黑马程序员…… Java中的管道流

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1.引言

Java I/O系统是建立在数据流概念之上的，而在UNIX/Linux中有一个类似的概念，就是管道，它具有将一个程序的输出当作另一个程序的输入的能力。在Java中，可以使用管道流进行线程之间的通信，输入流和输出流必须相连接，这样的通信有别于一般的Shared Data通信，其不需要一个共享的数据空间。

2.相关类及其关系

1）字节流：

分为管道输出流（PipedOutputStream）和管道输入流（PipedInputStream），利用 java.io.PipedOutputStream和java.io.PipedInputStream可以实现线程之间的二进制信息传输。如果要进行管道输出，则必须把输出流连在输入流上。 java.io.PipedOutputStream是java.io.OutputStream的直接子类，而java.io. PipedInputStream是java.io.InputStream的直接子类。PipedOutputStream和PipedInputStream往往成对出现、配合使用。举例说明：

TestPipe.Java

import java.io.IOException;

publicclass TestPipe {

   public static void main(String[] args) {

       Send s = new Send(); 
        Receive r = new Receive(); 
        try {

//输出管道流.connect(输入管道流);
           s.getPos().connect(r.getPis()); // 连接管道
        } catch (IOException e) { 
            e.printStackTrace(); 
        } 
        new Thread(s).start(); // 启动线程
        new Thread(r).start(); // 启动线程
    } 
}

Receive.java

importjava.io.IOException;
import java.io.PipedInputStream;

classReceive implements Runnable { //实现Runnable接口
    private PipedInputStream pis = null;

   public Receive() { 
        this.pis = newPipedInputStream(); // 实例化输入流
    }

   public void run() { 
        byte b[] = new byte[1024]; 
        int len = 0; 
        try { 
            len = this.pis.read(b); // 接收数据
        } catch (IOException e) { 
            e.printStackTrace(); 
        } 
        try { 
            this.pis.close(); 
        } catch (IOException e) { 
            e.printStackTrace(); 
        } 
        System.out.println("接收的内容为：" + new String(b, 0, len));
    }

   public PipedInputStream getPis() { 
        return pis; 
    } 
}

Send.java

importjava.io.IOException;
import java.io.PipedOutputStream; 
class Send implements Runnable { 
    // 实现Runnable接口
    private PipedOutputStream pos = null;//管道输出流

   public Send() { 
        this.pos = newPipedOutputStream();// 实例化输出流
    }

   public void run() { 
        String str = "HelloWorld!!!"; 
        try { 
            this.pos.write(str.getBytes());// 输出信息
        } catch (IOException e) { 
            e.printStackTrace(); 
        } 
        try { 
            this.pos.close(); // 关闭输出流
        } catch (IOException e) { 
            e.printStackTrace(); 
        } 
    }

   public PipedOutputStream getPos() { // 通过线程类得到输出流
        return pos; 
    } 
}

我们可以看到使用管道流，通过connect方法进行连接，实现了Send线程和Receive线程之间的通信。

注意：

PipedInputStream中实际是用了一个1024字节固定大小的循环缓冲区。写入PipedOutputStream的数据实际上保存到对应的 PipedInputStream的内部缓冲区。从PipedInputStream执行读操作时，读取的数据实际上来自这个内部缓冲区。如果对应的 PipedInputStream输入缓冲区已满，任何企图写入PipedOutputStream的线程都将被阻塞。而且这个写操作线程将一直阻塞，直至出现读取PipedInputStream的操作从缓冲区删除数据。这也就是说往PipedOutputStream写数据的线程Send若是和从PipedInputStream读数据的线程Receive是同一个线程的话，那么一旦Send线程发送数据过多（大于1024字节），它就会被阻塞，这就直接导致接受数据的线程阻塞而无法工作（因为是同一个线程嘛），那么这就是一个典型的死锁现象，这也就是为什么javadoc中关于这两个类的使用时告诉大家要在多线程环境下使用的原因了。

