LauncherJava - Overview

Overview

LauncherJava is the Java-based component responsible for actually launching Minecraft. It acts as an intermediary between the C++ launcher and the Minecraft JVM process.

Launch Script Format

The launcher communicates via a text-based protocol:

mod legacyjavafixer-1.0
mainClass net.minecraft.launchwrapper.Launch
param --username
param PlayerName
param --version
param ProjT Launcher
param --gameDir
param /path/to/minecraft
param --assetsDir
param /path/to/assets
param --assetIndex
param 1.20.4
param --uuid
param xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
param --accessToken
param [TOKEN]
param --userType
param msa
windowTitle ProjT Launcher: Instance Name
windowParams 854x480
userName PlayerName
sessionId token:[TOKEN]:[UUID]
launcher standard

Commands

Command	Description
`mod <name>`	Load a mod (tweaker)
`mainClass <class>`	Set main class
`param <value>`	Add launch parameter
`windowTitle <title>`	Set window title
`windowParams <WxH>`	Set window size
`userName <name>`	Player username
`sessionId <id>`	Session identifier
`launcher <type>`	Execute launch
`abort`	Cancel launch

Launcher Types

Type	Use Case
`standard`	All Minecraft versions (recommended)
`legacy`	Minecraft < 1.6 (deprecated)

Differences

Standard: Universal launcher, works with all versions
Legacy: Supports custom window icon/title but only for old versions

Building

LauncherJava is built as part of the main CMake build:

# Built automatically with main project
cmake --build build

# Or build separately
cd launcherjava
./gradlew build

Debugging

Enable Debug Output

debug on
mainClass ...

Attach Profiler

Since LauncherJava waits for the launcher command, you can:

Start LauncherJava
Attach your profiler (VisualVM, JProfiler, etc.)
Send the launcher command