Minecraft Launcher

The Minecraft launcher is the login and downloader front-end to the stand-alone Minecraft: Java Edition client. It is available for Windows, macOS and Linux.

The launcher 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 copy-restriction by forcing the user to login to a paid account when running for the first time.

It is possible for the launcher to run all release versions of the Java Edition of Minecraft, as well as most Beta versions, and a small fraction of Classic, Indev, Infdev and Alpha versions.

Not every version of the game is included (to see a list of all versions not included in the launcher, see missing launcher versions). The current launcher is the seventh launcher for Minecraft: Java Edition, and the third that was built to automatically update itself.

Launch
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 If the selected version does not exist or the latest installed version is outdated when "Use latest version" is selected in the profile, the launcher will download all Minecraft components and Java binary files.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.

Launcher

 * Manage multiple profiles (stored in " launcher_profiles.json "). Each profile can control:
 * Game version. Includes access to outdated development builds and previous releases. Older/custom versions can also be added manually.
 * 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)
 * Edit profile pictures or custom made ones
 * The launcher doesn't store the password locally. Instead of this, it uses a 'refresh token'
 * The launcher supports multiple instances (as the game also does)
 * 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)
 * 39 languages available (December 2019)
 * Crashes can be reported to Mojang, through Hopper
 * Crashes are indexed through a database for information on how to fix the issue
 * Launcher Settings
 * Switching accounts or logging off
 * Changing skins
 * Downloads servers on versions
 * Help button
 * Shows the player the launcher version information when the player clicks the launcher version

Backward compatibility
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 one Infdev version are available under the name "old_alpha". No Indev versions are currently available from the launcher.

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 backing up any save data and playing through a separate directory in the case of corruption.

To enable the old versions, click the "Installations", turn the "Historical Versions", make a new profile, select any of the versions in the "Version" box, then click "Save".

Skins


The player can change or add their custom skin within the Minecraft launcher. To change or add their skin, the player must click the "Skins" button, then click the "Browse" button to navigate through their file system for their skin file. The file must have a resolution of 64×64 in order for the launcher to accept it as a skin file. Additionally, the skin file must be in a  format. After the player has found the skin file, they must click the "Save" button to confirm their in-game skin.


 * The player can reset their skin to the Steve or Alex defaults by clicking "Click to reset".
 * The player can choose between player models - either Classic (Steve) or Slim (Alex).

Installations
When the player opens the launcher for the first time, it creates a profile that selects the latest version of Minecraft - it cannot be removed.

However, the player may create a game profile of their own in the launcher. In order to do so, they must click on the "Installations" button, then click "New..." to create one. After this, the player can name their profile as they want it. If the player leaves it, it will be named " ". Then, optionally, the player can select a profile picture from the launcher or a custom one. However, it must be a 128×128  picture. After, the player can select the version they wish to play, and click "Create" to confirm their profile. If the player wants to edit a profile, they can just click it. The same applies if the player wishes to delete a profile. Simply click the three dots button and press Delete. A popup appears; press Delete.
 * 2019LauncherProfileSettings.png The player can download server versions by clicking "Server".
 * If the player wants to resize the game, they can turn "Resolution" on to change the game window size.
 * The player can turn "More options" on to show more profile options.
 * If the player wants to change the  file path, they can turn on the "Game directory" option. Now, navigate to the folder where the player wishes to relocate the   folder. The player can also open the Minecraft profile by clicking the green arrow-shaped "Go to folder" button.
 * If the player turned "Snapshots" check on, it creates a "Latest snapshot" profile automatically.
 * The player can manually set a logging configuration (see the Debugging article on wiki.vg for more info)
 * The player can also duplicate a profile by clicking the stacked 3 dots button while hovering a profile.

Command Line usage
As of 2.1.1432, the launcher has 9 command-line options available:
 * - This option deletes the  and   folders, including their contents, from the working directory.
 * - This option, followed by a path (relative or absolute), will change the location (and/or name) of the  folder.
 * - This option, followed by a path (relative or absolute), will change the location (and/or name) of the  folder.
 * - This option adds support for Windows 10. Since the launcher now supports Windows 10 fully, this option is unnecessary and seems to have no effect.
 * - This option, followed by a path (relative or absolute), will change the location (and/or name) of the  folder.
 * - This option adds support for Windows 10. Since the launcher now supports Windows 10 fully, this option is unnecessary and seems to have no effect.
 * - This option adds support for Windows 10. Since the launcher now supports Windows 10 fully, this option is unnecessary and seems to have no effect.

The  option can be used to create a portable installation of the game. For example,  (or  ) can be used to make the launcher use the current directory (no matter what the current folder is called).

Trivia

 * In the top left corner of the 2016–19 launcher, a barely visible button could be found which randomly shows either a creeper face or a shrug kaomoji ¯\_(ツ)_/¯. It glowed white when clicked, but had no use at all. This was removed in the 2019 redesign of the launcher.
 * The creeper face as well as shrug were used for the language selection menu during the beta phase of the launcher, but when a globe was added instead both were simply moved downwards to be kept as an Easter egg.
 * Hovering long enough over the "Play" button in the 2016–19 launcher will cause random mobs to appear on the right side of the window.