A test for remapping when using URLClassLoader.addURL(URL)

A little test for remapping libraries loaded using URLClassLoader.addURL(URL).

This is a (super) stripped down version of a library like Libby, which allows to load libraries dynamically in the plugin class loader. It works by calling the addURL method on the plugin class loader (the method is called via reflection after having obtained access to the java.net module, for more info see URLClassLoaderHelper).

How to run

An already built binary is provided in this repo (addurl-test-plugin-1.0.jar), otherwise just clone the repo and use mvn package to build from source. The final plugin is located inside AddURLTestPlugin/target.

Copy the plugin inside the plugins folder and start your server with the following parameter: --add-opens java.base/java.net=ALL-UNNAMED.

Enter your server and break a block. If everything works, a (client-side only) zombie should have spawned at (0, 0, 0), otherwise an exception should have been thrown.

Example exception

java.lang.NoClassDefFoundError: net/minecraft/world/level/World
	at addurl-test-plugin-1.0.jar/com.fren_gor.addURLTestPlugin.BlockBreakListener.onBlockBreak(BlockBreakListener.java:12) ~[addurl-test-plugin-1.0.jar:?]
	at com.destroystokyo.paper.event.executor.asm.generated.GeneratedEventExecutor1.execute(Unknown Source) ~[?:?]
	at org.bukkit.plugin.EventExecutor$2.execute(EventExecutor.java:77) ~[paper-api-1.20.6-R0.1-SNAPSHOT.jar:?]
	at co.aikar.timings.TimedEventExecutor.execute(TimedEventExecutor.java:81) ~[paper-api-1.20.6-R0.1-SNAPSHOT.jar:git-Paper-71]
	at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:70) ~[paper-api-1.20.6-R0.1-SNAPSHOT.jar:?]
	at io.papermc.paper.plugin.manager.PaperEventManager.callEvent(PaperEventManager.java:54) ~[paper-1.20.6.jar:git-Paper-71]
	at io.papermc.paper.plugin.manager.PaperPluginManagerImpl.callEvent(PaperPluginManagerImpl.java:131) ~[paper-1.20.6.jar:git-Paper-71]
	at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:627) ~[paper-api-1.20.6-R0.1-SNAPSHOT.jar:?]
	at net.minecraft.server.level.ServerPlayerGameMode.destroyBlock(ServerPlayerGameMode.java:378) ~[paper-1.20.6.jar:git-Paper-71]
	at net.minecraft.server.level.ServerPlayerGameMode.destroyAndAck(ServerPlayerGameMode.java:337) ~[paper-1.20.6.jar:git-Paper-71]
	at net.minecraft.server.level.ServerPlayerGameMode.handleBlockBreakAction(ServerPlayerGameMode.java:213) ~[paper-1.20.6.jar:git-Paper-71]
	at net.minecraft.server.network.ServerGamePacketListenerImpl.handlePlayerAction(ServerGamePacketListenerImpl.java:1845) ~[paper-1.20.6.jar:git-Paper-71]
	at net.minecraft.network.protocol.game.ServerboundPlayerActionPacket.handle(ServerboundPlayerActionPacket.java:51) ~[paper-1.20.6.jar:git-Paper-71]
	at net.minecraft.network.protocol.game.ServerboundPlayerActionPacket.handle(ServerboundPlayerActionPacket.java:20) ~[paper-1.20.6.jar:git-Paper-71]
	at net.minecraft.network.protocol.PacketUtils.lambda$ensureRunningOnSameThread$0(PacketUtils.java:55) ~[paper-1.20.6.jar:git-Paper-71]
	at net.minecraft.server.TickTask.run(TickTask.java:18) ~[paper-1.20.6.jar:git-Paper-71]
	at net.minecraft.util.thread.BlockableEventLoop.doRunTask(BlockableEventLoop.java:151) ~[paper-1.20.6.jar:git-Paper-71]
	at net.minecraft.util.thread.ReentrantBlockableEventLoop.doRunTask(ReentrantBlockableEventLoop.java:24) ~[paper-1.20.6.jar:git-Paper-71]
	at net.minecraft.server.MinecraftServer.doRunTask(MinecraftServer.java:1517) ~[paper-1.20.6.jar:git-Paper-71]
	at net.minecraft.server.MinecraftServer.doRunTask(MinecraftServer.java:198) ~[paper-1.20.6.jar:git-Paper-71]
	at net.minecraft.util.thread.BlockableEventLoop.pollTask(BlockableEventLoop.java:125) ~[paper-1.20.6.jar:git-Paper-71]
	at net.minecraft.server.MinecraftServer.pollTaskInternal(MinecraftServer.java:1494) ~[paper-1.20.6.jar:git-Paper-71]
	at net.minecraft.server.MinecraftServer.pollTask(MinecraftServer.java:1417) ~[paper-1.20.6.jar:git-Paper-71]
	at net.minecraft.util.thread.BlockableEventLoop.managedBlock(BlockableEventLoop.java:135) ~[paper-1.20.6.jar:git-Paper-71]
	at net.minecraft.server.MinecraftServer.waitUntilNextTick(MinecraftServer.java:1383) ~[paper-1.20.6.jar:git-Paper-71]
	at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1244) ~[paper-1.20.6.jar:git-Paper-71]
	at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:326) ~[paper-1.20.6.jar:git-Paper-71]
	at java.base/java.lang.Thread.run(Thread.java:1583) ~[?:?]
Caused by: java.lang.ClassNotFoundException: net.minecraft.world.level.World
	at org.bukkit.plugin.java.PluginClassLoader.loadClass0(PluginClassLoader.java:197) ~[paper-api-1.20.6-R0.1-SNAPSHOT.jar:?]
	at org.bukkit.plugin.java.PluginClassLoader.loadClass(PluginClassLoader.java:164) ~[paper-api-1.20.6-R0.1-SNAPSHOT.jar:?]
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:526) ~[?:?]
	... 28 more

A full log of a failing execution is available here.

How it works

The plugin contains the jar to load (built by the AddURLTest module), which is extracted into the plugin data folder before being loaded using addURL.

AddURLTest just contains a class with the test method, which spawns the client side zombie. The BlockBreakEvent listener just calls such method.

Since the test method uses spigot-mapped NMS code, the zombie will be successfully spawned only if the loaded jar is remapped when loaded.