应用：过滤器模式

使用这个模式的典型例子是Unix的shell命令。这个模式的好处在于过滤器无需知道它与何种东西进行连接，并且这可以实现并行，而且系统的可扩展性可以根据添加删除或者改变Filter进行增强。

在这举一个不断计算平均值的例子，producer作为前端的数据源，不断产生随机数，通过pipe进入filter进行数据处理，然后通过第二个pipe就行后端处理。

importjava.util.*;
import java.io.*;

publicclass PipeTest
/* 建立3个线程（Producer、Filter、Consumer）类和两组通信管道，通过多线程将管道1的数据传送到管道2中，实现管道的通信。
* Producer => pout1->pin1 => Filter(pin1->pout2) => pout2->pin2 =>Consumer
*/ 
{ 
    public static void main(Stringargs[]) { 
        try { 
            PipedOutputStream pout1 = newPipedOutputStream(); 
            PipedInputStream pin1 = newPipedInputStream(pout1);

           PipedOutputStream pout2 = newPipedOutputStream(); 
            PipedInputStream pin2 = newPipedInputStream(pout2);

           /* construct threads */

           Producer prod = newProducer(pout1); 
            Filter filt = newFilter(pin1, pout2); 
            Consumer cons = newConsumer(pin2);

           /* start threads */

           prod.start(); 
            filt.start(); 
            cons.start(); 
        } catch (IOException e) { 
        } 
    } 
}

//前端：该类的作用是产生随机数，并将其放到管道1的输出流中
class Producer extends Thread { 
    private DataOutputStream out;//DataOutputStream是用于写入一些基本类型数据的类，此类的实例用于生成伪随机数流
    private Random rand = new Random();

   public Producer(OutputStream os) { 
        out = new DataOutputStream(os); 
    }

   public void run() { 
        while (true) { 
            try { 
                double num =rand.nextDouble(); 
                // 将double值直接写入流
                out.writeDouble(num); 
                System.out.println("写入流中的值是：" +num); 
                out.flush(); 
               sleep(Math.abs(rand.nextInt()%10));//随机休眠一段时间
            } catch (Exception e) { 
               System.out.println("Error:  " + e); 
            } 
        } 
    } 
}

//过滤器，起数据处理作用，读取管道1中输入流的内容，并将其放到管道2的输出流中
class Filter extends Thread { 
    private DataInputStream in; 
    private DataOutputStream out; 
    private double total = 0; 
    private int count = 0;

   public Filter(InputStream is, OutputStreamos) { 
        in = new DataInputStream(is); 
        out = new DataOutputStream(os); 
    }

   public void run() { 
        for (;;) { 
            try { 
                double x =in.readDouble(); // 读取流中的数据
                total += x; 
                count++; 
                if (count != 0) { 
                    double d = total /count; 
                    out.writeDouble(d);// 将得到的数据平均值写入流
                } 
            } catch (IOException e) { 
               System.out.println("Error:  " + e); 
            } 
        } 
    } 
}

//后端：读取管道2输入流的内容
class Consumer extends Thread { 
    private double old_avg = 0; 
    private DataInputStream in;

   public Consumer(InputStream is) { 
        in = new DataInputStream(is); 
    }

   public void run() { 
        for (;;) { 
            try { 
                double avg =in.readDouble(); 
                if (Math.abs(avg -old_avg) > 0.01) { 
                   System.out.println("现在的平均值是：  " + avg); 
                    System.out.println();
                    old_avg = avg; 
                } 
            } catch (IOException e) { 
               System.out.println("Error:  " + e); 
            } 
        } 
    }

}

2）字符流

Java利用 java.io.PipedWriter和java.io.PipedReader在线程之间传输字符信息。与 java.io.PipedOutputStream和java.io.PipedInputStream类似，java.io.PipedWriter是java.io.Writer的直接子类，java.io.PipedReader是java.io.Reader的直接子类。PipedWriter拥有一个允许指定输入管道字符流的构造方法，而PipedReader拥有一个允许指定输出管道字符流的构造方法。从而使得PipedWriter和PipedReader往往成对出现、配合使用。

