🔧 ModLoaderFixes

Fix Forge, NeoForge and Minecraft servers and clients that break on current Java, under load or behind a proxy. ☕

A native Java agent that repairs these as the game loads. It changes no game files and needs no .NET. ✨

🩹 What it fixes

Forge 1.7.2 servers stop before Minecraft loads on Java 8u20 and later:

[main/INFO] [LaunchWrapper]: Calling tweak class cpw.mods.fml.common.launcher.FMLInjectionAndSortingTweaker
[main/ERROR] [LaunchWrapper]: Unable to launch
java.util.ConcurrentModificationException
	at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:911) ~[?:1.8.0_504]
	at java.util.ArrayList$Itr.remove(ArrayList.java:875) ~[?:1.8.0_504]
	at net.minecraft.launchwrapper.Launch.launch(Launch.java:117) [launchwrapper-1.9.jar:?]

Forge 1.7.2 and 1.7.10, PaperSpigot 1.7.10 and vanilla 1.7.2 to 1.7.10 servers now and then never answer a login, and the client or proxy waits until it gives up:

[Netty IO #0/DEBUG] [net.minecraft.network.NetworkManager]: Set listener of net.minecraft.network.NetworkManager@1ab07dd4 to net.minecraft.server.network.NetHandlerLoginServer@2bcfb381
[Netty IO #0/DEBUG] [net.minecraft.util.MessageDeserializer]:  IN: [LOGIN:0] net.minecraft.network.login.client.C00PacketLoginStart[]

Forge 1.7.2 and 1.7.10 clients sometimes drop their connection as they join, while the game is busy:

[Netty Client IO #0] [ERROR] java.lang.RuntimeException: Timeout waiting for client thread to catch up!
at cpw.mods.fml.client.FMLClientHandler.waitForPlayClient(FMLClientHandler.java:843)
at cpw.mods.fml.common.FMLCommonHandler.waitForPlayClient(FMLCommonHandler.java:584)
at cpw.mods.fml.common.network.handshake.NetworkDispatcher.clientListenForServerHandshake(NetworkDispatcher.java:154)
at cpw.mods.fml.common.network.handshake.FMLHandshakeClientState$1.accept(FMLHandshakeClientState.java:33)
at cpw.mods.fml.common.network.handshake.FMLHandshakeClientState$1.accept(FMLHandshakeClientState.java:27)
at cpw.mods.fml.common.network.handshake.HandshakeMessageHandler.userEventTriggered(HandshakeMessageHandler.java:40)
...
at cpw.mods.fml.common.network.handshake.NetworkDispatcher.handlerAdded(NetworkDispatcher.java:132)
...
at cpw.mods.fml.common.network.handshake.NetworkDispatcher.insertIntoChannel(NetworkDispatcher.java:119)
at cpw.mods.fml.common.network.handshake.NetworkDispatcher.clientToServerHandshake(NetworkDispatcher.java:124)
at cpw.mods.fml.common.network.internal.FMLNetworkHandler.fmlClientHandshake(FMLNetworkHandler.java:66)
at net.minecraft.client.network.NetHandlerLoginClient.func_147390_a(NetHandlerLoginClient.java:97)

Forge 1.7.2 and 1.7.10 clients now and then drop a connection as they join a server again, when the server answers before the game has set the connection up:

[Netty Client IO #1/ERROR]: ChannelRegistrationHandler exception
java.lang.ClassCastException: Cannot cast net.minecraft.client.network.NetHandlerLoginClient to net.minecraft.client.network.NetHandlerPlayClient
	at java.lang.Class.cast(Class.java:3369)
	at cpw.mods.fml.common.network.FMLNetworkEvent$CustomPacketRegistrationEvent.<init>(FMLNetworkEvent.java:93)
	at cpw.mods.fml.client.FMLClientHandler.fireNetRegistrationEvent(FMLClientHandler.java:887)

Forge and vanilla clients from 1.7 to 1.12.2 sometimes crash with their sound system when its resources reload while it still loads, as Forge's second reload at startup can:

[Thread-8] java.lang.IllegalStateException: Only one OpenAL context may be instantiated at any one time.
	at org.lwjgl.openal.AL.create(AL.java:113)
	at paulscode.sound.libraries.LibraryLWJGLOpenAL.init(LibraryLWJGLOpenAL.java:164)
[Sound Library Loader]         SoundSystem did not load after 30 seconds.
[Thread-7] java.lang.UnsatisfiedLinkError: org.lwjgl.openal.AL10.nalGetSourcei(II)I

Forge 1.7.2 and 1.7.10 and vanilla 1.7.2 to 1.7.10 clients sometimes drop their connection a few milliseconds after they open it:

[Client thread] [INFO] Connecting to localhost, 25565
Internal Exception: io.netty.handler.codec.EncoderException: java.io.IOException: Can't serialize unregistered packet

Forge 1.12 to 1.12.2 and 1.13.2 and vanilla 1.8 to 1.14.3 clients sometimes crash as they start to connect:

