IO输入/输出从PipedInputStream/PipedOutputStream谈起（源码分析）
转载：http://miaoxiaodong78.blog.163.com/blog/static/18765136200702285946971/
从PipedInputStream/PipedOutputStream谈起
江苏 无锡 缪小东
本篇主要从分析PipeInputStrem和PipedOutputStream谈起。谈及软件设计的变化，以及如何将软件拆分、组合，适配……
1 源代码分析
下面将详细分析PipedInputStream和PipedOutputStream的源代码。
1.1 PipedInputStream
package java.io;
//PipedInputStream必须和PipedOutputStream联合使用。即必须连接输入部分。
//其原理为：PipedInputStream内部有一个Buffer，
//PipedInputStream可以使用InputStream的方法读取其Buffer中的字节。
//PipedInputStream中Buffer中的字节是PipedOutputStream调用PipedInputStream的方法放入的。
public class PipedInputStream extends InputStream {
boolean closedByWriter = false; //标识有读取方或写入方关闭
volatile boolean closedByReader = false;
boolean connected = false; //是否建立连接
Thread readSide; //标识哪个线程
Thread writeSide;
protected static final int PIPE_SIZE = 1024; //缓冲区的默认大小
protected byte buffer[] = new byte[PIPE_SIZE]; //缓冲区
protected int in = -1; //下一个写入字节的位置。0代表空，in==out代表满
protected int out = 0; //下一个读取字节的位置
public PipedInputStream(PipedOutputStream src) throws IOException { //给定源的输入流
connect(src);
}
public PipedInputStream() { } //默认构造器，下部一定要connect源
public void connect(PipedOutputStream src) throws IOException { //连接输入源
src.connect(this); //调用源的connect方法连接当前对象
}
protected synchronized void receive(int b) throws IOException { //只被PipedOuputStream调用
checkStateForReceive(); //检查状态，写入
writeSide = Thread.currentThread(); //永远是PipedOuputStream
if (in == out) awaitSpace(); //输入和输出相等，等待空间
if (in < 0) {
in = 0;
out = 0;
}
buffer[in++] = (byte)(b & 0xFF); //放入buffer相应的位置
if (in >= buffer.length) { in = 0; } //in为0表示buffer已空
}
synchronized void receive(byte b[], int off, int len) throws IOException {
checkStateForReceive();
writeSide = Thread.currentThread(); //从PipedOutputStream可以看出
int bytesToTransfer = len;
while (bytesToTransfer > 0) {
if (in == out) awaitSpace(); //满了，会通知读取的；空会通知写入
int nextTransferAmount = 0;
if (out < in) {
nextTransferAmount = buffer.length - in;
} else if (in < out) {
if (in == -1) {
in = out = 0;
nextTransferAmount = buffer.length - in;
} else {
nextTransferAmount = out - in;
}
}
if (nextTransferAmount > bytesToTransfer) nextTransferAmount = bytesToTransfer;
assert(nextTransferAmount > 0);
System.arraycopy(b, off, buffer, in, nextTransferAmount);
bytesToTransfer -= nextTransferAmount;
off += nextTransferAmount;
in += nextTransferAmount;
if (in >= buffer.length) { in = 0; }
}
}
private void checkStateForReceive() throws IOException { //检查当前状态，等待输入
if (!connected) {
throw new IOException("Pipe not connected");
} else if (closedByWriter || closedByReader) {
throw new IOException("Pipe closed");
} else if (readSide != null && !readSide.isAlive()) {
throw new IOException("Read end dead");
}
}
private void awaitSpace() throws IOException { //Buffer已满，等待一段时间
while (in == out) { //in==out表示满了，没有空间
checkStateForReceive(); //检查接受端的状态
notifyAll(); //通知读取端
try {
wait(1000);
} catch (InterruptedException ex) {
throw new java.io.InterruptedIOException();
}
}
}
synchronized void receivedLast() { //通知所有等待的线程（）已经接受到最后的字节
closedByWriter = true; //
notifyAll();
}
public synchronized int read() throws IOException {
if (!connected) { //检查一些内部状态
throw new IOException("Pipe not connected");
} else if (closedByReader) {
