static final int BUF_LENGTH_MASK = (BUF_LENGTH - 1);
	private static final int MAX_WRITER = 5;
	private static long file_pointer;
	private static int read_pos;
	private static int fill_bytes;
	private static byte[] buf;			//同时作写线程同步锁
	private static int[] unit_bytes;	//同时作读线程互斥锁
	private static int alloc_pos;
	private static URL url;
	private static boolean isalive = true;
	private static int writer_count;
	private static long file_length;
	private static long frame_bytes;
	private static int free_unit = UNIT_COUNT;	// "信号量"计数器
	
	public BuffRandAcceURL(String sURL) throws Exception {
		this(sURL,MAX_WRITER);
	}
	
	public BuffRandAcceURL(String sURL, int download_threads) throws Exception {
		buf = new byte[BUF_LENGTH];
		unit_bytes = new int[UNIT_COUNT];
		url = new URL(sURL);

		// 创建"写"线程
		writer_count = download_threads;
		for (int i = 0; i < download_threads; i++) {
			new Writer(this, url, buf, i + 1);
			Thread.sleep(200);
		}

		try_cache();
	}
	
	/*
	 * 缓冲
	 */
	private void try_cache() throws InterruptedException {
		int cache_size = BUF_LENGTH;
		int bi = unit_bytes[read_pos >> UNIT_LENGTH_BITS];
		if(bi != 0)
			cache_size -= UNIT_LENGTH - bi;
		while (fill_bytes < cache_size) {
			if (writer_count == 0 || isalive == false)
				return;
			synchronized (unit_bytes) {
				unit_bytes.wait(200);	//wait(200)错过通知也可结束循环?
			}
		}
	}

	private int try_reading(int i, int len) throws Exception {
		int n = (i + 1) & (UNIT_COUNT - 1);
		int r = (unit_bytes[i] > 0) ? (unit_bytes[i] + unit_bytes[n]) : unit_bytes[i];
		if (r < len) {
			if (writer_count == 0 || isalive == false)
				return r;
			try_cache();
		}
		
		return len;
	}
	
	/*
	 * 各个"写"线程互斥等待空闲块. 空闲块按由小到大的顺序分配.
	 */
	public int tryWriting() throws InterruptedException {
		int ret = -1;
		synchronized (buf) {
			while (free_unit == 0 && isalive)
				buf.wait();
			
			if(alloc_pos >= file_length || isalive == false)
				return -1;
			ret = alloc_pos;
			alloc_pos += UNIT_LENGTH;
			free_unit--;
		}
		return ret;
	}
	
	/*
	 * "写"线程向buf[]写完数据后调用,通知"读"线程
	 */
	public void updateBuffer(int i, int len) {
		synchronized (unit_bytes) {
			unit_bytes[i] = len;
			fill_bytes += len;
			unit_bytes.notify();
		}
	}
	
	/*
	 * "写"线程准备退出时调用
	 */
	public void updateWriterCount() {
		synchronized (unit_bytes) {
			writer_count--;
			unit_bytes.notify();
		}
	}
	
	public int getWriterCount() {
		return writer_count;
	}
	
	/*
	 * read方法内调用
	 */
	public void notifyWriter() {
		synchronized (buf) {
			buf.notifyAll();
		}
	}
	
	/*
	 * 被某个"写"线程调用,用于终止其它"写"线程;isalive也影响"读"线程流程
	 */
	public void terminateWriters() {
		synchronized (unit_bytes) {	
			if (isalive) {
				isalive = false;
				System.out.println("\n读取文件超时。重试 " + HttpReader.MAX_RETRY
						+ " 次后放弃，请您稍后再试。");
			}
			unit_bytes.notify();
		}
		notifyWriter();
	}
	
	public int read(byte[] b, int off, int len) throws Exception {
		int i = read_pos >> UNIT_LENGTH_BITS;
		
		// 1.等待有足够内容可读
		if(try_reading(i, len) < len || isalive == false)
			return -1;

		// 2.读取
		int tail = BUF_LENGTH - read_pos; // write_pos != BUF_LENGTH
		if (tail < len) {
			System.arraycopy(buf, read_pos, b, off, tail);
			System.arraycopy(buf, 0, b, off + tail, len - tail);
		} else
			System.arraycopy(buf, read_pos, b, off, len);

		fill_bytes -= len;
		file_pointer += len;
		read_pos += len;
		read_pos &= BUF_LENGTH_MASK;
		unit_bytes[i] -= len;
		if (unit_bytes[i] < 0) {
			int ni = read_pos >> UNIT_LENGTH_BITS;
			unit_bytes[ni] += unit_bytes[i];
			unit_bytes[i] = 0;
			free_unit++;
			notifyWriter();
		} else if (unit_bytes[i] == 0) {
			free_unit++;	// 空闲块"信号量"计数加1
			notifyWriter(); 	// 3.通知
		}
		// 如果下一块未填满,意味着文件读完,第1步已处理一次读空两块的情况

		return len;
	}

}

 

本文是JAVA开源项目jmp123源代码的一部分，单独成文旨在与朋友们交流文中提出的问题模型的解决方法。

【jmp123下载地址】http://jmp123.sourceforge.net/

评论

7 楼 qn_lf 2010-08-18  

不用被评为隐藏帖吧 楼主贴这么多 没功劳也有苦劳

6 楼 lfp001 2010-08-18  

<div class="quote_title">ahuango 写道</div>
<div class="quote_div">最好能贴一下使用代码，否则真的很难有耐心看下去。 有无从入手的感觉 </div>
<p> </p>
<div class="quote_title">xiayh1002 写道</div>
<div class="quote_div">代码最好能下载运行一下，多线程光这么看代码，还比较难理解</div>
<p> </p>
<div class="quote_title">beginLi 写道</div>
<div class="quote_div">楼主 开源是有好处的..你的播放器就舍不得拿出来？ </div>
<p> </p>
<p>这儿中介对以缓冲方式读取远程文件的方法的一介简要介绍。更详细的讲解在本站论坛和本站我的博客上有，如果你有兴趣，请看《（四）用JAVA编写MP3解码器——读取文件》</p>
<p>本站论坛 <a href="http://www.iteye.com/topic/740090">http://www.iteye.com/topic/740090</a></p>
<p>本站博客 <a href="http://lfp001.iteye.com/blog/740090">http://lfp001.iteye.com/blog/740090</a></p>

5 楼 beginLi 2010-08-18  

楼主 开源是有好处的..你的播放器就舍不得拿出来？

4 楼 xiayh1002 2010-08-18  

代码最好能下载运行一下，多线程光这么看代码，还比较难理解

3 楼 yangguo 2010-08-18  

bigkai_13 写道

这哥们最先研究肯定是C/C++ 或者是PHP。

+1.变量命名不符java的规范。

2 楼 bigkai_13 2010-08-17  

这哥们最先研究肯定是C/C++ 或者是PHP。

1 楼 ahuango 2010-08-17  

最好能贴一下使用代码，否则真的很难有耐心看下去。 有无从入手的感觉