Minecraft Launcher

The Minecraft launcher is the login and downloader front-end to the stand-alone client. It is responsible for downloading the main Java packages, including minecraft.jar, which holds the code and resources of the game such as textures, and LWJGL. It also acts somewhat like a basic and easily bypassable copy-restriction by forcing the user to login to a paid account when running for the first time. It is possible to use the launcher to run versions of Minecraft from nearly any era, however some versions are still not included on the launcher, such as Indev, Minecraft 2.0 and any of the Beta 1.9 pre-releases. The current launcher is the fourth launcher for Minecraft, and the first that was built to automatically update itself.

Behavior


Every time the launcher is executed, it checks a specific directory where Minecraft stores its files called .minecraft. It checks a file called "launcher_profiles.json" for login credentials. If the file does not exist, is corrupt, or no user is logged in, the user must login with a valid premium account, otherwise the user will be directed to play the Demo version or purchase the game.

When logging in, the launcher will first try to verify the login. If a valid "launcher_profiles.json" file exists but there is no Internet connection, the launcher will offer a "Play offline" mode if a player was logged in when it was last closed. If the connection is successful, the launcher will then, depending on profile options, compare the latest version in the latest "versions" directory to the latest version reported by the server. If the selected version does not exist or the latest installed version is outdated when "Use latest version" is selected in the profile, the launcher will download all Minecraft components and Java binary files. It will also synchronize everything in the Assets folder (see Resource pack).

Finally, upon a successful update or installation, the launcher will execute the Minecraft jar (via "javaw.exe", Oracle's Java Virtual Machine executable.) It is possible however for there to be missing ".jar" files, in which case the launcher will attempt to replace the missing files automatically but will stop if there is no connection.

Currently, Minecraft launcher supports only English. Users cannot select their preferred language.

Features

 * Manage multiple profiles (stored in "launcher_profiles.json"). Each profile can control:
 * Game version. Includes access to outdated development builds and previous releases. You can also add older/custom versions manually . See this video for instructions.
 * Mojang account used to play the game
 * Command line arguments for Java (such as heap size)
 * Game Directory (where the game files are saved on disk)
 * And so on.
 * The launcher doesn't store your password locally. Instead of this, it uses a 'refresh token'.
 * Automatic updates for launcher.
 * Ability to update libraries, such as LWJGL.
 * Offline mode. (The first time launch requires an Internet connection to download required resources)
 * Currently only available in English.
 * Crashes can be reported to Mojang, through Hopper.
 * Crashes are indexed through a database for information on how to fix the issue.

Time Machine
The Minecraft launcher has the ability to play old Alpha and Beta versions of the game. Currently, snapshots, Alpha, Beta, and all Release versions are available to play. Four Pre-classic versions, four Classic versions, and an Infdev version are already added under the names "old-alpha version". However, Indev is still missing.

Progress *note: as of 1.10.2

To enable the old versions either click the "Edit Profile" button or, preferably, create a new one. Under the "Version Selection" section choose to allow use of Alpha and/or Beta versions by ticking the appropriate boxes. Next, select the desired version from the drop-down menu and save the profile.

However, these versions are very out of date and may be unstable. Any bugs, glitches, missing features, or crashes that result from instability will never be resolved as only later versions are supported. Mojang strongly recommends that you play them in separate directories than your other saves to avoid corruption.

Command-line usage
MinecraftDev.jar (all systems):

As of the launcher released with Minecraft 1.6, there are other commands for the launcher. These are shown if you do