Client.jar

Not to be confused with the launcher marked for "Linux / Other", minecraft.jar is the main code source for Minecraft. It is located in the ../.minecraft/bin directory, or the ../minecraft/bin directory in OS X. It can be opened using an archive opening program (e.g. WinZip, WinRAR, 7-Zip, etc.).

Files in minecraft.jar
In minecraft.jar many files can be found, they can be categorized like this:
 * Sprite files: Files that contain sprites (2D textures) used by Minecraft, all in .png format.
 * Class files: Compiled Java code that Minecraft uses for program logic. These files are named like 'aka' or 'gb' because they have been run through an obfuscation tool to hide the names of things. However, they can still be disassembled to understand their functionality.
 * Language files: Language files are used to change the messages in the game (achievements, tooltips, menu). It does not affect in-game chat.
 * Text files: Files that contain easy to edit text, such as splashes and texture pack descriptions, another text file is achievements/Map.txt that contain code for the creation of the achievements menu background.
 * The META-INF directory

The META-INF directory
The meta-information directory contains information about the game files, rather than information explicitly required by Java to run the program.

This directory is notable because it must be deleted or modified, removing or validating invalid checksums, to use mods. The META-INF folder contains a digital certificate (MOJANG_C.DSA), as well as MANIFEST.MF and MOJANG_C.SF which contain checksums of all the Java files. The certificate is issued to prove the code is safe, and is what lets the web applet version of Minecraft run without Java safety dialogs. When the files are modified, the certificate is invalidated (the checksum changes), which will stop it from running. Removing META_INF removes the certificate, so it will run again, but if you try to use the browser version of Minecraft, you might get warning dialogs.

Deleting the META-INF directory has no important side effects; however, it prevents running minecraft.jar directly (from a terminal or with a script), and could theoretically open the door for "viruses" in the form of poorly or maliciously written modifications. Be careful when modding your game. See the Mods page for details.

Removing this folder in order to successfully install mods is not required in development versions.

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