In java we have enum types, they might be great to describe some constant stuff, like days of week.
But unfortunately some people use them to describe something that might change, like types of users, types of monsters in some game, etc. And this is just wrong, enums should be constant, they should never change or break.
Now let’s imagine that we want to create some game with support for modding, and we used enum for that monster thing… and now it is impossible to add new monster type from mod level. (or isn’t?)

So I will show you two things in this post: how to add new constant to already existing enum, and how we can replace enums with something different in our code.

Hacking into enums

REMEMBER: this is dirty hack, you should never use it, unless there is no other option.
Let’s create a simple enum:

public enum Monster {
    ZOMBIE(Zombie.class, "zombie"),
    ORK(Ork.class, "ork"),
    WOLF(Wolf.class, "wolf");
    private final Class<? extends Entity> entityClass;
    private final String                  entityId;
    Monster(Class<? extends Entity> entityClass, String entityId) {
        this.entityClass = entityClass;
        this.entityId = "monster:" + entityId;
    }
    public Class<? extends Entity> getEntityClass() { return this.entityClass; }
    public String getEntityId() { return this.entityId; }
    public Entity create() {
        try { return entityClass.newInstance(); }
        catch (InstantiationException | IllegalAccessException e) { throw new InternalError(e); }
    }
}

(I don’t format code like that normally, just for blog)
Using normal reflections to create new enum instance will just fail, but there are two other ways to do it:

Reflections

But first… Why does normal reflection fail to create our enum instance? We can just go to source of Constructor and see what is going here

public T newInstance(Object ... initargs) throws InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException
{
    if (!override) {
        if (!Reflection.quickCheckMemberAccess(clazz, modifiers)) {
            Class<?> caller = Reflection.getCallerClass();
            checkAccess(caller, clazz, null, modifiers);
    }}
    if ((clazz.getModifiers() & Modifier.ENUM) != 0) throw new IllegalArgumentException("Cannot reflectively create enum objects");
    ConstructorAccessor ca = constructorAccessor;
    if (ca == null) { ca = acquireConstructorAccessor(); }
    return (T) ca.newInstance(initargs);
}

And as we can see there is just that little, simple, check: if ((clazz.getModifiers() & Modifier.ENUM) != 0), and we need to skip it.
So easiest way to do so, is just call that code below this check using reflections:

public static void reflectionWay() throws Throwable {
    Class<Monster> monsterClass = Monster.class;
    // first we need to find our constructor, and make it accessible
    Constructor<?> constructor = monsterClass.getDeclaredConstructors()[0];
    constructor.setAccessible(true);

    // this is this same code as in constructor.newInstance, but we just skipped all that useless enum checks ;)
    Field constructorAccessorField = Constructor.class.getDeclaredField("constructorAccessor");
    constructorAccessorField.setAccessible(true);
    // sun.reflect.ConstructorAccessor -> itnernal class, we should not use it, if you need use it, it would be better to actually not import it, but use it only via reflections. (as package may change, and will in java 9)
    ConstructorAccessor ca = (ConstructorAccessor) constructorAccessorField.get(constructor);
    if (ca == null) {
        Method acquireConstructorAccessorMethod = Constructor.class.getDeclaredMethod("acquireConstructorAccessor");
        acquireConstructorAccessorMethod.setAccessible(true);
        ca = (ConstructorAccessor) acquireConstructorAccessorMethod.invoke(constructor);
    }
    // note that real constructor contains 2 additional parameters, name and ordinal
    Monster enumValue = (Monster) ca.newInstance(new Object[]{"CAERBANNOG_RABBIT", 4, CaerbannogRabbit.class, "caerbannograbbit"});// you can call that using reflections too, reflecting reflections are best part of java ;)
}

And done! kind of… Monster.values() still does return only 3 objects, we need to fix that too.
Every enum class have static field: T[] $VALUES (note that name can be changed if code was obfuscated, if you want to handle such cases, you should lookup for first field of T[]/Monster[] type)
Bad thing is that this field is private, static, and… final, but we can fix that too:

static void makeAccessible(Field field) throws Exception {
    field.setAccessible(true);
    Field modifiersField = Field.class.getDeclaredField("modifiers");
    modifiersField.setAccessible(true);
    modifiersField.setInt(field, field.getModifiers() & ~ Modifier.FINAL);
}

Yey, more reflections on reflections! So now we just need to create own fixed array of values:

