I add a public boolean field to PotionEffect via ASM and I wanted to know how can I access that field while writing my code. Do I need reflection?

Both solutions sound good. when you say reflection you mean that unreflected getter/setter thing you taught me using

MethodHandle

and invokeExact, right?

no need to do that. if u make the clas implement ur interface via asm u can just cast to it and call the getter and setter. should be performance saving

This is what I did, according to the teachings of diesieben07

 

/* in my actual code, the methods here span 3 classes that contain other methods and stuff, I simplified the code for clarity 
* (hopefully) putting all relevant methods to this use case together here and changing the names of some identifiers for
* readability. Because of that, something may have broke, because I'm doing theses edits directly in the textArea of this reply.
*/

private final static MethodHandle myFieldGetter = createGetterForField(PotionEffect.class, "gw_myField", "gw_myField");
private final static MethodHandle myFieldSetter = createSetterForField(PotionEffect.class, "gw_myField", "gw_myField");

public static MethodHandle createGetterForField(Class c, String forgeFieldName, String srgFieldName)
{
String fieldName = isDeobfEnvironment() ? forgeFieldName: srgFieldName;
Field field = null;
try
{
	field = c.getDeclaredField(fieldName);
}
catch (NoSuchFieldException e)
{
	// TODO this is unlikely to happen;
}
catch (SecurityException e)
{
	// TODO this is unlikely to happen;
}

field.setAccessible(true);

try
{
	return MethodHandles.publicLookup().unreflectGetter(field);
}
catch (IllegalAccessException e)
{
	return null;
}

public static MethodHandle createSetterForField(Class c, String forgeFieldName, String srgFieldName)
{
Field field = null;
String fieldName = FMLHelper.isDeobfEnvironment() ? forgeFieldName: srgFieldName;
try
{
	field = c.getDeclaredField(fieldName);
}
catch (NoSuchFieldException e)
{
}
catch (SecurityException e)
{
}

field.setAccessible(true);

try
{
	return MethodHandles.publicLookup().unreflectSetter(field);
}
catch (IllegalAccessException e)
{
	return null;
}
}

public static void setMyField(PotionEffect instance, boolean value) throws Throwable
{
myFieldSetter.invokeExact((PotionEffect)instance, value);
}

public static boolean getMyField(PotionEffect instance) throws Throwable
{
return (boolean) myFieldGetter.invokeExact((PotionEffect)instance);
}

public static boolean isDeobfEnvironment()
{
return (Boolean) Launch.blackboard.get("fml.deobfuscatedEnvironment");
}

Yes, I agree, that's a very good point. I usually end up ignoring a few when coding. In my real code I log those marked as "TODO" in this example. I just didn't want to clutter the solution with anything non essential. I was going to write something similar to your advice in the TODO comment, but I chose that short comment instead for simplicity.