Java IO：PipedReader和PipedWriter使用详解及源码分析
使用方法
PipedReader和PipedWriter即管道输入流和输出流，可用于线程间管道通信。它们和PipedInputStream/PipedOutputStream区别是前者操作的是字符后者是字节。
方法介绍
PipedReader提供的API如下：
//构造方法 PipedReader(PipedWriter src) //使用默认的buf的大小和传入的pw构造pr PipedReader(PipedWriter src, int pipeSize) //使用指定的buf的大小和传入的pw构造pr PipedReader() //使用默认大小构造pr PipedReader(int pipeSize) //使用指定大小构造pr //关闭流 void close() //绑定Writer void connect(PipedWriter src) //是否可读 synchronized boolean ready() //读取一个字符 synchronized int read() //读取多个字符到cbuf synchronized int read(char cbuf[], int off, int len) //Writer调用, 向Reader缓冲区写数据 synchronized void receive(int c) synchronized void receive(char c[], int off, int len) synchronized void receivedLast()
PipedWriter提供的API如下：
//构造方法 PipedWriter(PipedReader snk) PipedWriter() //绑定Reader Writer synchronized void connect(PipedReader snk) //关闭流 void close() //刷新流,唤醒Reader synchronized void flush() //写入1个字符,实际是写到绑定Reader的缓冲区 void write(int c) //写入多个字符到Reader缓冲区 void write(char cbuf[], int off, int len)
使用示例
/** * 写线程 */ public class Producer extends Thread { //输出流 private PipedWriter writer = new PipedWriter(); public Producer(PipedWriter writer) { this.writer = writer; } @Override public void run() { try { StringBuilder sb = new StringBuilder(); sb.append("Hello World!"); writer.write(sb.toString()); } catch (Exception e) { e.printStackTrace(); } } } /** * 读取线程 */ public class Consumer extends Thread{ //输入流 private PipedReader reader = new PipedReader(); public Consumer(PipedReader reader) { this.reader = reader; } @Override public void run() { try { char [] cbuf = new char[20]; reader.read(cbuf, 0, cbuf.length); System.out.println("管道流中的数据为: " + new String(cbuf)); } catch (Exception e) { e.printStackTrace(); } } } @org.junit.Test public void testPipedReaderWriter() { /** * 管道流通信核心是,Writer和Reader公用一块缓冲区,缓冲区在Reader中申请, * 由Writer调用和它绑定的Reader的Receive方法进行写. * * 线程间通过管道流通信的步骤为 * 1 建立输入输出流 * 2 绑定输入输出流 * 3 Writer写 * 4 Reader读 */ PipedReader reader = new PipedReader(); PipedWriter writer = new PipedWriter(); Producer producer = new Producer(writer); Consumer consumer = new Consumer(reader); try { writer.connect(reader); producer.start(); consumer.start(); } catch (Exception e) { e.printStackTrace(); } }
运行结果如下：
管道流中的数据为: Hello World!
源码分析
按照演示程序运行过程分析源码，主要有构造方法、connect、writer写、reader读等。
构造方法
PipedWriter构造方法
PipedWriter构造方法有两个，区别是是否指定需要连接的PipedReader对象。
/** * Creates a piped writer connected to the specified piped * reader. Data characters written to this stream will then be * available as input from <code>snk</code>. * * @param snk The piped reader to connect to. * @exception IOException if an I/O error occurs. */ public PipedWriter(PipedReader snk) throws IOException { connect(snk); } /** * Creates a piped writer that is not yet connected to a * piped reader. It must be connected to a piped reader, * either by the receiver or the sender, before being used. * * @see java.io.PipedReader#connect(java.io.PipedWriter) * @see java.io.PipedWriter#connect(java.io.PipedReader) */ public PipedWriter() { }
PipedReader 构造方法
PipedReader构造方法有四个，区别是是否指定要连接的PipedWriter对象以及缓冲区大小设置，默认缓冲区大小为1024。
/** * Creates a <code>PipedReader</code> so that it is connected * to the piped writer <code>src</code> and uses the specified * pipe size for the pipe's buffer. Data written to <code>src</code> * will then be available as input from this stream. * @param src the stream to connect to. * @param pipeSize the size of the pipe's buffer. * @exception IOException if an I/O error occurs. * @exception IllegalArgumentException if {@code pipeSize <= 0}. * @since 1.6 */ public PipedReader(PipedWriter src, int pipeSize) throws IOException { initPipe(pipeSize); //设置缓冲区大小 connect(src); //连接对应的PipedWriter } public PipedReader(PipedWriter src) throws IOException { this(src, DEFAULT_PIPE_SIZE); //默认缓冲区大小 } /** * Creates a <code>PipedReader</code> so that it is not yet * {@link #connect(java.io.PipedWriter) connected} and uses * the specified pipe size for the pipe's buffer. * It must be {@linkplain java.io.PipedWriter#connect( * java.io.PipedReader) connected} to a <code>PipedWriter</code> * before being used. * * @param pipeSize the size of the pipe's buffer. * @exception IllegalArgumentException if {@code pipeSize <= 0}. * @since 1.6 */ public PipedReader(int pipeSize) { initPipe(pipeSize); //指定大小 } public PipedReader() { initPipe(DEFAULT_PIPE_SIZE); //默认1024 }
connect方法
