管道Pipe

　　java.nio.channels包中含有一个名为Pipe（管道）的类。广义上讲，管道就是一个用来在两个实体之间单向传输数据的导管。管道的概念对于Unix（和类Unix）操作系统的用户来说早就很熟悉了。Unix系统中，管道被用来连接一个进程的输出和另一个进程的输入。Pipe类实现一个管道范例，不过它所创建的管道是进程内（在Java虚拟机进程内部）而非进程间使用的。

参见图3-10。

/*
 * @(#)Pipe.java    1.21 05/11/17
 *
 * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package java.nio.channels;

import java.io.IOException;
import java.nio.channels.spi.*;

/**
 * A pair of channels that implements a unidirectional pipe.
 *
 * <p> A pipe consists of a pair of channels: A writable {@link
 * Pipe.SinkChannel </code>sink<code>} channel and a readable {@link
 * Pipe.SourceChannel </code>source<code>} channel.  Once some bytes are
 * written to the sink channel they can be read from source channel in exactly
 * the order in which they were written.
 *
 * <p> Whether or not a thread writing bytes to a pipe will block until another
 * thread reads those bytes, or some previously-written bytes, from the pipe is
 * system-dependent and therefore unspecified.  Many pipe implementations will
 * buffer up to a certain number of bytes between the sink and source channels,
 * but such buffering should not be assumed.  </p>
 *
 *
 * @author Mark Reinhold
 * @author JSR-51 Expert Group
 * @version 1.21, 05/11/17
 * @since 1.4
 */

public abstract class Pipe {

    /**
     * A channel representing the readable end of a {@link Pipe}.  </p>
     *
     * @since 1.4
     */
    public static abstract class SourceChannel
    extends AbstractSelectableChannel
    implements ReadableByteChannel, ScatteringByteChannel
    {
    /**
     * Constructs a new instance of this class.
     */
    protected SourceChannel(SelectorProvider provider) {
        super(provider);
    }

    /**
     * Returns an operation set identifying this channel‘s supported
     * operations.
     *
     * <p> Pipe-source channels only support reading, so this method
     * returns {@link SelectionKey#OP_READ}.  </p>
     *
     * @return  The valid-operation set
     */
    public final int validOps() {
        return SelectionKey.OP_READ;
    }

    }

    /**
     * A channel representing the writable end of a {@link Pipe}.  </p>
     *
     * @since 1.4
     */
    public static abstract class SinkChannel
    extends AbstractSelectableChannel
    implements WritableByteChannel, GatheringByteChannel
    {
    /**
     * Initializes a new instance of this class.
     */
    protected SinkChannel(SelectorProvider provider) {
        super(provider);
    }

    /**
     * Returns an operation set identifying this channel‘s supported
     * operations.
     *
     * <p> Pipe-sink channels only support writing, so this method returns
     * {@link SelectionKey#OP_WRITE}.  </p>
     *
     * @return  The valid-operation set
     */
    public final int validOps() {
        return SelectionKey.OP_WRITE;
    }

    }

    /**
     * Initializes a new instance of this class.
     */
    protected Pipe() { }

    /**
     * Returns this pipe‘s source channel.  </p>
     *
     * @return  This pipe‘s source channel
     */
    public abstract SourceChannel source();

    /**
     * Returns this pipe‘s sink channel.  </p>
     *
     * @return  This pipe‘s sink channel
     */
    public abstract SinkChannel sink();

    /**
     * Opens a pipe.
     *
     * <p> The new pipe is created by invoking the {@link
     * java.nio.channels.spi.SelectorProvider#openPipe openPipe} method of the
     * system-wide default {@link java.nio.channels.spi.SelectorProvider}
     * object.  </p>
     *
     * @return  A new pipe
     *
     * @throws  IOException
     *          If an I/O error occurs
     */
    public static Pipe open() throws IOException {
    return SelectorProvider.provider().openPipe();
    }

}

　　Pipe实例是通过调用不带参数的Pipe.open( )工厂方法来创建的。Pipe类定义了两个嵌套的通道类来实现管路。这两个类是Pipe.SourceChannel（管道负责读的一端）和Pipe.SinkChannel（管道负责写的一端）。这两个通道实例是在Pipe对象创建的同时被创建的，可以通过在Pipe对象上分别调用source( )和sink( )方法来取回。

