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。
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