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();
}
}
}