[Client thread] [INFO] Connecting to localhost, 25565
Description: Ticking screen
java.lang.NullPointerException: Ticking screen
	at em$a.a(SourceFile:387)
	at em.m(SourceFile:220)
	at em.a(SourceFile:229)
	at beg.e(SourceFile:91)

Minecraft clients from 1.12 sometimes stay on their connecting screen forever when Mojang's list of blocked servers is cut off, and every later connection of the game fails too:

[Server Connector #1/ERROR]: Caught previously unhandled exception :
java.lang.ExceptionInInitializerError: null
Caused by: java.io.UncheckedIOException: java.net.SocketException: Connection reset
	at java.io.BufferedReader$1.hasNext(BufferedReader.java:574) ~[?:?]
	at com.mojang.patchy.MojangBlockListSupplier.createBlockList(MojangBlockListSupplier.java:25) ~[patchy-2.2.10.jar:?]

Forge 1.12 servers drop players at login under load:

[Server thread/INFO] [minecraft/NetHandlerPlayServer]: Player lost connection: Internal Exception: io.netty.handler.codec.EncoderException: java.io.IOException: Can't serialize unregistered packet

Forge 1.12 and 1.12.1 clients crash when a proxy such as BungeeCord or Void moves them to another server:

[Netty Epoll Client IO #1] [INFO] Injecting existing registry data into this client instance
[main] [FATAL] net.minecraft.util.ReportedException: Ticking entity
	at net.minecraft.world.World.func_72939_s(World.java:1766)
	at net.minecraft.client.Minecraft.func_71407_l(Minecraft.java:1841)
Caused by: java.lang.NullPointerException
	at net.minecraft.entity.player.EntityPlayer.func_192028_j(EntityPlayer.java:589)
	at net.minecraft.entity.player.EntityPlayer.func_70636_d(EntityPlayer.java:574)

Forge 1.13.2 to 1.20.1 and NeoForge 1.20.1 servers drop players at login under load:

[Server thread/INFO] [minecraft/ServerLoginNetHandler]: com.mojang.authlib.GameProfile@6ac532df[id=<null>,name=Player,properties={},legacy=false] (/172.17.0.1:43658) lost connection: Internal Exception: java.util.ConcurrentModificationException

NeoForge 1.20.2 and 1.20.3 servers, and the first betas for 1.20.4, now and then end a player's login handshake before its last answer and drop the player:

[Server thread/DEBUG] [net.neoforged.neoforge.network.HandshakeHandler/FMLHANDSHAKE]: Handshake complete!
[Netty Epoll Server IO #4/DEBUG] [net.minecraft.network.Connection/]: Failed to sent packet
io.netty.handler.codec.DecoderException: Badly compressed packet - size of 2 is below server threshold of 256
	at net.minecraft.network.CompressionDecoder.decode(CompressionDecoder.java:45)

Forge 1.13.2 to 1.20.1 and NeoForge 1.20.1 to 1.20.3 servers, and the first NeoForge betas for 1.20.4, now and then drop a player just as its login handshake completes:

[Server thread/DEBUG] [net.minecraftforge.network.HandshakeHandler]: Handshake complete!
[Netty Epoll Server IO #2/DEBUG] [net.minecraft.network.Connection]: Failed to sent packet
java.lang.NullPointerException: null
	at net.minecraftforge.network.HandshakeHandler.lambda$biConsumerFor$1(HandshakeHandler.java:154)
	at java.util.function.BiConsumer.lambda$andThen$0(Unknown Source)

Forge 1.13.2 to 1.15.2 now and then crashes while it loads a world or its mods:

java.lang.NullPointerException: null
	at cpw.mods.modlauncher.TransformerClassWriter.getCommonSuperClass(TransformerClassWriter.java:102) ~[modlauncher-2.1.5.jar:?] {}
	at org.objectweb.asm.SymbolTable.addMergedType(SymbolTable.java:1209) ~[asm-6.2.jar:?] {}

Forge 1.13.2 to 1.19.4 sometimes fails its first start while it corrects fml.toml, or writes keys twice:

com.electronwill.nightconfig.core.io.ParsingException: Invalid TOML data: entry "[disableOptimizedDFU]" defined twice in its table.
Exception in thread "main" java.util.ConcurrentModificationException
	at com.electronwill.nightconfig.core.ConfigSpec.correct(ConfigSpec.java:655)
	at net.minecraftforge.fml.loading.FMLConfig.loadFrom(FMLConfig.java:54)

Forge 1.13.2 to 1.21.4 and NeoForge 1.20.1 sometimes crash while they load mods:

java.lang.ClassCastException: class java.util.HashMap$Node cannot be cast to class java.util.HashMap$TreeNode (java.util.HashMap$Node and java.util.HashMap$TreeNode are in module java.base of loader 'bootstrap')
	at java.util.HashSet.add(HashSet.java:221)
	at net.minecraftforge.registries.ObjectHolderRegistry.addHandler(ObjectHolderRegistry.java:64)

Forge 1.14.2 and 1.14.3 clients sometimes hang at startup and never reach their menu. A thread dump shows the client thread waiting in EventListenerHelper.getListenerListInternal while a modloading-worker thread holds EventBus's lock.

