ConstantifyCS.java

package fr.umlv.constantify;
import static java.lang.invoke.MethodHandles.constant;
import static java.lang.invoke.MethodHandles.dropArguments;
import static java.lang.invoke.MethodType.methodType;

import java.lang.invoke.MethodHandle;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.MethodType;
import java.lang.invoke.MutableCallSite;

public class ConstantifyCS extends MutableCallSite {
  private static final MethodHandle FALLBACK;
  static {
    try {
      FALLBACK = MethodHandles.lookup().findVirtual(ConstantifyCS.class, "fallback",
          methodType(Object.class, Object.class));
    } catch (NoSuchMethodException | IllegalAccessException e) {
      throw new AssertionError(e);
    }
  }

  private ConstantifyCS(MethodType type) {
    super(type);
    setTarget(FALLBACK.bindTo(this).asType(type));
  }

  private Object fallback(Object value) {
    Class<?> clazz = type().parameterType(0);
    setTarget(dropArguments(constant(clazz, value), 0, clazz));
    return value;
  }

  public static MethodHandle constantify(Class<?> type) {
    return new ConstantifyCS(methodType(type, type)).dynamicInvoker();
  }
}

Main.java

import java.lang.invoke.MethodHandle;

import fr.umlv.constantify.ConstantifyCS;

public class Main {
  private static final MethodHandle CST = ConstantifyCS.constantify(int.class);
  
  static int f(int i) {
    try {
      return (int) CST.invokeExact(i);
    } catch (Throwable e) {
      throw new AssertionError(e);
    }
  }
  
  public static void main(String[] args) {
    System.out.println(f(2));
    System.out.println(f(2));
    System.out.println(f(3));
  }
}