PipedOutputStream与PipedInputStream
1、PipedInputStream与PipedOutputStream简介
PipedInputStream与PipedOutputStream，分别是管道入流和管道输出流。他们的作用是线程间的通信，必须配套使用。输入输出都是针对程序本身而言的，A线程将数据写入PipedOutputStream，数据会自动的传输到PipedInputStream的缓冲区供B线程读取。
2、PipedOutputStream源码
//jdk1.7.40 package java.io; import java.io.*; public class PipedOutputStream extends OutputStream { // 与PipedOutputStream通信的PipedInputStream对象 private PipedInputStream sink; // 构造函数，指定配对的PipedInputStream public PipedOutputStream(PipedInputStream snk) throws IOException { connect(snk); } // 构造函数 public PipedOutputStream() { } // 将“管道输出流” 和 “管道输入流”连接。 public synchronized void connect(PipedInputStream snk) throws IOException { if (snk == null) { throw new NullPointerException(); } else if (sink != null || snk.connected) { throw new IOException("Already connected"); } // 设置“管道输入流” sink = snk; // 初始化“管道输入流”的写位置 // int是PipedInputStream中定义的，代表“管道输入流”的读写位置 snk.in = -1; // 初始化“管道输出流”的读位置。 // out是PipedInputStream中定义的，代表“管道输出流”的读写位置 snk.out = 0; // 设置“管道输入流”和“管道输出流”为已连接状态 // connected是PipedInputStream中定义的，用于表示“管道输入流与管道输出流”是否已经连接 snk.connected = true; } // 将int类型b写入“管道输出流”中。 // 将b写入“管道输出流”之后，它会将b传输给“管道输入流” public void write(int b) throws IOException { if (sink == null) { throw new IOException("Pipe not connected"); } sink.receive(b); } // 将字节数组b写入“管道输出流”中。 // 将数组b写入“管道输出流”之后，它会将其传输给“管道输入流” public void write(byte b[], int off, int len) throws IOException { if (sink == null) { 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; } // “管道输入流”接收数据 sink.receive(b, off, len); } // 清空“管道输出流”。 // 这里会调用“管道输入流”的notifyAll()； // 目的是让“管道输入流”放弃对当前资源的占有，让其它的等待线程(等待读取管道输出流的线程)读取“管道输出流”的值。 public synchronized void flush() throws IOException { if (sink != null) { synchronized (sink) { sink.notifyAll(); } } } // 关闭“管道输出流”。 // 关闭之后，会调用receivedLast()通知“管道输入流”它已经关闭。 public void close() throws IOException { if (sink != null) { sink.receivedLast(); } } }
3、PipeOutputStream源码
package java.io; public class PipedInputStream extends InputStream { // “管道输出流”是否关闭的标记 boolean closedByWriter = false; // “管道输入流”是否关闭的标记 volatile boolean closedByReader = false; // “管道输入流”与“管道输出流”是否连接的标记 // 它在PipedOutputStream的connect()连接函数中被设置为true boolean connected = false; Thread readSide; // 读取“管道”数据的线程 Thread writeSide; // 向“管道”写入数据的线程 // “管道”的默认大小 private static final int DEFAULT_PIPE_SIZE = 1024; protected static final int PIPE_SIZE = DEFAULT_PIPE_SIZE; // 缓冲区 protected byte buffer[]; //下一个写入字节的位置。in==out代表缓冲区满或者说缓冲区被读空 protected int in = -1; //下一个读取字节的位置。 protected int out = 0; // 构造函数：指定与“管道输入流”关联的“管道输出流” public PipedInputStream(PipedOutputStream src) throws IOException { this(src, DEFAULT_PIPE_SIZE); } // 构造函数：指定与“管道输入流”关联的“管道输出流”，以及“缓冲区大小” public PipedInputStream(PipedOutputStream src, int pipeSize) throws IOException { initPipe(pipeSize); connect(src); } // 构造函数：默认缓冲区大小是1024字节 public PipedInputStream() { initPipe(DEFAULT_PIPE_SIZE); } // 构造函数：指定缓冲区大小是pipeSize public PipedInputStream(int pipeSize) { initPipe(pipeSize); } // 初始化“管道”：新建缓冲区大小 private void initPipe(int pipeSize) { if (pipeSize <= 0) { throw new IllegalArgumentException("Pipe Size <= 0"); } buffer = new byte[pipeSize]; } // 将“管道输入流”和“管道输出流”绑定。 // 实际上，这里调用的是PipedOutputStream的connect()函数 public void connect(PipedOutputStream src) throws IOException { src.connect(this); } // 接收int类型的数据b。 // 它只会在PipedOutputStream的write(int b)中会被调用 protected synchronized void receive(int b) throws IOException { // 检查管道状态 checkStateForReceive(); // 获取“写入管道”的线程 writeSide = Thread.currentThread(); // 若缓冲区满则等待 if (in == out) awaitSpace(); if (in < 0) { in = 0; out = 0; } // 将b保存到缓冲区 buffer[in++] = (byte)(b & 0xFF); if (in >= buffer.length) { in = 0; } } // 接收字节数组b。 synchronized void receive(byte b[], int off, int len) throws IOException { // 检查管道状态 checkStateForReceive(); // 获取“写入管道”的线程 writeSide = Thread.currentThread(); int bytesToTransfer = len; while (bytesToTransfer > 0) { // 若“写入管道”的数据正好全部被读取完，则等待。 if (in == out) awaitSpace(); int nextTransferAmount = 0; // 如果“管道中被读取的数据，少于写入管道的数据”； // 则设置nextTransferAmount=“buffer.length - in” if (out < in) { nextTransferAmount = buffer.length - in; } else if (in < out) { // 如果“管道中被读取的数据，大于/等于写入管道的数据”，则执行后面的操作 // 若in==-1(即管道的写入数据等于被读取数据)，此时nextTransferAmount = buffer.length - in; // 否则，nextTransferAmount = out - in; if (in == -1) { in = out = 0; nextTransferAmount = buffer.length - in; } else { nextTransferAmount = out - in; } } if (nextTransferAmount > bytesToTransfer) nextTransferAmount = bytesToTransfer; // assert断言的作用是，若nextTransferAmount <= 0，则终止程序。 