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();  
}

 
　　
 
三、Writer类：下载线程，负责向buf[]写数据。
 
        /** *//**
* http://www.bt285.cn http://www.5a520.cn
*/
package instream;  
import java.io.IOException;  
import java.net.URL;  
 
public final class Writer implements Runnable {  
    private static boolean isalive = true;  
    private byte[] buf;  
    private IWriterCallBack icb;  
    protected int index;            //buf[]内"块"索引号  
    protected long start_pos;       //index对应的文件位置(相对于文件首的偏移量)  
    protected int await_count;      //用于判断:下载速度足够就退出一个"写"线程  
    private HttpReader hr;  
      
    public Writer(IWriterCallBack call_back, URL u, byte[] b, int i) {  
        hr = new HttpReader(u);  
        if(HttpReader.getContentLength() == 0)  //实例化HttpReader对象都不成功  
            return;  
        icb = call_back;  
        buf = b;  
        Thread t = new Thread(this,"dt_"+i);  
        t.setPriority(Thread.NORM_PRIORITY + 1);  
        t.start();  
    }  
      
    public void run() {  
        int write_bytes=0, write_pos=0, rema = 0, retry = 0;  
        boolean cont = true;  
        while (cont) {  
            try {  
                // 1.等待空闲块  
                if(retry == 0) {  
                    if (icb.tryWriting(this) == false)  
                        break;  
                    write_bytes = 0;  
                    rema = BuffRandAcceURL.UNIT_LENGTH;  
                    write_pos = index << BuffRandAcceURL.UNIT_LENGTH_BITS;  
                }  
                  
                // 2.定位  
                hr.seek(start_pos);  
 
                // 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, write_pos, w)) == -1) {  
                        cont = false;  
                        break;  
                    }  
                    rema -= w;  
                    write_pos += w;  
                    start_pos += w;  
                    write_bytes += w;  
                }  
                  
                //4.通知"读"线程  
                retry = 0;  
                icb.updateBuffer(index, write_bytes);  
            } catch (InterruptedException e) {  
                isalive = false;  
                icb.terminateWriters();  
                break;  
            } catch (IOException e) {  
                if(++retry == HttpReader.MAX_RETRY) {  
                    isalive = false;  
                    icb.terminateWriters();  
                    break;  
                }  
            }  
        }  
        icb.updateWriterCount();  
        try {  
            hr.close();  
        } catch (Exception e) {}  
        hr = null;  
        buf = null;  
        icb = null;  
    }  
 
}

 
四、IRandomAccess接口：
 
随机读取文件接口，BuffRandAcceURL类和BuffRandAcceFile类实现接口方法。BuffRandAcceFile类实现读取本地磁盘文件，这儿就不给出其源码了。
 
        package instream;  
 
public interface IRandomAccess {  
    public int read() throws Exception;  
    public int read(byte b[]) throws Exception;  
    public int read(byte b[], int off, int len) throws Exception;  
    public int dump(int src_off, byte b[], int dst_off, int len) throws Exception;  
    public void seek(long pos) throws Exception;  
    public long length();  
    public long getFilePointer();  
    public void close();  
}

 
五、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？以下只是我的一次尝试，请指正。
 
        /** *//**
* http://www.5a520.cn  http://www.bt285.cn
*/
package instream;  
 
import java.net.URL;  
import java.net.URLDecoder;  
import decode.Header;  
import tag.MP3Tag;  
import tag.TagThread;  
 
public final class BuffRandAcceURL implements IRandomAccess, IWriterCallBack {  
    public static final int UNIT_LENGTH_BITS = 15;                  //32K  
    public static final int UNIT_LENGTH = 1 << UNIT_LENGTH_BITS;  
    public static final int BUF_LENGTH = UNIT_LENGTH << 4;            //16块  
    public static final int UNIT_COUNT = BUF_LENGTH >> UNIT_LENGTH_BITS;  
    public static final int BUF_LENGTH_MASK = (BUF_LENGTH - 1);  
    private static final int MAX_WRITER = 8;  
    private static long file_pointer;  
    private static int read_pos;  
    private static int fill_bytes;  
    private static byte[] buf;      //同时也作读写同步锁:buf.wait()/buf.notify()  
    private static int[] buf_bytes;  
    private static int buf_index;  
    private static int alloc_pos;  
    private static URL url = null;  
    private static boolean isalive = true;  
    private static int writer_count;  
    private static int await_count;  
    private long file_length;  
    private long frame_bytes;  
      
