Launching the game

Once the launcher has authenticated the user and downloaded the game, its next task is to start it. For this, it requires a few informations found in the version manifest.

Java version

Until the Electron-based launcher, Minecraft stayed on the version of Java that had the most userbase, jumping from Java 6 in the early days, to Java 7 then 8.

Once the Electron-based launcher released, the version manifest embedded a javaVersion key that let it downloads the correct JVM for any given Minecraft version. This has the advantages that it does not expects anything about the user's computer (Even if the player does not have Java installed, the game will run).

This JVM is downloaded to $MINECRAFT_FOLDER/runtime/[javaVersion.component]/[OS]

Assets

The assets are expected to be downloaded according to the Assets section of the game files page, but also expects an $MINECRAFT_FOLDER/assets/indexes folder containing the JSON of the assets indexes available in the version manifest.

Arguments

This section is a WIP as not all arguments explanation are available. If you know about them please contribute!

Running Minecraft requires several arguments.

On newer releases you can find them in the manifest under the arguments.jvm and arguments.game keys. Those are array of either a string corresponding to the argument or an object containing the value and the rules to check before appending the given argument. In case of an object, the value key can be either a string or an array of string.

The first version using this key is 1.12.2. For those before this one, you can check the minecraftArguments key. It's a string referencing all the arguments passed to the game. With the exception of the log4j config, no JVM arguments are passed to the game.

Here's a list of them with their corresponding description:

JVM Arguments

Argument	Description	Usage
-Djava.library.path=${natives_directory}	The natives folder path	Load the natives
-Dminecraft.launcher.brand=${launcher_name}: 309	The launcher name	@TODO: find outNeed to dig deeper
-Dminecraft.launcher.version=${launcher_version}: 309	The launcher version	@TODO: find outNeed to dig deeper
-cp ${classpath}	List of all the absolute path for the jars to load (libraries + client.jar) separated by :
-Dos.name=Windows 10 -Dos.version=10.0	Rule to be added only on Windows 10	@TODO: find outNeed to dig deeper
-XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump	Apparently used to make the game run smoothly on Intel Graphics. No real information on thisNeed to dig deeper
-XstartOnFirstThread	Rule to be added only on OSX	Makes the JVM starts with thread 0, required on OSX
-Xss1M	-	Sets the JVM stack size to 1M

Game Arguments

Argument	Description	Usage
--clientId ${clientid}	@TODO: find outNeed to dig deeper	@TODO: find outNeed to dig deeper
--xuid ${auth_xuid}	@TODO: find outNeed to dig deeper	@TODO: find outNeed to dig deeper
--username ${auth_player_name}	Sets the player username	Used when logging in offline-mode, @TODO: find out if its useful in online-mode
--version ${version_name}	Sets the version name of the game	@TODO: find outNeed to dig deeper
--gameDir ${game_directory}	The instance directory	The game will locate every runtime files there (e.g. options.txt, world saves, ...)
--assetsDir ${assets_root}	Should contain {indexes/, objects/, virtual/}	The asset directory used by the game
--assetsIndex ${assets_index_name}	Should contain the asset indexes JSON file for any installed version	Let the games locate the assets based on its internal name in the assets/objects
--uuid ${auth_uuid}	The player UUID	@TODO: find outNeed to dig deeper
--accessToken ${auth_access_token}	The Mojang Access Token or the final token in the Microsoft authentication scheme	Authenticate with the game
--userType ${user_type}	@TODO: find outNeed to dig deeper	@TODO: find outNeed to dig deeper
--versionType ${version_type}	Not sure, some launcher sets it to "release" / "snapshot" (MultiMC), other sets it as their brand name (SKCraft)
--demo	Not sure if it's still available	Run the game in demo-mode (When the account does not own the game)
--width ${resolution_width}	Rule adding it only for "feature: has_custom_resolution"Need to dig deeper	Sets the game's width
--height ${resolution_height}	Rule adding it only for "feature: has_custom_resolution"Need to dig deeper	Sets the game's height

Legacy Arguments

Argument	Description	Usage
--tweakClass net.minecraft.launchwrapper.AlphaVanillaTweaker	The class comes from net.minecraft.launchwrapper library	Wrap the starting applet into a frame and setups up the params correctly
--session ${auth_session}	No longer used ? Need to check how was implemented the newer auth in older versions	The session ID used to authenticate with Mojang while connecting to servers
--userProperties ${user_properties}	@TODO: find outNeed to dig deeper	@TODO: find outNeed to dig deeper
--workDir ${game_directory}	Only in a few versions between 13w16a and 13w23b (including release 1.5.2)	Same as newer --gameDir
--gameDir ${game_directory}	-	Same as newer --gameDir
--versionType ${version_type}	-	Same as newer --versionType
--userType ${user_type}	-	Same as newer --userType
--assetIndex ${assets_index_name}	-	Same as newer --assetIndex
--version ${version_name}	-	Same as newer --version
--username ${auth_player_name}	-	Same as newer --username
--uuid ${uuid}	-	Same as newer --uuid
--accessToken ${accessToken}	-	Same as newer --accessToken
--assetIndex ${assets_index_name}	-	Same as newer --assetIndex
--assetsDir ${assets_root}	-	Same as newer --assetsDir
--assetsDir ${game_assets}	Used until 13w48b (Last release = 1.7.2)	Probably the /assets/virtual/legacy for pre-1.6 assetsNeed to check

Main class

The main class is also referenced in the version manifest.

Classname	Description
net.minecraft.client.main.Main	Used by most versions
net.minecraft.launchwrapper.Launch	Used until 1.5.2, except 13w16b that for some reasons uses net.minecraft.client.main.Main
com.mojang.minecraft.RubyDung	Used for really early versions
com.mojang.rubydung.RubyDung	Used for really early versions

It can be something else for custom instances, e.g. Forge instances.

Building the final command

Once we have everything ready we can build the final command to start the game.

%JAVA_PATH%/bin/java [JVM Args] [main_class] [Game / Legacy Args]

A frame with the game will popup and the logs will be available on standard output as expected