　　此时，您可能在想管道到底有什么作用。您不能使用Pipe在操作系统级的进程间建立一个类Unix管道（您可以使用SocketChannel来建立）。Pipe的source通道和sink通道提供类似java.io.PipedInputStream和java.io.PipedOutputStream所提供的功能，不过它们可以执行全部的通道语义。请注意，SinkChannel和SourceChannel都继承了AbstractSelectableChannel（所以也间接地继承了SelectableChannel），这意味着pipe通道可以同选择器一起使用 。

　　管道可以被用来仅在同一个Java虚拟机内部传输数据。虽然有更加有效率的方式来在线程之间传输数据，但是使用管道的好处在于封装性。生产者线程和用户线程都能被写道通用的Channel API中。根据给定的通道类型，相同的代码可以被用来写数据到一个文件、socket或管道。选择器可以被用来检查管道上的数据可用性，如同在socket通道上使用那样地简单。这样就可以允许单个用户线程使用一个Selector来从多个通道有效地收集数据，并可任意结合网络连接或本地工作线程使用。因此，这些对于可伸缩性、冗余度以及可复用性来说无疑都是意义重大的。

　　Pipes的另一个有用之处是可以用来辅助测试。一个单元测试框架可以将某个待测试的类连接到管道的“写”端并检查管道的“读”端出来的数据。它也可以将被测试的类置于通道的“读”端并将受控的测试数据写进其中。两种场景对于回归测试都是很有帮助的。

　　管路所能承载的数据量是依赖实现的（implementation-dependent）。唯一可保证的是写到SinkChannel中的字节都能按照同样的顺序在SourceChannel上重现。

　　例3-11诠释了如何使用管道。

/**
 *
 */
package test.nio.pipe;

import java.nio.ByteBuffer;
import java.nio.channels.Channels;
import java.nio.channels.Pipe;
import java.nio.channels.ReadableByteChannel;
import java.nio.channels.WritableByteChannel;
import java.util.Random;

/**
 * * Test Pipe objects using a worker thread. ** Created April, 2002 * @author
 * Ron Hitchens ([email protected])
 */
public class PipeTest {
    public static void main(String[] argv) throws Exception {
        // Wrap a channel around stdout
        WritableByteChannel out = Channels.newChannel(System.out);
        // Start worker and get read end of channel
        ReadableByteChannel workerChannel = startWorker(10);
        ByteBuffer buffer = ByteBuffer.allocate(100);
        while (workerChannel.read(buffer) >= 0) {
            buffer.flip();
            out.write(buffer);
            buffer.clear();
        }
    }

    // This method could return a SocketChannel or
    // FileChannel instance just as easily
    private static ReadableByteChannel startWorker(int reps) throws Exception {
        Pipe pipe = Pipe.open();
        Worker worker = new Worker(pipe.sink(), reps);
        worker.start();
        return (pipe.source());
    }// -----------------------------------------------------------------

    /**
     * * A worker thread object which writes data down a channel. * Note: this
     * object knows nothing about Pipe, uses only a * generic
     * WritableByteChannel.
     */
    private static class Worker extends Thread {
        WritableByteChannel channel;
        private int reps;

        Worker(WritableByteChannel channel, int reps) {
            this.channel = channel;
            this.reps = reps;
        }

        // Thread execution begins here
        public void run() {
            ByteBuffer buffer = ByteBuffer.allocate(100);
            try {
                for (int i = 0; i < this.reps; i++) {
                    doSomeWork(buffer);
                    // channel may not take it all at once
                    while (channel.write(buffer) > 0) {
                        // empty
                    }
                }
                this.channel.close();
            } catch (Exception e) {
                // easy way out; this is demo code
                e.printStackTrace();
            }
        }

        private String[] products = { "No good deed goes unpunished",
                "To be, or what?", "No matter where you go, there you are",
                "Just say \"Yo\"", "My karma ran over my dogma" };
        private Random rand = new Random();

        private void doSomeWork(ByteBuffer buffer) {
            int product = rand.nextInt(products.length);
            buffer.clear();
            buffer.put(products[product].getBytes());
            buffer.put("\r\n".getBytes());
            buffer.flip();
        }
    }
}

　　例3-11 工作线程对一个管道进行写操作

以上内容出自 : NIO 一书