How to use Reflection to change vanilla values

[JimiIT92]

As by title, i don't know how to use reflection to change a vanilla value. In my particular case i need to change this values

swingProgressInt

from the EntityLivingBase class to make a faster animation when using a specific item (in my case a Katana).

I tried this code

Field field;
	try {
		field = Class.forName("net.minecraft.entity.EntityLivingBase").getDeclaredField("swingProgressInt");
		field.setAccessible(true);

		field.setInt(arg0, 4);
	} catch (NoSuchFieldException e) {
		// TODO Auto-generated catch block
		e.printStackTrace();
	} catch (SecurityException e) {
		// TODO Auto-generated catch block
		e.printStackTrace();
	} catch (ClassNotFoundException e) {
		// TODO Auto-generated catch block
		e.printStackTrace();
	}

but i don't understand what to type in as arg0. The function takes an Object and an int. the in value is the new value of the variable i guess but the object i don't understand what it is

 

[JimiIT92]

Someone in another post told me to do that Anyway i tried changing that value in the function by doing

attacker.swingProgressInt = 2;

but it doesn't affect the animation (is neither slower or faster)

[JimiIT92]

Ok, so i've overrited this method

@Override
public boolean onLeftClickEntity(ItemStack stack, EntityPlayer player, Entity entity)
{
	player.swingProgressInt = 1;
	return this.hitEntity(stack, (EntityLivingBase) entity, player);
}

and my katana is faster (wich is what i want), BUT if i hit an entity it doesn't deal damage

[LordMastodon]

Perhaps try adding super.onLeftClickEntity. Not sure if that'll work, but try it.

[JimiIT92]

Ok, so i've looked into the super method wich is only a return false statement. So i changed my function to this and it worked

@Override
public boolean onLeftClickEntity(ItemStack stack, EntityPlayer player, Entity entity)
{
	player.swingProgressInt = 3;
	return false;
}

 

Also fun fact: it seems that i can make my item only faster but not slower.

[LordMastodon]

What about decimal values or negative numbers?

[JimiIT92]

Negative numbers makes my katana to not swing at all. I think that the exact value to set could be find in ths function

private int getArmSwingAnimationEnd()
    {
        return this.isPotionActive(Potion.digSpeed) ? 6 - (1 + this.getActivePotionEffect(Potion.digSpeed).getAmplifier()) * 1 : (this.isPotionActive(Potion.digSlowdown) ? 6 + (1 + this.getActivePotionEffect(Potion.digSlowdown).getAmplifier()) * 2 : 6);
    }

but since is a private function i can't access that to see what value it returns. I tried using the exact value that is supposed to return if the haste or slowdown effect is applied (level 1) (5 for digSpeed and 8 for digSlowdown) but nothing happens (instead with a value of 5 the katana do a weird animation).

[jeffryfisher]

Although you're no longer using reflection, I'd like to clarify something from your OP. You had this:

 

                try {
		field = Class.forName("net.minecraft.entity.EntityLivingBase").getDeclaredField("swingProgressInt");
		field.setAccessible(true);

		field.setInt(arg0, 4);
	} catch...

 

If I understand reflection correctly, then it is only necessary to getDeclaredField once and setAccessible once (perhaps in a constructor). Later, one may use the field to operate upon instances of its class to make value assignments many times. Do I have that right?

 

[Failender]

Although you're no longer using reflection, I'd like to clarify something from your OP. You had this:

 

                try {
		field = Class.forName("net.minecraft.entity.EntityLivingBase").getDeclaredField("swingProgressInt");
		field.setAccessible(true);

		field.setInt(arg0, 4);
	} catch...

 

If I understand reflection correctly, then it is only necessary to getDeclaredField once and setAccessible once (perhaps in a constructor). Later, one may use the field to operate upon instances of its class to make value assignments many times. Do I have that right?

 

yes.

[ItsAMysteriousYT]

There is an easier way made by forge. Its called ObfuscationReflectionHelper. In there use the methods getPrivateValue/setPrivatevalue etc

[JimiIT92]

Understand, thank you

[ItsAMysteriousYT]

Why not? Whats so bad about it?

[coalbricks]

Replying for future modders. ObfuscationReflectionHelper is perfectly fine if you use it efficiently. You can use it to get a Field object, which you can store in a static member.

Example:

public class Example {
	private static Field someField = null;

	static {
		try {
			someField = ObfuscationReflectionHelper.findField(SomeMinecraftClass.class, "field_12345_a"); // You need to pass the SRG name
			someField.setAccessible(true);
		} catch (Exception e) {
			e.printStackTrace();
		}
	}

	public void doStuff(SomeMinecraftClass minecraftThing) {
		try {
        		Object o = someField.get(minecraftThing);
		} catch (Exception e) {
			e.printStackTrace();
		}
	}
}

I would also advise against passing the fully qualified class name as a String if you can help it. It's better to use the Class object directly if you can, and in most cases you can.

 

You can find SRG names at http://mcpbot.bspk.rs/

Edited January 11, 2021 by coalbricks
Added link to MCPBot site

[coalbricks]

12 hours ago, diesieben07 said:

1. Your exception handling is inexcusable. This is not how you handle exceptions.
2. The Field should be stored in a static final field, to allow further JVM optimizations.

1. It's a basic example. And that exception handling is fine for the purposes of being a basic example. It's also fine in production code in certain cases which I don't want to debate here, although I will admit the doStuff() function should have a null guard.

2. Yes while it would be better to store it in a static final field, the JVM optimizations obtained by making the member final are minimal at best, unless the field is being used to perform reflective gets/sets frequently. This pattern works perfectly fine in MOST cases but if you really care you can separate the static initializer logic into a separate function and make the field final; in most cases it's just not worth the trouble.

Edited January 11, 2021 by coalbricks
Added a note about final fields