以典型KWIC系统为例，下边的代码演示了如何使用字符流并且使用了过滤器模式：ReadLineThread--Pipe1 --> ShiftThread -- Pipe2 --> SortLinesThread

importjava.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.PipedReader;
import java.io.PipedWriter;
import java.util.ArrayList;
import java.util.Collections;
import java.util.StringTokenizer;

public class KwicPipe {
    public static void main(String[]args) {
        try {
            //get the input and output path
            String src = args[0];
            String dest = args[1];


            //(writeToShiftThread =>readFromShiftThread) = Pipe1
            PipedReaderreadFromShiftThread = new PipedReader();
            PipedWriterwriteToShiftThread = new PipedWriter(readFromShiftThread);


            //(writeToSortLinesThread=> readFromSortLinesThread) = Pipe2
            PipedReaderreadFromSortLinesThread = new PipedReader();
            PipedWriterwriteToSortLinesThread = new PipedWriter(readFromSortLinesThread);


            //ReadLineThread --Pipe1--> ShiftThread -- Pipe2 --> SortLinesThread
            ReadLineThread R1 = newReadLineThread(writeToShiftThread,src);
            ShiftThread R2 = newShiftThread(readFromShiftThread,writeToSortLinesThread);
            SortLinesThread R3 = newSortLinesThread(readFromSortLinesThread,dest);


            //Start the three processingthread
            R1.start();
            R2.start();
            R3.start();
        }
        catch (IOException e) {
            System.out.println("NOI/O");
        }
    }
}

// read the content of kwici.dat and send the lines to another thread
class ReadLineThread extends Thread {
    PipedWriter PipeIn;
    String InputFilename= null;
    ReadLineThread(PipedWriterPlaceInPipe, String InputFilename) {
        PipeIn = PlaceInPipe;
        this.InputFilename =InputFilename;
    }
    private BufferedReaderfileopen(String InputFilename) {
        BufferedReader input_file = null;
        try {
            input_file = newBufferedReader(new FileReader(InputFilename));
        } catch (IOException e) {
           System.err.println(("File not open" + e.toString()));
            System.exit(1);
        }
        return input_file;
    }
    public void run() {
        try {
            String Input;
            BufferedReader TheInput =fileopen(InputFilename);
            while ( (Input =TheInput.readLine()) != null) {
               System.out.println(Input);
                PipeIn.write(Input +"\n"); // Read from the file and then write to the pipe1
            }
        }
        catch (FileNotFoundException e) {
            System.out.println("NOFILE ");
        }
        catch (IOException e) {
            System.out.println("NO I/O");
        }
    }
}