    public BuffRandAcceURL(String sURL) throws Exception {  
        this(sURL,MAX_WRITER);  
    }  
      
    public BuffRandAcceURL(String sURL, int download_threads) throws Exception {  
        buf = new byte[BUF_LENGTH];  
        buf_bytes = new int[UNIT_COUNT];  
        url = new URL(sURL);  
          
        //创建线程以异步方式解析ID3  
        new TagThread(url);  
          
        //打印当前文件名  
        try {  
            String s = URLDecoder.decode(sURL, "GBK");  
            System.out.println("start>> " + s.substring(s.lastIndexOf("/") + 1));  
            s = null;  
        } catch (Exception e) {  
            System.out.println("start>> " + sURL);  
        }  
          
        //创建"写"线程  
        for(int i = 0; i < download_threads; i++)  
            new Writer(this, url, buf, i+1);  
        frame_bytes = file_length = HttpReader.getContentLength();  
        if(file_length == 0) {  
            Header.strLastErr = "连接URL出错，重试 " + HttpReader.MAX_RETRY + " 次后放弃。";  
            throw new Exception("retry " + HttpReader.MAX_RETRY);  
        }  
        writer_count = download_threads;  
          
        //缓冲  
        try_cache();  
          
        //跳过ID3 v2  
        MP3Tag mP3Tag = new MP3Tag();  
        int v2_size = mP3Tag.checkID3V2(buf,0);  
        if (v2_size > 0) {  
            frame_bytes -= v2_size;  
            //seek(v2_size):  
            fill_bytes -= v2_size;  
            file_pointer = v2_size;  
            read_pos = v2_size;  
            read_pos &= BUF_LENGTH_MASK;  
            int units = v2_size >> UNIT_LENGTH_BITS;  
            for(int i = 0; i < units; i++) {  
                buf_bytes[i] = 0;  
                this.notifyWriter();  
            }  
            buf_bytes[units] -= v2_size;  
            this.notifyWriter();  
        }  
        mP3Tag = null;  
    }  
      
    private void try_cache() throws InterruptedException {  
        int cache_size = BUF_LENGTH;  
        if(cache_size > (int)file_length - alloc_pos)  
            cache_size = (int)file_length - alloc_pos;  
        cache_size -= UNIT_LENGTH;  
          
        //等待填写当前正在读的那"一块"缓冲区  
        /**//*if(fill_bytes >= cache_size && writer_count > 0) { 
            synchronized (buf) { 
                buf.wait(); 
            } 
            return; 
        }*/ 
          
        //等待填满缓冲区  
        while (fill_bytes < cache_size) {  
            if (writer_count == 0 || isalive == false)  
                return;  
            if(BUF_LENGTH > (int)file_length - alloc_pos)  
                cache_size = (int)file_length - alloc_pos - UNIT_LENGTH;  
            System.out.printf(" [缓冲%1$6.2f%%] ",(float)fill_bytes / cache_size * 100);  
            synchronized (buf) {  
                buf.wait();  
            }  
        }  
        System.out.printf(" ");  
    }  
      
    private int try_reading(int i, int len) throws Exception {  
        int n = (i == UNIT_COUNT - 1) ? 0 : (i + 1);  
        int r = (buf_bytes[i] == 0) ? 0 : (buf_bytes[i] + buf_bytes[n]);  
        while (r < len) {  
            if (writer_count == 0 || isalive == false)  
                return r;  
            try_cache();  
            r = (buf_bytes[i] == 0) ? 0 : (buf_bytes[i] + buf_bytes[n]);  
        }  
          
        return len;  
    }  
      
    /**//* 
     * 各个"写"线程互斥等待空闲块 
     */ 
    public synchronized boolean tryWriting(Writer w) throws InterruptedException {  
        await_count++;  
        while (buf_bytes[buf_index] != 0 && isalive) {  
            this.wait();  
        }  
          
