CraftBukkit开发：为什么会抛出NullPointerException？

Question

我正在编写一个简单的Bukkit / Spigot插件，该插件将大城市的建筑物内的守卫者替换为沉船。这是代码：

package ru.cardboardbox.shulkerspawner;
import org.bukkit.*;
import org.bukkit.block.Block;
import org.bukkit.entity.Entity;
import org.bukkit.entity.EntityType;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.entity.CreatureSpawnEvent;
import org.bukkit.plugin.java.JavaPlugin;
import org.bukkit.util.BlockVector;

import java.util.logging.Logger;

public final class main extends JavaPlugin implements Listener {
    World world_the_end = Bukkit.getWorld("world_the_end");
    private Logger log = Logger.getLogger("Minecraft");
    @Override
    public void onEnable() {
        log.info("[ShulkerSpawner] Good morning sunshine!");
        getServer().getPluginManager().registerEvents(this, this);
    }
    @EventHandler(ignoreCancelled = true)
    public void onSpawn(CreatureSpawnEvent e) {

        Location spawneeStandingOnBlockLocation = e.getLocation().add(new BlockVector(0,-1,0));
        Location spawneeRealLocation = e.getLocation();
        Block checkedBlock = spawneeStandingOnBlockLocation.getBlock();
        Material checkedBlockMaterial = checkedBlock.getType();
        Entity spawnedEntity = e.getEntity();
        log.info("[ShulkerSpawner] We're checking at " + spawneeRealLocation + ", the entity being an instance of " + e.getEntityType());
        log.info("[ShulkerSpawner] The entity is standing on the block " + spawneeStandingOnBlockLocation.toString() + ", which is " + checkedBlockMaterial.toString());

        if (e.getEntityType() == EntityType.ENDERMAN) {



            /////// NULL POINTER NEXT LINE
            Location endCityOrNull = world_the_end.locateNearestStructure(spawneeRealLocation, StructureType.END_CITY,3,true);



            if (endCityOrNull != null) {
                if (    checkedBlockMaterial == Material.PURPUR_BLOCK  ||
                        checkedBlockMaterial == Material.PURPUR_SLAB   ||
                        checkedBlockMaterial == Material.PURPUR_PILLAR ||
                        checkedBlockMaterial == Material.PURPUR_STAIRS )
                {
                    log.info("[ShulkerSpawner] Successfully(?) spawned a shulker at " + spawneeRealLocation.toString());
                    //It's cruft, but it's EXACTLY what I need. Don't ask why. I really need to replace endermen with shulkers.
                    world_the_end.spawnEntity(spawneeRealLocation,EntityType.SHULKER);
                    spawnedEntity.remove();
                } else {
                    log.info("[ShulkerSpawner] Entity is not on city blocks");
                }
            } else {
                log.info("[ShulkerSpawner] Entity is not near a city");
            }
        } else {
            log.info("[ShulkerSpawner] Entity is not an Enderman");
        }
    }


}

问题：碰碰碰碰碰碰的情况下，带有NullPointerException的标记行往往会抛出空白。方法locateNearestStructure是可为空的，并且在找不到结构时返回null。

