Read only mode of buffer

package com.tdkj.DataApi.nio;

import java.nio.ByteBuffer;

/** * @author hxy * @version 1.0 * @date 2021-03-11 10:56 */
public class ReadOnlyBuffer {
    

    public static void main(String[] args) {
    
        //buffer read only mode 
        // Create a buffer
        ByteBuffer byteBuffer = ByteBuffer.allocate(64);
        for (int i = 0; i <64 ; i++) {
    
            byteBuffer.put((byte)i);
        }
        // Read 
        byteBuffer.flip();

        // Will be buff Set to read only 
        ByteBuffer readOnlyBuffer = byteBuffer.asReadOnlyBuffer();

        while (readOnlyBuffer.hasRemaining()){
     //hasRemaining  Determine whether the next person has any data 
            System.out.println(readOnlyBuffer.get()); //get Would be  position The pointer automatically +1
        }

        //readOnlyBuffer.putInt(1);
        // If it is set to read-only, it cannot be written   Otherwise, the error will be reported 
        //java.nio.ReadOnlyBufferException
    }
}

Directly modify the data in the channel

package com.tdkj.DataApi.nio;

import java.io.RandomAccessFile;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;

/** * @author hxy * @version 1.0 * @date 2021-03-11 11:07 */
public class MappedByteBufferTest  {
    

    //MappedByteBuffer You can modify files directly in memory 
    // The operating system does not need to be copied once   Memory refers to off heap memory 

    public static void main(String[] args) throws Exception {
    
        RandomAccessFile randomAccessFile = new RandomAccessFile("D://file.txt", "rw");
        // Get the corresponding channel 
        FileChannel channel = randomAccessFile.getChannel();
        /** *  Parameters 1 FileChannel.MapMode.READ_WRITE  Use read-write mode  *  Parameters 2 0  You can directly modify the starting position of  *  Parameters 3(size) 5  Is the size mapped to memory （ Not the location of the index ）, namely   How many bytes are mapped from the file to memory  *  That is, the scope that can be directly modified is 0,1,2,3,4 5 Bytes  *  The actual type is  DirectByteBuffer **/
        MappedByteBuffer mappedByteBuffer = channel.map(FileChannel.MapMode.READ_WRITE,0,20);
        mappedByteBuffer.put(0,(byte)'a');
        mappedByteBuffer.put(1,(byte)'b');
        mappedByteBuffer.put(4,(byte)'c');
        mappedByteBuffer.put(7,(byte)'c');
        mappedByteBuffer.put(8,(byte)'c');
        mappedByteBuffer.put(9,(byte)'c');
            //20 It cannot be modified in this position   That is to say  0-20  but 20 It refers to the number of bytes that can be modified   Instead of Subscripts 
        //mappedByteBuffer.put(20,(byte)'c');
        // If there is Chinese in the file   Then it will also be modified   But it will become garbled 
        randomAccessFile.close();

    }
}