// read the lines from ReadLineThread and shift them. Send all the shiftedlines to SortLinesThread
class ShiftThread extends Thread {
    PipedReader PipeOut;
    PipedWriter PipeIn;
    ShiftThread(PipedReader ReadFromPipe,PipedWriter WriteToPipe) {
        PipeOut = ReadFromPipe;
        PipeIn = WriteToPipe;
    }
    public void run() {
        char[] cbuf = new char[80];
        int i, j;
        StringBuffer linebuff = newStringBuffer();
        try {
            // read from ReadLineThread
            i = PipeOut.read(cbuf, 0,80);
            while (i != -1) {               
                for (j = 0; j < i;j++) {
                    //if new line
                    if (cbuf[j]=='\n'){
                        // When reach theend of line,shift it
                       shiftline(linebuff.toString());
                        // empty thebuffer
                       linebuff.delete(0, linebuff.length());
                    }
                    else {
                        linebuff.append(cbuf[j]);
                    }
                }
                i = PipeOut.read(cbuf, 0,80); //get next buffer's worth
            }
        }
        catch (FileNotFoundException e) {
            System.out.println("NOFILE ");
        }
        catch (IOException e) {
            System.out.println("NOI/O or end of stream (ShiftThread terminated)");
        }
        /* BECAUSE
         * If a thread was providing datacharacters to the connected piped output,
         * but the thread is no longeralive, then an IOException is thrown. (javadoc)
         */
    }
    private void shiftline( String line )
    {
        String onetoken = new String ();
        StringTokenizer tokens =
            new StringTokenizer( line );
        ArrayList Tokens = newArrayList ();
        int count = tokens.countTokens();
        for ( int i = 0; i < count;i++)
        {
            onetoken =tokens.nextToken();
            if (!((onetoken.compareTo("a" ) == 0) && (onetoken.compareTo( "an" ) == 0)&& (onetoken.compareTo( "and" ) == 0) &&(onetoken.compareTo( "the" ) == 0)))
            {
                Tokens.add(onetoken);
            }
        }
        for ( int tokencount = 0;tokencount < count; tokencount++ )
        {
            StringBuffer linebuffer = newStringBuffer ();
            int index = tokencount;
            for ( int i = 0; i< count;i++ )
            {
                if (index >= count)
                    index = 0;
                linebuffer.append (Tokens.get(index)  );
                linebuffer.append ("");
                index++;
            }  //for i
            line = linebuffer.toString();
            // send the line to theSortLinesThread
            try {
                PipeIn.write(line+"\n");
            } catch (IOException e) {
                e.printStackTrace();
            }
        } // for token count
        return;
    }
}

class SortLinesThread extends Thread {
    PipedReader PipeOut;
    String OutputFilename;
    ArrayList  KwicList = new ArrayList();
    SortLinesThread(PipedReaderReadFromPipe, String OutputFilename) {
        PipeOut = ReadFromPipe;
        this.OutputFilename =OutputFilename;
    }
    public void run() {
        char[] cbuf = new char[80];
        int i, j;
        StringBuffer linebuff = newStringBuffer();
        try {
            // read from ShiftLineThread
            i = PipeOut.read(cbuf, 0,80);
            while (i != -1) { // I don'tknow we're using that (The method Read blocks until at least one character ofinput is available.)
                for (j = 0; j < i;j++) {
                    //if new line
                    if (cbuf[j]=='\n'){
                        // add it to theArrayList
                        KwicList.add(linebuff.toString());
                        // adn empty thebuffer
                       linebuff.delete(0, linebuff.length());
                    }
                    else {
                        //append thecharacter to the line
                        linebuff.append(cbuf[j]);
                    }
                }           
                i = PipeOut.read(cbuf, 0,80); //get next buffer's worth
            }
        }
        catch (FileNotFoundException e) {
            System.out.println("NOFILE ");
        }
        catch (IOException e) {
            System.out.println("NOI/O or end of stream (SortLinesThread terminated)");
        }
        /* BECAUSE
         * If a thread was providing datacharacters to the connected piped output,
         * but the thread is no longeralive, then an IOException is thrown. (javadoc)
         */
        // when the reading is finished,sort the ArrayList and diplay
        Collections.sort(KwicList);//sortwhen added
        displaylist ( KwicList);//Standard Output
        //Export to file
        try {
            export(KwicList,OutputFilename);
        } catch (Exception e) {
           System.out.println("Error Output File ");
        }
    }
    private void displaylist(ArrayList KwicList )
    {
        System.out.println ("\nList: " );
        for ( int count = 0; count             System.out.println(KwicList.get (count) );
    }
    private voidexport(ArrayList List, String oufFilename) throws Exception{
        BufferedWriter writer = null;
        try {
            writer = newBufferedWriter(new FileWriter(oufFilename));
        } catch (FileNotFoundException e){
           System.err.println(("File not open" + e.toString()));
            System.exit(1);
        }

        for (int count = 0; count              writer.write(List.get(count));
             writer.write("\r\n");
        }
        writer.flush();
        writer.close();
        System.out.println("ProcessedFinished");
    }
}