Forge, NeoForge and vanilla clients from 1.14 to 1.20.4 sometimes crash the JVM as they leave a world while a sound streams:

# A fatal error has been detected by the Java Runtime Environment:
#  SIGSEGV (0xb) at pc=0x00007522e862e144
# Problematic frame:
# C  [liblwjgl_stb.so+0x2e144]
Current thread: JavaThread "Sound engine"
J  org.lwjgl.stb.STBVorbis.nstb_vorbis_decode_frame_pushdata
J  net.minecraft.client.audio.OggAudioStream.func_216460_a

Forge, Paper, Spigot and vanilla 1.16 to 1.18.2 servers sometimes hang at startup and never finish loading their data packs. The log stops after Reloading ResourceManager, and a thread dump shows the main thread waiting while the worker threads sit idle:

"main" #1 ... WAITING (parking)
	at java.util.concurrent.CompletableFuture.get(CompletableFuture.java:1908)
	at net.minecraft.server.v1_16_R3.Main.main(Main.java:188)

Forge and vanilla 1.7 clients, and Forge, NeoForge and vanilla clients from 1.13, drop the chat a server sends just before it disconnects them, such as a proxy's explanation of a kick. 1.7 clients never show it. Clients from 1.13 drop it now and then, and from 1.14.4 they log Ignoring packet due to disconnection for it with debug logging.

Forge and NeoForge 1.20.1 to 1.21.4 clients sometimes crash before the game's window opens:

java.lang.RuntimeException: java.util.concurrent.ExecutionException: java.lang.NullPointerException: Cannot invoke "java.lang.Boolean.booleanValue()" because the return value of "net.minecraftforge.fml.loading.FMLConfig$ConfigValue.getConfigValue(com.electronwill.nightconfig.core.file.CommentedFileConfig)" is null
	at net.minecraftforge.fml.earlydisplay.DisplayWindow.setupMinecraftWindow(DisplayWindow.java:527)
	at net.minecraftforge.fml.loading.ImmediateWindowHandler.setupMinecraftWindow(ImmediateWindowHandler.java:54)

Forge clients from 1.20.1 and NeoForge 1.20.1 to 1.21.3 clients give up at startup when the game's window takes more than 10 seconds to open, as it can on a busy machine or one that renders in software:

[main/INFO]: Trying GL version 4.6
[pool-2-thread-1/ERROR]: ERROR DISPLAY
Failed to initialize the mod loading system and display.

Failure details:
Timed out trying to setup the Game Window.

Forge and NeoForge clients from 1.20.6, and vanilla clients from 26.1, sometimes drop their connection as they join or as a proxy moves them to another server:

[Render thread] [WARN] Client disconnected with reason: Internal Exception: io.netty.handler.codec.EncoderException: Pipeline has no outbound protocol configured, can't process packet ServerboundChunkBatchReceivedPacket
[Netty Epoll IO #0] [ERROR] io.netty.handler.codec.EncoderException: Sending unknown packet 'serverbound/minecraft:chunk_batch_received'

Minecraft, Forge, NeoForge and Paper servers from 1.21.11 now and then crash while they generate a chunk, often as a player arrives:

Description: Exception generating new chunk
java.lang.NullPointerException: Cannot invoke "net.minecraft.world.level.MoonPhase.index()" because "moonphase" is null
	at net.minecraft.server.level.ServerLevel.getMoonBrightness(ServerLevel.java:708)
	at net.minecraft.server.level.WorldGenRegion.getCurrentDifficultyAt(WorldGenRegion.java:365)
	at net.minecraft.world.level.NaturalSpawner.spawnMobsForChunkGeneration(NaturalSpawner.java:404)

🚀 Usage

Download the agent from the latest release:

curl -fsSL -O https://github.com/void-community/ModLoaderFixes/releases/latest/download/modloaderfixes.jar

Add it to the Java arguments of the server or the launcher profile:

java -javaagent:modloaderfixes.jar -jar forge-1.12.2-14.23.5.2859.jar nogui

🐳 With itzg/minecraft-server:

docker run -d -e EULA=TRUE -e TYPE=FORGE -e VERSION=1.12.2 \
  -e JVM_OPTS=-javaagent:/patcher/modloaderfixes.jar \
  -v "$PWD/modloaderfixes.jar:/patcher/modloaderfixes.jar:ro" \
  itzg/minecraft-server:java8

✅ The log shows a line for each fix as the agent applies it:

ModLoaderFixes: patched net/minecraftforge/fml/common/network/handshake/NetworkDispatcher (serverInitiateHandshake()I: 1 invocation of net/minecraft/network/NetworkManager.func_150723_a redirected)

📦 Requirements

• 🐧 Linux x64 or ARM64 with glibc 2.17 or later: CentOS 7, Ubuntu 18.04, Debian 10 and newer, and every itzg image. One jar serves both.
• To skip the jar, pass modloaderfixes-linux-x64.so or modloaderfixes-linux-arm64.so from the release to -agentpath with an absolute path.
• 🧪 CI tests every commit: real Forge and NeoForge servers start with the agent, many with 200 players logging in at once, and the client fixes run against reproductions of their bugs.

📄 License

MIT