Minecraft: Java Edition Extension

Minecraft: Java Edition game support for the ClassicLauncher.

Implements the full launch pipeline: version JSON resolution, library download with SHA-1 verification, native extraction, asset download, and pre-launch token refresh. Also provides a version API backed by Mojang's public version manifest and a tiled background renderer for the launcher UI.

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Supported game

Game	ID	Launch
Minecraft: Java Edition	minecraft_java	Full pipeline (version JSON -> libs -> natives -> assets -> process)

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Architecture

MinecraftJavaExtension         LauncherExtension entry point
 ├─ MinecraftJava.GAME          Game descriptor (JAR, version filters, API + launch factories)
 │   ├─ MinecraftJavaVersionApi    Fetches Mojang manifest; supplements with local versions
 │   └─ MinecraftJavaLaunchStrategy  10-step prepare + buildCommand
 │       ├─ MinecraftFileDownloader  Atomic download, SHA-1 verify, CDN redirects
 │       ├─ MinecraftLibraryResolver Library rules, platform filtering, classpath + natives
 │       ├─ NativesExtractor         ZIP extraction to per-launch temp directory
 │       ├─ AssetDownloader          Asset index + objects, SHA-1 skip-if-valid
 │       ├─ LogConfigDownloader      Log4j config from version JSON logging section
 │       ├─ VersionJsonParser        Gson deserialiser for mixed-type argument arrays
 │       ├─ ArgumentEvaluator        Rule evaluation, ${variable} substitution
 │       └─ Sha1Verifier             SHA-1 hex comparison, null/missing-safe
 └─ TiledBackgroundRenderer     Tiled background with darkness + saturation (from microsoft-auth)

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Launch pipeline

MinecraftJavaLaunchStrategy.prepare()
  1.  Resolve version ID (profile override, or latest filtered by snapshot/beta/alpha flags)
  2.  Download version JSON from Mojang (or use cached copy)
  3.  Parse version JSON (VersionJsonParser + Gson)
  4.  Download + SHA-1-verify client JAR
  5.  Resolve + download + SHA-1-verify all libraries (platform rules applied)
  6.  Extract native JARs to per-launch temp directory
  7.  Download asset index + all referenced objects
  8.  Download Log4j log config referenced in version JSON
  9.  Ensure valid Minecraft access token (delegates to MinecraftAccount.ensureAccessToken())
  10. Log "All files ready. Launching..."

MinecraftJavaLaunchStrategy.buildCommand()
  ├─ Evaluate JVM arguments (modern rule-gated or legacy hardcoded)
  ├─ Prepend -XstartOnFirstThread on macOS
  ├─ Substitute all ${variable} tokens (auth, classpath, natives, assets, resolution, ...)
  └─ Return [javaExe, ...jvmArgs, mainClass, ...gameArgs]

SHA-1 policy: files are re-downloaded only when both a sha1 and a url are present in the version JSON. Locally-installed versions without a url are never re-downloaded, preserving custom or patched installations.

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Version API: MinecraftJavaVersionApi

Fetches all available versions from Mojang's public manifest at https://launchermeta.mojang.com/mc/game/version_manifest_v2.json.

Also scans the local <gameDataDir>/versions/ directory and appends any locally-installed versions not present in the remote manifest. Local versions require both a <id>.json and <id>.jar file. Remote versions take precedence when IDs conflict.

Manifest type	VersionType.id	VersionType.name
release	release	Release
snapshot	snapshot	Snapshot
old_beta	old_beta	Beta
old_alpha	old_alpha	Alpha

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Required extensions

This extension requires the Microsoft Account extension, which provides:

• MinecraftAccount — the abstract auth contract used by the launch strategy
• TiledBackgroundRenderer — shared UI component for tiled backgrounds
• MicrosoftAccountProvider — the account provider this extension registers its game with

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Build

# Install the Launcher to your local Maven repo first (required — it is a provided dependency)
mvn -f ../Launcher/pom.xml install

# Install microsoft-auth to your local Maven repo (required — it is a provided dependency)
mvn -f ../MicrosoftAccountExtension/pom.xml install

# Build the extension JAR
mvn package

Output: target/classiclauncher-minecraft-java-1.0-SNAPSHOT.jar

Runtime dependencies (Gson, MinecraftAuth, lenni0451.commons) are not bundled. They are declared by the required Microsoft Account extension and downloaded by the Launcher's LibraryManager at extension install time.

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Install

1. Install the Microsoft Account extension first (it is a required dependency).
2. Host the built JAR and manifest.yml at a publicly accessible URL (or a local file:// URL for development).
3. In the Launcher, open Settings -> Extensions, paste the manifest URL, and click Install.
4. Restart the Launcher. Minecraft: Java Edition appears as the default game.

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Code formatting

All code must be formatted before commit. This project uses Spotless with the Eclipse JDT formatter (see eclipse-formatter.xml). IntelliJ IDEA picks up the committed .idea/codeStyles/ automatically.

# Check formatting
mvn spotless:check

# Apply formatting
mvn spotless:apply

It is recommended to install the pre-commit hook so formatting is applied automatically on every git commit:

pip install pre-commit   # or: brew install pre-commit
pre-commit install

See Formatting.md for the full style guide.

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Disclaimer

Minecraft, Minecraft: Java Edition, Mojang, and Microsoft are trademarks or registered trademarks of Microsoft Corporation and/or Mojang Studios. This project is not affiliated with, endorsed by, or sponsored by Microsoft Corporation or Mojang Studios. All trademarks are the property of their respective owners.