Java-IO之PipedReader和PipedWriter

PipedReader和PipedWriter与PipedInputStream和PipedOutputStream一样，都可以用于管道通信。PipedWriter是字符管道输出流，继承于Writer；PipedReader是字符管道输入流，继承于Reader，PipedWriter和PipedReader的作用是可以通过管道进行线程间的通讯。两者必须要配套使用。

示例程序：

public class Receiver extends Thread {

    // 管道输入流对象。
    // 它和“管道输出流(PipedWriter)”对象绑定，
    // 从而可以接收“管道输出流”的数据，再让用户读取。
    private PipedReader in = new PipedReader();

    // 获得“管道输入流对象”
    public PipedReader getReader()
    {
        return in;
    }

    @Override
    public void run(){
        readMessageOnce() ;
        //readMessageContinued() ;
    }

    // 从“管道输入流”中读取1次数据
    public void readMessageOnce(){
        // 虽然buf的大小是2048个字符，但最多只会从“管道输入流”中读取1024个字符。
        // 因为，“管道输入流”的缓冲区大小默认只有1024个字符。
        char[] buf = new char[2048];
        try {
            int len = in.read(buf);
            System.out.println(new String(buf,0,len));
            in.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    // 从“管道输入流”读取>1024个字符时，就停止读取
    public void readMessageContinued()
    {
        int total=0;
        while(true) {
            char[] buf = new char[1024];
            try {
                int len = in.read(buf);
                total += len;
                System.out.println(new String(buf,0,len));
                // 若读取的字符总数>1024，则退出循环。
                if (total > 1024)
                        break;
                } catch (IOException e) {
                    e.printStackTrace();
                }
        }

        try {
            in.close();
        } catch (IOException e) {
                e.printStackTrace();
        }
    }
}

public class Sender extends Thread {
    // 管道输出流对象。
    // 它和“管道输入流(PipedReader)”对象绑定，
    // 从而可以将数据发送给“管道输入流”的数据，然后用户可以从“管道输入流”读取数据。
    private PipedWriter out = new PipedWriter();
    // 获得“管道输出流”对象
    public PipedWriter getWriter(){
        return out;
    }

    @Override
    public void run(){
        writeShortMessage();
        //writeLongMessage();
    }

    // 向“管道输出流”中写入一则较简短的消息："this is a short message"
    private void writeShortMessage() {
        String strInfo = "this is a short message" ;
        try {
            out.write(strInfo.toCharArray());
            out.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    // 向“管道输出流”中写入一则较长的消息
    private void writeLongMessage() {
        StringBuilder sb = new StringBuilder();
        // 通过for循环写入1020个字符
        for (int i=0; i<102; i++)
            sb.append("0123456789");
            // 再写入26个字符。
            sb.append("abcdefghijklmnopqrstuvwxyz");
            // str的总长度是1020+26=1046个字符
            String str = sb.toString();
        try {
            // 将1046个字符写入到“管道输出流”中
            out.write(str);
            out.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

public class PipeTest {

    public static void main(String[] args) {
        Sender t1 = new Sender();   

        Receiver t2 = new Receiver();   

        PipedWriter out = t1.getWriter();   

        PipedReader in = t2.getReader();
        try {
            //管道连接。下面2句话的本质是一样。
            //out.connect(in);
            in.connect(out);   

            /**
            * Thread类的START方法：
            * 使该线程开始执行；Java 虚拟机调用该线程的 run 方法。
            * 结果是两个线程并发地运行；当前线程（从调用返回给 start 方法）和另一个线程（执行其 run 方法）。
            * 多次启动一个线程是非法的。特别是当线程已经结束执行后，不能再重新启动。
            */
            t1.start();
            t2.start();
        } catch (IOException e) {
             e.printStackTrace();
        }
    }
}

输出结果：

this is a short message

基于JDK8的PipedReader的源码：

public class PipedReader extends Reader {
    boolean closedByWriter = false;
    boolean closedByReader = false;
    boolean connected = false;

    /* REMIND: identification of the read and write sides needs to be
       more sophisticated.  Either using thread groups (but what about
       pipes within a thread?) or using finalization (but it may be a
       long time until the next GC). */
    Thread readSide;
    Thread writeSide;

    /**
     * The size of the pipe's circular input buffer.
     */
    //默认的输入缓冲大小为1024
    private static final int DEFAULT_PIPE_SIZE = 1024;

    /**
     * The circular buffer into which incoming data is placed.
     */
    //字符数组
    char buffer[];

    /**
     * The index of the position in the circular buffer at which the
     * next character of data will be stored when received from the connected
     * piped writer. <code>in<0</code> implies the buffer is empty,
     * <code>in==out</code> implies the buffer is full
     */
    //输入标志位
    int in = -1;

    /**
     * The index of the position in the circular buffer at which the next
     * character of data will be read by this piped reader.
     */
    //输出标志位，in==out表示为满
    int out = 0;
    //构造函数
    public PipedReader(PipedWriter src) throws IOException {
        this(src, DEFAULT_PIPE_SIZE);
    }
    //构造函数，有管道大小
    public PipedReader(PipedWriter src, int pipeSize) throws IOException {
        initPipe(pipeSize);
        connect(src);
    }
    //没有连接，必须链接才行
    public PipedReader() {
        initPipe(DEFAULT_PIPE_SIZE);
    }
    public PipedReader(int pipeSize) {
        initPipe(pipeSize);
    }
    //构建pipeSize大小的字符缓冲区
    private void initPipe(int pipeSize) {
        if (pipeSize <= 0) {
            throw new IllegalArgumentException("Pipe size <= 0");
        }
    buffer = new char[pipeSize];
    }
    //Reader与Writer连接
    public void connect(PipedWriter src) throws IOException {
        src.connect(this);
    }

