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。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。

本次修正了几处可能产生死锁的情况，源码中加入一些注解，略去了一些和本文无关的方法实现。

一、HttpReader类功能：HTTP协议从指定URL读取数据。

Java代码



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;

}

public int getData(byte[] b, int off, int len) throws IOException {

//...

}

public void close() {

//...

}

/*

* 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;

//System.out.println(Thread.currentThread().getName()+ ", cur_pos="+cur_pos);

}

}

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;
}

public int getData(byte[] b, int off, int len) throws IOException {
//...
}

public void close() {
//...
}

/*
* 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;
//System.out.println(Thread.currentThread().getName()+ ", cur_pos="+cur_pos);
}
}

二、IWriterCallBack接口

Java代码



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();

}

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。

Java代码



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;

}

}

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、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。

Java代码



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();

}

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？

Java代码



package instream;

import java.net.URL;

import java.net.URLDecoder;

import tag.ID3Tag;

/*

* FIFO方式共享环形缓冲区buf[]

* buf[]逻辑上分成16块, 每一块的长度UNIT_LENGTH=32K不小于最大帧长1732

*

* 同步: 写/写 -- 互斥等待空闲块

* 读/写 -- 并发访问buf[]

*

*/

public final class BuffRandAcceURL implements IRandomAccess, 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);

// 打印文件名

try {

String s = URLDecoder.decode(sURL, "GBK");

System.out.println(s.substring(s.lastIndexOf("/") + 1));

} catch (Exception e) {

System.out.println(sURL);

}

// 获取文件长度

// 为何同一URL得到的文件长度有时对有时又不对?

frame_bytes = file_length = url.openConnection().getContentLength();

if (file_length == -1)

throw new Exception("ContentLength=-1");

// 创建线程以异步方式解析tag

new TagThread(url, file_length);

// 创建"写"线程

// 线程被创建后立即连接URL开始下载,由于服务器限制了同一IP每秒最大连接次数,频繁连接

// 会被服务器拒绝,因此延时.

writer_count = download_threads;

for (int i = 0; i < download_threads; i++) {

new Writer(this, url, buf, i + 1);

Thread.sleep(200);

}

// 缓冲

try_cache();

// 跳过 ID3 v2

ID3Tag tag = new ID3Tag();

int v2_size = tag.checkID3V2(buf, 0);

if (v2_size > 0) {

frame_bytes -= v2_size;

seek(v2_size);

}

tag = null;

}

/*

* 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;

System.out.printf("/r[缓冲%1$6.2f%%] ",(float)fill_bytes / cache_size * 100);

synchronized (unit_bytes) {

unit_bytes.wait(200); //wait(200)错过通知也可结束循环?

}

}

System.out.printf("/r");

}

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
) : 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;管理空闲块采用类似于C++的信号量机制.

*/

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() throws Exception {

//...

}

public int read(byte b[]) throws Exception {

return read(b, 0, b.length);

}

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;

}

/*

* 从buf[]偏移src_off位置复制,不移动文件"指针",不发信号.

*/

public int dump(int src_off, byte b[], int dst_off, int len) throws Exception {

//...

}

public long length() {

//...

}

public long getFilePointer() {

//...

}

public void close() {

//...

}

/*

* 随机读取定位

*/

public void seek(long pos) throws Exception {

//...

}

}