assert(nextTransferAmount > 0); // 将数据写入到缓冲中 System.arraycopy(b, off, buffer, in, nextTransferAmount); bytesToTransfer -= nextTransferAmount; off += nextTransferAmount; in += nextTransferAmount; if (in >= buffer.length) { in = 0; } } } // 检查管道状态 private void checkStateForReceive() 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"); } } // 等待。 // 若“写入管道”的数据正好全部被读取完(例如，管道缓冲满)，则执行awaitSpace()操作； // 它的目的是让“读取管道的线程”管道产生读取数据请求，从而才能继续的向“管道”中写入数据。 private void awaitSpace() throws IOException { // 如果“管道中被读取的数据，等于写入管道的数据”时， // 则每隔1000ms检查“管道状态”，并唤醒管道操作：若有“读取管道数据线程被阻塞”，则唤醒该线程。 while (in == out) { checkStateForReceive(); /* full: kick any waiting readers */ notifyAll(); try { wait(1000); } catch (InterruptedException ex) { throw new java.io.InterruptedIOException(); } } } // 当PipedOutputStream被关闭时，被调用 synchronized void receivedLast() { closedByWriter = true; notifyAll(); } // 从管道(的缓冲)中读取一个字节，并将其转换成int类型 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"); } notifyAll(); 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) { /* now empty */ in = -1; } return ret; } // 从管道(的缓冲)中读取数据，并将其存入到数组b中 public synchronized int read(byte b[], int off, int len) throws IOException { if (b == null) { throw new NullPointerException(); } else if (off < 0 || len < 0 || len > b.length - off) { throw new IndexOutOfBoundsException(); } else if (len == 0) { return 0; } /* possibly wait on the first character */ int c = read(); if (c < 0) { return -1; } b[off] = (byte) c; int rlen = 1; while ((in >= 0) && (len > 1)) { int available; if (in > out) { available = Math.min((buffer.length - out), (in - out)); } else { available = buffer.length - out; } // A byte is read beforehand outside the loop if (available > (len - 1)) { available = len - 1; } System.arraycopy(buffer, out, b, off + rlen, available); out += available; rlen += available; len -= available; if (out >= buffer.length) { out = 0; } if (in == out) { /* now empty */ in = -1; } } return rlen; } // 返回不受阻塞地从此输入流中读取的字节数。 public synchronized int available() throws IOException { if(in < 0) return 0; else if(in == out) return buffer.length; else if (in > out) return in - out; else return in + buffer.length - out; } // 关闭管道输入流 public void close() throws IOException { closedByReader = true; synchronized (this) { in = -1; } } }
4、示例
Sender.java
package io; import java.io.*; public class Sender implements Runnable{ private PipedOutputStream out = new PipedOutputStream(); public PipedOutputStream getOutputStream() { return out; } private void writeShortMessage() { String info = "Hi, what's your name?"; try { out.write(info.getBytes()); out.close(); }catch(IOException e) { e.printStackTrace(); } } public void run() { writeShortMessage(); } }
Receiver.java
package io; import java.io.*; public class Receiver implements Runnable{ private PipedInputStream in = new PipedInputStream(); public PipedInputStream getInputStream() { return in; } private void readMessageOnce() { //默认缓冲区大小1024字节，一次最多读取1024字节 //想要读取更多字节要读取多次 byte[] b = new byte[2048]; try { int len = in.read(b, 0, b.length); System.out.println(new String(b,0,len)); in.close(); }catch(IOException e) { e.printStackTrace(); } } public void run() { readMessageOnce(); } }
PipedStreamTest.java
package io; import java.io.*; public class PipeStreamTest { public static void main(String[] args) { Sender s = new Sender(); Receiver r = new Receiver(); PipedOutputStream out = s.getOutputStream(); PipedInputStream in = r.getInputStream(); try { out.connect(in); new Thread(s).start(); new Thread(r).start(); }catch(IOException e) { e.printStackTrace(); } } }
输出结果：
Hi, what's your name?
结果分析：
实际上write(byte b[])是调用的PipedOutputStream.java中的write(byte b[], int off, int len)函数。查看write(byte b[], int off, int len)的源码，我们发现：它会调用 sink.receive(b, off, len); 进一步查看receive(byte b[], int off, int len)的定义，我们知道sink.receive(b, off, len)的作用就是：将“管道输出流”中的数据保存到“管道输入流”的缓冲中。而“管道输入流”的缓冲区buffer的默认大小是1024个字节。
