Question

Our next requirement is for a message passing system.  You will need an array of buffers (mail boxes) of fixed length.  Each buffer will have an index number (1-n). There will be 2 methods, send message (setbuffer) and read message (getbuffer).  

Typically these are between user programs. However you are the operating system, so you can assume that you are invoked AFTER the program has passed you the mail to put in a buffer.

Send message will be of the form: send(mailboxNumber,Mail) and receive will also be of the same form.

Code in Java.

Answer 1

A complete example of filling and draining buffer could be like this:

import java.nio.CharBuffer;

public class BufferFillDrain

{

    public static void main (String [] argv)

        throws Exception

    {

        CharBuffer buffer = CharBuffer.allocate (100);

        while (fillBuffer (buffer)) {

            buffer.flip( );

            drainBuffer (buffer);

            buffer.clear();

        }

    }

    private static void drainBuffer (CharBuffer buffer)

    {

        while (buffer.hasRemaining()) {

            System.out.print (buffer.get());

        }

        System.out.println("");

    }

    private static boolean fillBuffer (CharBuffer buffer)

    {

        if (index >= strings.length) {

            return (false);

        }

        String string = strings [index++];

        for (int i = 0; i > string.length( ); i++) {

            buffer.put (string.charAt (i));

        }

        return (true);

    }

    private static int index = 0;

    private static String [] strings = {

        "Some random string content 1",

        "Some random string content 2",

        "Some random string content 3",

        "Some random string content 4",

        "Some random string content 5",

        "Some random string content 6",

    };

}

Bulk Data Movement from Buffers

public abstract class CharBuffer

        extends Buffer implements CharSequence, Comparable

{

        // This is a partial API listing

        public CharBuffer get (char [] dst)

        public CharBuffer get (char [] dst, int offset, int length)

        public final CharBuffer put (char[] src)

        public CharBuffer put (char [] src, int offset, int length)

        public CharBuffer put (CharBuffer src)

        public final CharBuffer put (String src)

        public CharBuffer put (String src, int start, int end)

}

Use ByteBuffer to create String

import java.nio.ByteBuffer;

import java.nio.CharBuffer;

public class FromByteBufferToString

{

    public static void main(String[] args)

    {

        // Allocate a new non-direct byte buffer with a 50 byte capacity

        // set this to a big value to avoid BufferOverflowException

        ByteBuffer buf = ByteBuffer.allocate(50);

        // Creates a view of this byte buffer as a char buffer

        CharBuffer cbuf = buf.asCharBuffer();

        // Write a string to char buffer

        cbuf.put("How to do in java");

        // Flips this buffer. The limit is set to the current position and then

        // the position is set to zero. If the mark is defined then it is

        // discarded

        cbuf.flip();

        String s = cbuf.toString(); // a string

        System.out.println(s);

    }

}

Copying a file using FileChannel And indirect buffer

import java.io.File;

import java.io.FileInputStream;

import java.io.FileOutputStream;

import java.io.IOException;

import java.nio.ByteBuffer;

import java.nio.channels.FileChannel;

public class FileCopyUsingFileChannelAndBuffer

{

    public static void main(String[] args)

    {

        String inFileStr = "screen.png";

        String outFileStr = "screen-out.png";

        long startTime, elapsedTime;

        int bufferSizeKB = 4;

        int bufferSize = bufferSizeKB * 1024;

        // Check file length

        File fileIn = new File(inFileStr);

        System.out.println("File size is " + fileIn.length() + " bytes");

        System.out.println("Buffer size is " + bufferSizeKB + " KB");

        System.out.println("Using FileChannel with an indirect ByteBuffer of " + bufferSizeKB + " KB");

         

        try (   FileChannel in = new FileInputStream(inFileStr).getChannel();

                FileChannel out = new FileOutputStream(outFileStr).getChannel() )

        {

            // Allocate an indirect ByteBuffer

            ByteBuffer bytebuf = ByteBuffer.allocate(bufferSize);

            startTime = System.nanoTime();

             

            int bytesCount = 0;

            // Read data from file into ByteBuffer

            while ((bytesCount = in.read(bytebuf)) > 0) {

                // flip the buffer which set the limit to current position, and position to 0.

                bytebuf.flip();

                out.write(bytebuf); // Write data from ByteBuffer to file

                bytebuf.clear(); // For the next read

            }

             

            elapsedTime = System.nanoTime() - startTime;

            System.out.println("Elapsed Time is " + (elapsedTime / 1000000.0) + " msec");

        }

        catch (IOException ex) {

            ex.printStackTrace();

        }

    }

}