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# LauncherJava `launcherjava/`
> **Location**: `launcherjava/`
> **Language**: Java
> **Purpose**: Minecraft launching component
> **Latest Version**: 0.0.5-1
---
## 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.
---
## How It Works
```
┌─────────────┐ ┌──────────────┐ ┌─────────────┐
│ ProjT │────▶│ LauncherJava │────▶│ Minecraft │
│ Launcher │ │ (Java) │ │ JVM │
│ (C++) │ │ │ │ │
└─────────────┘ └──────────────┘ └─────────────┘
│ │
│ stdin │ exec
│ (script) │ (launch)
▼ ▼
```
### Launch Sequence
1. C++ launcher sends launch script via stdin
2. LauncherJava parses the script
3. Waits for `launcher` command
4. Executes Minecraft with configured parameters
---
## Launch Script Format
The launcher communicates via a text-based protocol:
```text
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:
```bash
# Built automatically with main project
cmake --build build
# Or build separately
cd launcherjava
./gradlew build
```
---
## Debugging
### Enable Debug Output
```text
debug on
mainClass ...
```
### Attach Profiler
Since LauncherJava waits for the `launcher` command, you can:
1. Start LauncherJava
2. Attach your profiler (VisualVM, JProfiler, etc.)
3. Send the `launcher` command
---
## License
Available under **GPL-3.0** (with classpath exception), sublicensed from its original **Apache-2.0** codebase.
---
## Related Documentation
- [Program Info](./program_info.md) — Application identity
- [JavaCheck](./javacheck.md) — Java detection
|
