I need to figure out the obfuscated name of several (10+) fields in vanilla classes. I attempted to use the FMLDeobfuscationRemapper to do this, but unfortunately it returns an empty map.

try {
        Method getFieldMapMethod = FMLDeobfuscatingRemapper.class.getDeclaredMethod("getFieldMap", String.class);
        getFieldMapMethod.setAccessible(true);
        Map<String, String> itemFieldMap = (Map<String, String>) getFieldMapMethod.invoke(FMLDeobfuscatingRemapper.INSTANCE, "net.minecraft.item.Item");
        System.out.println(itemFieldMap.keySet());
} catch (Exception e) {
        e.printStackTrace();
}

This prints out [].

I'm writing a mod to write certain fields of items and blocks to Json files, using the library json-io. I have to specify the fields I want outputted to the library.

That's unfortunate. Thanks for helping.