Create a new String from a ByteBuffer with only one char[] (so no Array.copyOf()) It would be nice if we could get rid of the CharBuffer as well, but, it's a start. String has a packaged scoped constructor - which doesn't use Array.copyOf(). It can be accessed through reflection.

ByteBufferToStringConverter.java

package be.idevelop.util;

/**
 * MIT License http://opensource.org/licenses/MIT
 * 
 * @author @willemsst
 */

import java.lang.reflect.*;
import java.nio.*;
import java.nio.charset.*;

public class ByteBufferToStringConverter {

    private static final Constructor<String> PACKAGE_SCOPED_STRING_CONSTRUCTOR;

    static {
        try {
            PACKAGE_SCOPED_STRING_CONSTRUCTOR = String.class.getDeclaredConstructor(char[].class, boolean.class);
            PACKAGE_SCOPED_STRING_CONSTRUCTOR.setAccessible(true);
        } catch (NoSuchMethodException e) {
            throw new IllegalStateException("Packaged scope String constructor not found", e);
        }
    }

    private ByteBufferToStringConverter() {
        throw new UnsupportedOperationException("Util class. Can not be instantiated.");
    }

    public static String convert(ByteBuffer byteBuffer, Charset charset) {
        return createNewString(convertToCharArray(byteBuffer, charset));
    }

    private static char[] convertToCharArray(ByteBuffer byteBuffer, Charset charset) {
        char[] chars = new char[byteBuffer.remaining()];
        if (byteBuffer.remaining() > 0) {
            CharsetDecoder decoder = charset.newDecoder();
            CharBuffer charBuffer;
            try {
                charBuffer = decoder.decode(byteBuffer);
            } catch (CharacterCodingException e) {
                throw new IllegalStateException("Character decoding failed for charset " + charset.displayName(), e);
            }
            chars = charBuffer.array();
        }
        return chars;
    }

    private static String createNewString(char[] chars) {
        try {
            return PACKAGE_SCOPED_STRING_CONSTRUCTOR.newInstance(chars, true);
        } catch (InstantiationException | IllegalAccessException | InvocationTargetException e) {
            throw new IllegalStateException("Could not invoke packaged scoped String constructor");
        }
    }
}