    /**
     * Receives a char of data. This method will block if no input is
     * available.
     */
    //接受数据，
    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();
        while (in == out) {
            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;
        }
    }

    /**
     * Receives data into an array of characters.  This method will
     * block until some input is available.
     */
    synchronized void receive(char c[], int off, int len)  throws IOException {
        while (--len >= 0) {
            receive(c[off++]);
        }
    }

    /**
     * Notifies all waiting threads that the last character of data has been
     * received.
     */
    synchronized void receivedLast() {
        closedByWriter = true;
        notifyAll();
    }

    /**
     * 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) {
            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();
            try {
                wait(1000);
            } catch (InterruptedException ex) {
                throw new java.io.InterruptedIOException();
            }
        }
        int ret = buffer[out++];
        if (out >= buffer.length) {
            out = 0;
        }
        if (in == out) {
            /* now empty */
            in = -1;
        }
        return ret;
    }

    /**
     * Reads up to <code>len</code> characters of data from this piped
     * stream into an array of characters. Less than <code>len</code> characters
     * will be read if the end of the data stream is reached or if
     * <code>len</code> exceeds the pipe's buffer size. This method
     * blocks until at least one character of input is available.
     *
     * @param      cbuf     the buffer into which the data is read.
     * @param      off   the start offset of the data.
     * @param      len   the maximum number of characters read.
     * @return     the total number of characters read into the buffer, or
     *             <code>-1</code> if there is no more data because the end of
     *             the stream has been 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(char cbuf[], int off, int len)  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");
        }

        if ((off < 0) || (off > cbuf.length) || (len < 0) ||((off + len) > cbuf.length) || ((off + len) < 0)) {
            throw new IndexOutOfBoundsException();
        } else if (len == 0) {
            return 0;
        }

        /* possibly wait on the first character */
        int c = read();
        if (c < 0) {
            return -1;
        }
        cbuf[off] =  (char)c;
        int rlen = 1;
        while ((in >= 0) && (--len > 0)) {
            cbuf[off + rlen] = buffer[out++];
            rlen++;
            if (out >= buffer.length) {
                out = 0;
            }
            if (in == out) {
                /* now empty */
                in = -1;
            }
        }
        return rlen;
    }

    /**
     * Tell whether this stream is ready to be read.  A piped character
     * stream is ready if the circular buffer is not empty.
     *
     * @exception  IOException  if the pipe is
     *                  <a href=PipedInputStream.html#BROKEN> <code>broken</code></a>,
     *                  {@link #connect(java.io.PipedWriter) unconnected}, or closed.
     */
    //是否准备好可以读
    public synchronized boolean ready() 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");
        }
        if (in < 0) {
            return false;
        } else {
            return true;
        }
    }

    /**
     * Closes this piped stream and releases any system resources
     * associated with the stream.
     *
     * @exception  IOException  if an I/O error occurs.
     */
    //关闭管道流，释放资源
    public void close()  throws IOException {
        in = -1;
        closedByReader = true;
    }
}

基于JDK8的PipedWriter源码：

public class PipedWriter extends Writer {

        /* REMIND: identification of the read and write sides needs to be
       more sophisticated.  Either using thread groups (but what about
       pipes within a thread?) or using finalization (but it may be a
       long time until the next GC). */
        private PipedReader sink;

    /* This flag records the open status of this particular writer. It
     * is independent of the status flags defined in PipedReader. It is
     * used to do a sanity check on connect.
     */
    private boolean closed = false;

    /**
     * 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.
     */
    //构造函数，连接Reader
    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() {
    }
    //连接Reader，初始化in和out
    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;
        snk.in = -1;
        snk.out = 0;
        snk.connected = true;
    }
    //写字符
    public void write(int c)  throws IOException {
        if (sink == null) {
            throw new IOException("Pipe not connected");
        }
        sink.receive(c);
    }
    public void write(char cbuf[], int off, int len) throws IOException {
        if (sink == null) {
            throw new IOException("Pipe not connected");
        } else if ((off | len | (off + len) | (cbuf.length - (off + len))) < 0) {
            throw new IndexOutOfBoundsException();
        }
        sink.receive(cbuf, off, len);
    }

    /**
     * Flushes this output stream and forces any buffered output characters
     * to be written out.
     * This will notify any readers that characters are waiting in the pipe.
     *
     * @exception  IOException  if the pipe is closed, or an I/O error occurs.
     */
    //刷新，强制缓存写出
    public synchronized void flush() throws IOException {
        if (sink != null) {
            if (sink.closedByReader || closed) {
                    throw new IOException("Pipe closed");
            }
            synchronized (sink) {
                sink.notifyAll();
            }
        }
    }

    /**
     * Closes this piped output stream and releases any system resources
     * associated with this stream. This stream may no longer be used for
     * writing characters.
     *
     * @exception  IOException  if an I/O error occurs.
     */
    public void close()  throws IOException {
        closed = true;
        if (sink != null) {
            sink.receivedLast();
        }
    }
}