Custom String

ClassHacking.java

package me.yamakaja.unsafe.classhacking;

import com.sun.management.HotSpotDiagnosticMXBean;
import sun.misc.Unsafe;

import java.lang.invoke.MethodHandle;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.MethodType;
import java.lang.management.ManagementFactory;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;

/**
 * Created by Yamakaja on 9/6/17.
 */
public class ClassHacking {

    public static final int MODIFIER_OFFSET = 152;
    public static final int ACCESS_FLAG_OFFSET = 156;

    public static void main(String[] args) {
        Unsafe unsafe;
        try {
            Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
            theUnsafe.setAccessible(true);
            unsafe = (Unsafe) theUnsafe.get(null);
        } catch (IllegalAccessException | NoSuchFieldException e) {
            e.printStackTrace();
            return;
        }

        System.out.println(ManagementFactory.getPlatformMXBean(HotSpotDiagnosticMXBean.class).getVMOption("UseCompressedOops").getValue());

        long klassPointer = (unsafe.getInt("", 8L) & 0xFFFFFFFFL) << 3;

        unsafe.putInt(klassPointer + MODIFIER_OFFSET, unsafe.getInt(klassPointer + MODIFIER_OFFSET) & ~Modifier.FINAL);
        unsafe.putInt(klassPointer + ACCESS_FLAG_OFFSET, unsafe.getInt(klassPointer + ACCESS_FLAG_OFFSET) & ~Modifier.FINAL);

        System.out.println(Modifier.toString(unsafe.getInt(klassPointer + MODIFIER_OFFSET)));
        System.out.println(Modifier.toString(unsafe.getInt(klassPointer + ACCESS_FLAG_OFFSET)));

        System.out.println(Modifier.toString(String.class.getModifiers()));

        Class<?> clazz = new ASMClassLoader().load("me.yamakaja.unsafe.classhacking.SubStringImpl", StringGenerator.generateStringClass());

        System.out.println("Loaded class!");

        try {
            String instance = (String) clazz.newInstance();
            System.out.println("Instance created!");
            System.out.println(instance);
        } catch (InstantiationException | IllegalAccessException e) {
            e.printStackTrace();
        }

    }

    private static class ASMClassLoader extends ClassLoader {

        public ASMClassLoader() {
            super(ASMClassLoader.class.getClassLoader());
        }

        public Class<?> load(String name, byte[] data) {
            return this.defineClass(name, data, 0, data.length);
        }

    }

}

StringGenerator.java

package me.yamakaja.unsafe.classhacking;

import org.objectweb.asm.*;

import static org.objectweb.asm.Opcodes.*;

/**
 * Created by Yamakaja on 9/6/17.
 */
public class StringGenerator {

    public static byte[] generateStringClass() {
        ClassWriter cw = new ClassWriter(0);
        MethodVisitor mv;

        cw.visit(52, ACC_PUBLIC + ACC_SUPER, "me/yamakaja/unsafe/classhacking/SubStringImpl", null, "java/lang/String", null);

        cw.visitSource("SubString.java", null);

        {
            mv = cw.visitMethod(ACC_PUBLIC, "<init>", "()V", null, null);
            mv.visitCode();
            Label l0 = new Label();
            mv.visitLabel(l0);
            mv.visitLineNumber(6, l0);
            mv.visitVarInsn(ALOAD, 0);
            mv.visitMethodInsn(INVOKESPECIAL, "java/lang/String", "<init>", "()V", false);
            mv.visitInsn(RETURN);
            Label l1 = new Label();
            mv.visitLabel(l1);
            mv.visitLocalVariable("this", "Lme/yamakaja/unsafe/classhacking/SubString;", null, l0, l1, 0);
            mv.visitMaxs(1, 1);
            mv.visitEnd();
        }
        {
            mv = cw.visitMethod(ACC_PUBLIC, "toString", "()Ljava/lang/String;", null, null);
            mv.visitCode();
            Label l0 = new Label();
            mv.visitLabel(l0);
            mv.visitLineNumber(10, l0);
            mv.visitLdcInsn("Linus Torvalds, a custom string!");
            mv.visitInsn(ARETURN);
            Label l1 = new Label();
            mv.visitLabel(l1);
            mv.visitLocalVariable("this", "Lme/yamakaja/unsafe/classhacking/SubString;", null, l0, l1, 0);
            mv.visitMaxs(1, 1);
            mv.visitEnd();
        }
        {
            mv = cw.visitMethod(ACC_PUBLIC, "substringX", "(I)Ljava/lang/String;", null, null);
            mv.visitCode();
            Label l0 = new Label();
            mv.visitLabel(l0);
            mv.visitLdcInsn("Result of substring array!");
            mv.visitInsn(ARETURN);

            Label l1 = new Label();
            mv.visitLabel(l1);
            mv.visitLocalVariable("this", "Lme/yamakaja/unsafe/classhacking/SubString;", null, l0, l1, 0);
            mv.visitLocalVariable("start", "I", null, l0, l1, 1);
            mv.visitMaxs(1, 2);
            mv.visitEnd();
        }
        cw.visitEnd();

        return cw.toByteArray();
    }

}