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 sixth 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 If the selected version does not exist or the latest installed version is outdated when "Use latest version" is selected in the profile, the launcher will download all Minecraft components and Java binary files.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, the Minecraft launcher supports 41 languages.

Launcher

 * 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 Includes access to outdated development builds and previous releases.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)
 * Edit profile pictures or custom made ones
 * The launcher doesn't store your password locally. Instead of this, it uses a 'refresh token'.
 * The launcher does not support multiple instances, whereas the game 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)
 * 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.
 * Launcher Settings.
 * Switching Accounts or logging off.
 * Changing Skins.
 * Downloads servers on versions.
 * Help button.
 * Tiny Creeper face button (when you click it, it glows but it does nothing).
 * Shows you the launcher version information when you click the launcher version.

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 "Launch Options" then turn the "Historical Versions" on then open your profile or make a new one, next select any of Alpha/Beta versions on the Version box then click "Save" button.

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.

Skins
You can change or add your custom skin within the Minecraft Launcher. To change or add your skin click the "Skins" button, then click the "Browse" button to navigate through your file system for your 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 you have found the skin file, click the "Save" button to confirm your in-game skin.
 * You can reset your skin to the Steve or Alex defaults by clicking "Click to reset".
 * You can choose between player models - either Classic (Steve) or Slim (Alex).

Profiles
When you open the launcher for first time, it creates a profile that selects the latest version of Minecraft - it cannot be removed.

However, you may create a game profile of your own in the launcher. Click on the "Launch Options" button, then click "Add New" to create one. After this, name your profile as you want it. If you leave it, it will be named "Unnamed Configuration". Then, optionally, select a profile picture from the launcher or a custom one. However, it must be a 128×128  picture. Select the version you wish to play, and click "Save" to confirm your profile. If you want to edit a profile, just click it. The same applies if you wish to delete a profile. Simply click the profile you want to delete and then click the "Delete [X]" button to remove it.
 * 2016LauncherProfileSettings.jpg can download server versions by clicking "Download Server".
 * If you want to resize the game, turn "Resolution" on to change the game window size.
 * You can turn "Advanced Settings" on to show more profile options.
 * If you want to change the  file path, turn the on the "Game directory" option. Now, navigate to the folder where you wish to relocate the   folder. You can also open the Minecraft profile by clicking the green arrow-shaped "Go to folder" button.
 * If you turned "Snapshot Versions" on, it creates a "Latest Snapshot" profile automatically.

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

Trivia

 * Before beta testing on the new launcher, it was supposed to select launcher language in left front, after that it was replaced by creeper face button that does nothing, just glows.
 * If you stay long enough on the "Play" button, some mobs will appear on the corner.