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;
}
}
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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();
}
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