force bukkit to render all maps by id on player join (fuck you)

disclaimer.md

if you aren't going to use CraftBukkit give up now

i also do not care if "you probably shouldn't use craftbukkit non-reflectively" as i'd prefer not to waste my time writing an implementation that's actually useful. i'll depend on what i need to depend to get what i need. this is simply a cheat on top of a cheat, and is probably good insight on how you could achieve this.

PlayerJoinListener.java

import java.lang.reflect.Field;
import java.util.ArrayList;
import java.util.List;

import org.bukkit.craftbukkit.v1_16_R3.entity.CraftPlayer;
import org.bukkit.craftbukkit.v1_16_R3.map.CraftMapView;
import org.bukkit.craftbukkit.v1_16_R3.map.RenderData;

import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.player.PlayerJoinEvent;

public class PlayerJoinListener implements Listener {
  
    // CraftMapView#renderCache
    private static Field craftBukkitRenderCache;
  
    static {
       try {
          craftBukkitRenderCache = CraftMapView.class.getDeclaredField("renderCache");
          craftBukkitRenderCache.setAccessible(true);
       } catch (NoSuchFieldException e) {
          // do something
       }
    }
  
  
    @EventHandler
    public void onPlayerJoin(PlayerJoinEvent event) {
       // We need CraftPlayer for this invocation
       CraftPlayer player = (CraftPlayer) event.getPlayer();
       // Supply this, we're going to do this in a lambda
       List<Integer> mapsYouWantToRender = getTheMapsYouWantToRender();
       // Lambda for iteration
       mapsYouWantToRender.forEach(map -> {
           // get a map by the current iterated map id. you store these somewhere, i'd hope
           MapView view = Bukkit.getMap(map);
           // don't do anything if it's null
           if (view != null) {
              // cast to get the mapview we need
              CraftMapView craftMapView = (CraftMapView) view;
              try {
                  // steal the cache
                  Map<CraftPlayer, RenderData> cache = (Map<CraftPlayer, RenderData>) field.get(craftMapView);
                  // check to see if the cache contains the player who just logged in
                  if (!cache.containsKey(player)) {
                      // no? lets render the mapview to the player
                      craftMapView.render(player);
                      // update the map (send it back)
                      player.sendMap(view);
                  }
              } catch (IllegalAccessException e) {
                  // whatever
              }
           }
       });
    }
  
}

SomeRenderer.java

import org.bukkit.map.MapRenderer;
import org.bukkit.map.MapCanvas;
import org.bukkit.map.MapView;

public class SomeRenderer extends MapRenderer {
  
   public SomeRenderer() {
       // The first trick of this is feeding "true" into the super call
       // Since MapRenderer is really a CraftRenderer, this sets CraftRenderer#isContextual to true
       // We need to trick the actual render implementation so that the player it caches is not null,
       // so we need CraftRenderer#isContextual to return true.
       super(true); 
   }
  
  
   @Override
    public void render(MapView mapView, MapCanvas mapCanvas, Player player) {
       // Do something
    }
  
}