Question

I want to test MappedByteBuffer‘s READ_WRITE mode. But I get an exception:

Exception in thread "main" java.nio.channels.NonWritableChannelException
 at sun.nio.ch.FileChannelImpl.map(FileChannelImpl.java:755)
 at test.main(test.java:13)

I have no idea have to fix it.
Thanks in advance.

now I fix the program and there is no exception. But the system returns a sequence of Garbage characters, but in fact there is just a string “asdfghjkl” in the file in.txt . I guess may be the coding scheme cause this problem, but i do not know how to verify it and fix it.

import java.io.File;
import java.nio.channels.*;
import java.nio.MappedByteBuffer;
import java.io.RandomAccessFile;

class test{
    public static void main(String[] args) throws Exception{

    File f= new File("./in.txt");
    RandomAccessFile in = new RandomAccessFile(f, "rws");
    FileChannel fc = in.getChannel();
    MappedByteBuffer mbb = fc.map(FileChannel.MapMode.READ_WRITE, 0, f.length());

    while(mbb.hasRemaining())
        System.out.print(mbb.getChar());
    fc.close();
    in.close();

    }
};

Answer 1

FileInputStream is for reading only and you are using FileChannel.MapMode.READ_WRITE. It should be READ for FileInputStream. Use RandomAccessFile raf = new RandomAccessFile(f, "rw"); for READ_WRITE map.

EDIT:
The character in your file is probably 8 bit, and java uses 16 bit characters. Hence the getChar will read two bytes instead of single byte.

Use get() method and cast it to char to get your desired character:

while(mbb.hasRemaining())
        System.out.print((char)mbb.get());

Alternatively, since the file is probably US-ASCII based, you can use CharsetDecoder to obtain a CharBuffer, for instance:

import java.io.*;
import java.nio.*;
import java.nio.channels.*;
import java.nio.charset.*;

public class TestFC {
        public static void main(String a[]) throws IOException{
                File f = new File("in.txt");
                try(RandomAccessFile in = new RandomAccessFile(f, "rws"); FileChannel fc = in.getChannel();) {
                        MappedByteBuffer mbb = fc.map(FileChannel.MapMode.READ_WRITE, 0, f.length());

                        Charset charset = Charset.forName("US-ASCII");
                        CharsetDecoder decoder = charset.newDecoder();
                        CharBuffer cb = decoder.decode(mbb);

                        for(int i=0; i<cb.limit();i++) {
                                System.out.print(cb.get(i));
                        }
                }
        }
}

will give you desired results as well.