踢球者：分配时将引发NullPointerException。对于我们要查找的结构不会生成的世界，不会抛出该异常（因此它将始终返回null）。

问题：为什么会发生，如何解决？

    [08:56:54] [Server thread/INFO]: [ShulkerSpawner] We're checking at Location{world=CraftWorld{name=world_the_end},x=1667.5,y=102.0,z=35.5,pitch=0.0,yaw=1.818178}, the entity being an instance of ENDERMAN
[08:56:54] [Server thread/INFO]: [ShulkerSpawner] The entity is standing on the block Location{world=CraftWorld{name=world_the_end},x=1667.5,y=101.0,z=35.5,pitch=0.0,yaw=1.818178}, which is PURPUR_BLOCK
[08:56:54] [Server thread/ERROR]: Could not pass event CreatureSpawnEvent to shulkerspawner v1.0-SNAPSHOT
java.lang.NullPointerException: null
    at ru.cardboardbox.shulkerspawner.main.onSpawn(main.java:39) ~[?:?]
    at com.destroystokyo.paper.event.executor.asm.generated.GeneratedEventExecutor7.execute(Unknown Source) ~[?:?]
    at org.bukkit.plugin.EventExecutor.lambda$create$1(EventExecutor.java:69) ~[patched_1.15.2.jar:git-Paper-129]
    at co.aikar.timings.TimedEventExecutor.execute(TimedEventExecutor.java:80) ~[patched_1.15.2.jar:git-Paper-129]
    at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:70) ~[patched_1.15.2.jar:git-Paper-129]
    at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:559) ~[patched_1.15.2.jar:git-Paper-129]
    at org.bukkit.craftbukkit.v1_15_R1.event.CraftEventFactory.callCreatureSpawnEvent(CraftEventFactory.java:631) ~[patched_1.15.2.jar:git-Paper-129]
    at org.bukkit.craftbukkit.v1_15_R1.event.CraftEventFactory.doEntityAddEventCalling(CraftEventFactory.java:552) ~[patched_1.15.2.jar:git-Paper-129]
    at net.minecraft.server.v1_15_R1.WorldServer.addEntity0(WorldServer.java:1214) ~[patched_1.15.2.jar:git-Paper-129]
    at net.minecraft.server.v1_15_R1.WorldServer.addEntity(WorldServer.java:1121) ~[patched_1.15.2.jar:git-Paper-129]
    at net.minecraft.server.v1_15_R1.SpawnerCreature.spawnMobs(SpawnerCreature.java:123) ~[patched_1.15.2.jar:git-Paper-129]
    at net.minecraft.server.v1_15_R1.ChunkProviderServer.lambda$tickChunks$7(ChunkProviderServer.java:723) ~[patched_1.15.2.jar:git-Paper-129]
    at it.unimi.dsi.fastutil.longs.Long2ObjectLinkedOpenHashMap$1.forEach(Long2ObjectLinkedOpenHashMap.java:1661) ~[patched_1.15.2.jar:git-Paper-129]
    at com.google.common.collect.Iterables$UnmodifiableIterable.forEach(Iterables.java:105) ~[patched_1.15.2.jar:git-Paper-129]
    at net.minecraft.server.v1_15_R1.ChunkProviderServer.tickChunks(ChunkProviderServer.java:659) ~[patched_1.15.2.jar:git-Paper-129]
    at net.minecraft.server.v1_15_R1.ChunkProviderServer.tick(ChunkProviderServer.java:602) ~[patched_1.15.2.jar:git-Paper-129]
    at net.minecraft.server.v1_15_R1.WorldServer.doTick(WorldServer.java:406) ~[patched_1.15.2.jar:git-Paper-129]
    at net.minecraft.server.v1_15_R1.MinecraftServer.b(MinecraftServer.java:1245) ~[patched_1.15.2.jar:git-Paper-129]
    at net.minecraft.server.v1_15_R1.DedicatedServer.b(DedicatedServer.java:430) ~[patched_1.15.2.jar:git-Paper-129]
    at net.minecraft.server.v1_15_R1.MinecraftServer.a(MinecraftServer.java:1112) ~[patched_1.15.2.jar:git-Paper-129]
    at net.minecraft.server.v1_15_R1.MinecraftServer.run(MinecraftServer.java:934) ~[patched_1.15.2.jar:git-Paper-129]
    at java.lang.Thread.run(Thread.java:835) [?:?]

关于包含空引用的world_the_end的建议：它奇怪地覆盖了世界的所有维度，并且在整个世界中都可以正常工作。只有在《末日》中才会这样抛出。 The End的文件夹称为“ world_the_end”，它是否默认为默认世界，我是否完全误解了bukkit世界如何工作？

Answer 1

您可以尝试在服务器中获得所有优势，并按照以下步骤确定终点：

public World getTheEnd() {
    for(World w: Bukkit.getServer().getWorlds()) {
        if(w.getEnvironment().equals(World.Environment.THE_END)) {
            return w;
        }
    }
    return null;
}

在Spigot / Bukkit服务器中，World对象的参数类型为World.Environment，可以是：

• World.Environment.NORMAL
• World.Environment.NETHER
• World.Environment.THE_END

请注意，仅当您的服务器只有一个终端世界时，此解决方案才有效。否则，函数getTheEnd()将返回（可能）不可预测的结束世界。

请让我知道这是否解决了您的问题。