A note to readers

This tutorial does not provide complete source code. It presents the ideas and key code fragments only. For the rationale, see the tutorial notes.

A Minecraft launcher needs to read extensive configuration from a version manifest (version.json). This chapter examines the core manifest structure and identifies the fields that matter during launch.

---

Manifest structure overview

Modern Minecraft version manifests are stored as JSON and normally contain the following key parts (nonessential fields are omitted):

{
  "arguments": {
    "game": [
      "--username",
      "${auth_player_name}",
      "--version",
      "${version_name}",
      "--gameDir",
      "${game_directory}",
      "--assetsDir",
      "${assets_root}",
      "--assetIndex",
      "${assets_index_name}",
      "--uuid",
      "${auth_uuid}",
      "--accessToken",
      "${auth_access_token}",
      "--clientId",
      "${clientid}",
      "--xuid",
      "${auth_xuid}",
      "--versionType",
      "${version_type}"
      // ... more game arguments
    ],
    "jvm": [
      "--sun-misc-unsafe-memory-access=allow",
      "--enable-native-access=ALL-UNNAMED",
      "-Djava.library.path=${natives_directory}/java",
      "-Djna.tmpdir=${natives_directory}/jna",
      "-Dorg.lwjgl.system.SharedLibraryExtractPath=${natives_directory}/lwjgl",
      "-Dio.netty.native.workdir=${natives_directory}/netty",
      "-Dminecraft.launcher.brand=${launcher_name}",
      "-Dminecraft.launcher.version=${launcher_version}",
      "-cp",
      "${classpath}"
      // ... more JVM arguments
    ]
  },
  "assetIndex": {
    "id": "32",
    "sha1": "49da57a9512de46382d2fe4b68af047fea7a16f9",
    "size": 586366,
    "totalSize": 477011740,
    "url": "https://piston-meta.mojang.com/v1/packages/49da57a9512de46382d2fe4b68af047fea7a16f9/32.json"
  },
  "downloads": {
    "client": {
      "sha1": "2dc72797acbc1b63fc16a11c4ac393605f453754",
      "size": 39193383,
      "url": "https://piston-data.mojang.com/v1/objects/2dc72797acbc1b63fc16a11c4ac393605f453754/client.jar"
    }
  },
  "id": "26.2",
  "javaVersion": {
    "component": "java-runtime-epsilon",
    "majorVersion": 25
  },
  "libraries": [
    {
      "downloads": {
        "artifact": {
          "path": "at/yawk/lz4/lz4-java/1.10.1/lz4-java-1.10.1.jar",
          "sha1": "f541d7f910fe3d76f38f799c507c48cc81b12ecb",
          "size": 910232,
          "url": "https://libraries.minecraft.net/at/yawk/lz4/lz4-java/1.10.1/lz4-java-1.10.1.jar"
        }
      },
      "name": "at.yawk.lz4:lz4-java:1.10.1"
    }
    // ... more dependency libraries
  ],
  "mainClass": "net.minecraft.client.main.Main",
  "releaseTime": "2026-06-16T12:03:33+00:00",
  "time": "2026-06-16T12:03:33+00:00",
  "type": "release"
}

📝 Note: The example uses JSON5 syntax so that it can contain comments. Actual manifests are plain JSON and contain no comments.

---

Key fields explained

A launcher only needs to focus on the following fields; the remaining fields, such as releaseTime, do not affect launch logic.

Field path	Type	Description
arguments.game	string[]	Game-engine launch arguments such as the player name and game directory. They contain placeholders.
arguments.jvm	string[]	Java virtual-machine arguments such as library paths and system properties. They also contain placeholders.
assetIndex.id	string	Asset-index identifier, such as "32", used to locate the associated asset-index JSON file.
assetIndex.sha1	string	SHA-1 hash of the asset-index file, used to verify file integrity.
downloads.client	object	Download information for the main game JAR, including sha1, size, and url.
id	string	Version name, such as "26.2". Many launchers display it or use it in directory names.
javaVersion.majorVersion	number	Recommended Java major version, such as 25. An older version may not start the game.
libraries	object[]	List of dependency libraries. Each library includes a name Maven coordinate and download information.
mainClass	string	Game entry-point class, such as net.minecraft.client.main.Main, used as the Java command class name.
type	string	Version type, such as "release" or "snapshot".

---

A special note about dependency libraries

A Maven library's sha1 location is not fixed

The SHA-1 hash of a library can appear in different places in manifests from different versions. Two common forms are shown below.

Form one (nested under downloads.artifact):

{
  "libraries": [
    {
      "downloads": {
        "artifact": {
          "sha1": "f541d7f910fe3d76f38f799c507c48cc81b12ecb"
        }
      }
    }
  ]
}

Form two (directly on the library object):

{
  "libraries": [
    {
      "sha1": "ada2141c0cc52ee8f5c48cd5fa4ce0e794f22236"
    }
  ]
}

💡 Recommendation: Support both structures when parsing. Look for downloads.artifact.sha1 first, then fall back to the top-level sha1 when it is absent.

Changelog

9/21/26, 5:27 AM

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• 6ead8-docs: reorganize and localize documentationon 9/21/26

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Contributors: Wuchang325