PipedWriter和PipedReader都有connect方法，两者作用相同。实际上PipedReader的connect方法是调用PipedWriter中connect方法实现的。
PipedWriter connect方法
/** * Connects this piped writer to a receiver. If this object * is already connected to some other piped reader, an * <code>IOException</code> is thrown. * <p> * If <code>snk</code> is an unconnected piped reader and * <code>src</code> is an unconnected piped writer, they may * be connected by either the call: * <blockquote><pre> * src.connect(snk)</pre></blockquote> * or the call: * <blockquote><pre> * snk.connect(src)</pre></blockquote> * The two calls have the same effect. * * @param snk the piped reader to connect to. * @exception IOException if an I/O error occurs. */ public synchronized void connect(PipedReader snk) throws IOException { if (snk == null) { throw new NullPointerException(); } else if (sink != null || snk.connected) { throw new IOException("Already connected"); } else if (snk.closedByReader || closed) { throw new IOException("Pipe closed"); } sink = snk; //绑定对应的PipedReader snk.in = -1; //写入操作下标 snk.out = 0; //读取操作下标 snk.connected = true; //连接状态 }
PipedReader connect方法
/** * Causes this piped reader to be connected * to the piped writer <code>src</code>. * If this object is already connected to some * other piped writer, an <code>IOException</code> * is thrown. * @param src The piped writer to connect to. * @exception IOException if an I/O error occurs. */ public void connect(PipedWriter src) throws IOException { src.connect(this); //调用PipedWriter的方法 }
PipedWriter write方法
write有写入一个字符和写入多个字符两种重载方法，实现原理都一样，调用和它绑定的PipedReader的receive方法向缓冲区写数据，下面分析写入多个字符的write方法。
/** * Writes the specified <code>char</code> to the piped output stream. * If a thread was reading data characters from the connected piped input * stream, but the thread is no longer alive, then an * <code>IOException</code> is thrown. * <p> * Implements the <code>write</code> method of <code>Writer</code>. * * @param c the <code>char</code> to be written. * @exception IOException if the pipe is * <a href=PipedOutputStream.html#BROKEN> <code>broken</code></a>, * {@link #connect(java.io.PipedReader) unconnected}, closed * or an I/O error occurs. */ public void write(int c) throws IOException { if (sink == null) { throw new IOException("Pipe not connected"); } sink.receive(c); //调用PipedReader的receive方法 } /** * Receives a char of data. This method will block if no input is * available. */ synchronized void receive(int c) throws IOException { 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"); } writeSide = Thread.currentThread(); //获取当前线程 while (in == out) { //满，唤醒读者。（有点疑惑） if ((readSide != null) && !readSide.isAlive()) { throw new IOException("Pipe broken"); } /* full: kick any waiting readers */ notifyAll(); try { wait(1000); } catch (InterruptedException ex) { throw new java.io.InterruptedIOException(); } } if (in < 0) { in = 0; out = 0; } buffer[in++] = (char) c; if (in >= buffer.length) { in = 0; } }
PipedReader read方法
read方法同样有读取一个字符和读取多个字符两种重载方法，下面分析读取一个字符的read。
/** * Reads the next character of data from this piped stream. * If no character is available because the end of the stream * has been reached, the value <code>-1</code> is returned. * This method blocks until input data is available, the end of * the stream is detected, or an exception is thrown. * * @return the next character of data, or <code>-1</code> if the end of the * stream is reached. * @exception IOException if the pipe is * <a href=PipedInputStream.html#BROKEN> <code>broken</code></a>, * {@link #connect(java.io.PipedWriter) unconnected}, closed, * or an I/O error occurs. */ public synchronized int read() throws IOException { if (!connected) { 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(); int trials = 2; while (in < 0) { //缓冲区为空 if (closedByWriter) { /* closed by writer, return EOF */ return -1; } if ((writeSide != null) && (!writeSide.isAlive()) && (--trials < 0)) { throw new IOException("Pipe broken"); } /* might be a writer waiting */ notifyAll(); //唤醒写者 try { wait(1000); } catch (InterruptedException ex) { throw new java.io.InterruptedIOException(); } } int ret = buffer[out++]; //读 if (out >= buffer.length) { out = 0; } if (in == out) { //所有字符都被读取 /* now empty */ in = -1; } return ret; }
参考：
[1] http://www.2cto.com/kf/201312/263319.html
[2] http://www.cnblogs.com/skywang12345/p/io_20.html
https://blog.csdn.net/sk199048/article/details/51260757