Field $VALUESField = Monster.class.getDeclaredField("$VALUES");
makeAccessible($VALUESField);
// just copy old values to new array and add our new field.
Monster[] oldValues = (Monster[]) $VALUESField.get(null);
Monster[] newValues = new Monster[oldValues.length + 1];
System.arraycopy(oldValues, 0, newValues, 0, oldValues.length);
newValues[oldValues.length] = enumValue;
$VALUESField.set(null, newValues);

And done! Intellij <3
Intellij <3
(Note that we can’t really add new field to enum, so it would never be like real enum field, adding new field is only possible using some bytecode manipulation AND only before class was loaded) But again… there are still few things we CAN fix!
Monster.class.getEnumConstants() - this still might return old array. (this array is created on first use)
Enum.valueOf(Monster.class, "CAERBANNOG_RABBIT") - this also might fail.
As you already know how to fix fields like that I will just skip that part (as it will be this same code as above), and only tell you where you need to apply your fixes:
private volatile transient T[] enumConstants = null; - in Class.class, note that it can be null.
private volatile transient Map<String, T> enumConstantDirectory = null; - in Class.class, note that it can be null too.
Also note that you can just set that values back to null so java will regenerate them on next use.

Unsafe

This same thing can be done using unsafe (also should not be used):

public static void unsafeWay() throws Throwable {
    Constructor<?> constructor = Unsafe.class.getDeclaredConstructors()[0];
    constructor.setAccessible(true);
    Unsafe unsafe = (Unsafe) constructor.newInstance();
    Monster enumValue = (Monster) unsafe.allocateInstance(Monster.class);
}

Looks simpler? But notice that we didn’t even pass anything to constructor of monster, this is how Unsafe works, it does NOT call the constructor, it just allocate new object, and this is very unsafe, as any action on that object might now throw unexpected errors, we need to manually simulate constructor!
So we just need to manually get all fields and set their values to something that make sense.

Field ordinalField = Enum.class.getDeclaredField("ordinal");
makeAccessible(ordinalField);
ordinalField.setInt(enumValue, 5);

Field nameField = Enum.class.getDeclaredField("name");
makeAccessible(nameField);
nameField.set(enumValue, "LION");

Field entityClassField = Monster.class.getDeclaredField("entityClass");
makeAccessible(entityClassField);
entityClassField.set(enumValue, Lion.class);

Field entityIdField = Monster.class.getDeclaredField("entityId");
makeAccessible(entityIdField);
entityIdField.set(enumValue, "Lion");

And then again, you need to add that value to all that arrays and maps (using this same code as for previous method).
__
Whole code: Gist: EnumHack.java

Own enum

But what if we want to have something similar to enum? I created weird, but working, library that use new StackWalker API (java 9) that allows you to create simple and dynamic enums like that:

public class SampleEnum extends DynamicEnum<SampleEnum> {
    public static final SampleEnum A = $("heh");
    private final String someProperty;
    SampleEnum(String someProperty) {this.someProperty = someProperty;}
    public String getSomeProperty() {return this.someProperty;}
    public static SampleEnum[] values() {return DynamicEnum.values(SampleEnum.class);}
    public static SampleEnum valueOf(String name) {return DynamicEnum.valueOf(SampleEnum.class, name);}
}

Or like this:

    static class EnumExample extends DynamicEnum<EnumExample>
    {
        public static final EnumExample A = $();
        public static final EnumExample B = $();
        public static final EnumExample C = new EnumExample() { public int doSomething() {return 7;} };
        public int doSomething() {return 5;}
        public static EnumExample[] values() {return DynamicEnum.values(EnumExample.class);}
        public static EnumExample valueOf(String name) {return DynamicEnum.valueOf(EnumExample.class, name);}
    }

And it works! you can do EnumExample.A.name() and it returns valid A name!
And ofc new values can be added:

EnumExample d = new EnumExample();
EnumExample e = new EnumExample() {};
Assert.assertSame(3, EnumExample.addEnumElement("D", d)); // returns assigned ordinal
Assert.assertSame(4, EnumExample.addEnumElement("E", e));

How it works? A bit tricky, but not that hacky as previous reflections: Github link
You can also find it in diorite repository in a bit cleaner form (LazyValue is used, and reflections are handled by special library).

It is possible to implement that for java 8, but then you need to use internal API Reflections.getCallerClass() to track where method was invoked. __

Also… I need some ideas for new posts! :D

If you want me to check something you can't understand, or explain some weird java behavior, feel free to e-mail me at: gotofinaldev@gmail.com or just write comment below!

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