        //下载速度足够就结束一个"写"线程  
        if(writer_count > 1 && w.await_count >= await_count &&  
                w.await_count >= writer_count)  
            return false;  
          
        if(alloc_pos >= file_length)  
            return false;  
        w.await_count = await_count;  
        await_count--;  
        w.start_pos = alloc_pos;  
        w.index = buf_index;  
        alloc_pos += UNIT_LENGTH;  
        buf_index = (buf_index == UNIT_COUNT - 1) ? 0 : buf_index + 1;  
        return isalive;  
    }  
      
    public void updateBuffer(int i, int len) {  
        synchronized (buf) {  
            buf_bytes[i] = len;  
            fill_bytes += len;  
            buf.notify();  
        }  
    }  
      
    public void updateWriterCount() {  
        synchronized (buf) {  
            writer_count--;  
            buf.notify();  
        }  
    }  
      
    public synchronized void notifyWriter() {  
        this.notifyAll();  
    }  
      
    public void terminateWriters() {  
        synchronized (buf) {  
            if (isalive) {  
                isalive = false;  
                Header.strLastErr = "读取文件超时。重试 " + HttpReader.MAX_RETRY  
                        + " 次后放弃，请您稍后再试。";  
            }  
            buf.notify();  
        }  
          
        notifyWriter();       
    }  
      
    public int read() throws Exception {  
        int iret = -1;  
        int i = read_pos >> UNIT_LENGTH_BITS;  
        // 1."等待"有1字节可读  
        while (buf_bytes[i] < 1) {  
            try_cache();  
            if (writer_count == 0)  
                return -1;  
        }  
        if(isalive == false)  
            return -1;  
 
        // 2.读取  
        iret = buf[read_pos] & 0xff;  
        fill_bytes--;  
        file_pointer++;  
        read_pos++;  
        read_pos &= BUF_LENGTH_MASK;  
        if (--buf_bytes[i] == 0)  
            notifyWriter();     // 3.通知  
 
        return iret;  
    }  
      
    public int read(byte b[]) throws Exception {  
        return read(b, 0, b.length);  
    }  
 
    public int read(byte[] b, int off, int len) throws Exception {  
        if(len > UNIT_LENGTH)  
            len = UNIT_LENGTH;  
        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_len = BUF_LENGTH - read_pos; // write_pos != BUF_LENGTH  
        if (tail_len < len) {  
            System.arraycopy(buf, read_pos, b, off, tail_len);  
            System.arraycopy(buf, 0, b, off + tail_len, len - tail_len);  
        } else 
            System.arraycopy(buf, read_pos, b, off, len);  
 
        fill_bytes -= len;  
        file_pointer += len;  
        read_pos += len;  
        read_pos &= BUF_LENGTH_MASK;  
        buf_bytes[i] -= len;  
        if (buf_bytes[i] < 0) {  
            int ni = read_pos >> UNIT_LENGTH_BITS;  
            buf_bytes[ni] += buf_bytes[i];  
            buf_bytes[i] = 0;  
            notifyWriter();  
        } else if (buf_bytes[i] == 0)  
            notifyWriter();  
          
        return len;  
    }  
      
    /**//* 
     * 从src_off位置复制,不移动文件"指针" 
     */ 
    public int dump(int src_off, byte b[], int dst_off, int len) throws Exception {  
        int rpos = read_pos + src_off;  
        if(try_reading(rpos >> UNIT_LENGTH_BITS, len) < len || isalive == false)  
            return -1;  
        int tail_len = BUF_LENGTH - rpos;  
        if (tail_len < len) {  
            System.arraycopy(buf, rpos, b, dst_off, tail_len);  
            System.arraycopy(buf, 0, b, dst_off + tail_len, len - tail_len);  
        } else 
            System.arraycopy(buf, rpos, b, dst_off, len);  
        // 不发信号  
 
        return len;  
    }  
      
    public long length() {  
        return file_length;  
    }  
      
    public long getFilePointer() {  
        return file_pointer;  
    }  
 
    public void close() {  
        //  
    }  
      
    //  
    public void seek(long pos) throws Exception {  
        //  
    }  
      
}