throw new IOException("Pipe closed");
} else if (writeSide != null && !writeSide.isAlive()&& !closedByWriter && (in < 0)) {
throw new IOException("Write end dead");
}
readSide = Thread.currentThread(); //当前线程读取
int trials = 2; //重复两次？？？？
while (in < 0) {
if (closedByWriter) { return -1; } //输入断关闭返回-1
if ((writeSide != null) && (!writeSide.isAlive()) && (--trials < 0)) { //状态错误
throw new IOException("Pipe broken");
}
notifyAll(); // 空了，通知写入端可以写入
try {
wait(1000);
} catch (InterruptedException ex) {
throw new java.io.InterruptedIOException();
}
}
int ret = buffer[out++] & 0xFF; //
if (out >= buffer.length) { out = 0; }
if (in == out) { in = -1; } //没有任何字节
return ret;
}
public synchronized int read(byte b[], int off, int len) throws IOException {
if (b == null) { //检查输入参数的正确性
throw new NullPointerException();
} else if (off < 0 || len < 0 || len > b.length - off) {
throw new IndexOutOfBoundsException();
} else if (len == 0) {
return 0;
}
int c = read(); //读取下一个
if (c < 0) { return -1; } //已经到达末尾了，返回-1
b[off] = (byte) c; //放入外部buffer中
int rlen = 1; //return-len
while ((in >= 0) && (--len > 0)) { //下一个in存在，且没有到达len
b[off + rlen] = buffer[out++]; //依次放入外部buffer
rlen++;
if (out >= buffer.length) { out = 0; } //读到buffer的末尾，返回头部
if (in == out) { in = -1; } //读、写位置一致时，表示没有数据
}
return rlen; //返回填充的长度
}
public synchronized int available() throws IOException { //返回还有多少字节可以读取
if(in < 0)
return 0; //到达末端，没有字节
else if(in == out)
return buffer.length; //写入的和读出的一致，表示满
else if (in > out)
return in - out; //写入的大于读出
else
return in + buffer.length - out; //写入的小于读出的
}
public void close() throws IOException { //关闭当前流，同时释放与其相关的资源
closedByReader = true; //表示由输入流关闭
synchronized (this) { in = -1; } //同步化当前对象，in为-1
}
}
1.2 PipedOutputStream
// PipedOutputStream一般必须和一个PipedInputStream连接。共同构成一个pipe。
//它们的职能是：
package java.io;
import java.io.*;
public class PipedOutputStream extends OutputStream {
private PipedInputStream sink; //包含一个PipedInputStream
public PipedOutputStream(PipedInputStream snk)throws IOException { //带有目的地的构造器
connect(snk);
}
public PipedOutputStream() { } //默认构造器，必须使用下面的connect方法连接
public synchronized void connect(PipedInputStream snk) throws IOException {
if (snk == null) { //检查输入参数的正确性
throw new NullPointerException();
} else if (sink != null || snk.connected) {
throw new IOException("Already connected");
}
sink = snk; //一系列初始化工作
snk.in = -1;
snk.out = 0;
snk.connected = true;
}
public void write(int b) throws IOException { //向流中写入数据
if (sink == null) { throw new IOException("Pipe not connected"); }
sink.receive(b); //本质上是，调用PipedInputStream的receive方法接受此字节
}
public void write(byte b[], int off, int len) throws IOException {
if (sink == null) { //首先检查输入参数的正确性
throw new IOException("Pipe not connected");
} else if (b == null) {
throw new NullPointerException();
} else if ((off < 0) || (off > b.length) || (len < 0) || ((off + len) > b.length) || ((off + len) < 0)) {
throw new IndexOutOfBoundsException();
} else if (len == 0) {
return;
}
sink.receive(b, off, len); //调用PipedInputStream的receive方法接受
}
public synchronized void flush() throws IOException { //flush输出流
if (sink != null) {
synchronized (sink) { sink.notifyAll(); } //本质是通知输入流，可以读取
}
}
public void close() throws IOException { //关闭流同时释放相关资源