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.

The previous chapter explained the general manifest structure and its fields. This chapter explains how to assemble launch arguments into the final command.

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Overview

Building launch arguments has four main steps:

1. Build JVM arguments
2. Build game arguments
3. Build the classpath
4. Replace placeholders

Each step has its own logic, explained below.

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1. Build JVM arguments

Process

• Read each argument from the manifest's arguments.jvm array.
• Remove spaces from each argument with replace(" ", ""), then append it to jvm_args.

Mermaid flowchart

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2. Build game arguments

Process

• Read arguments from the manifest's arguments.game array.
• Remove spaces and append them to jvm_args as well. JVM and game arguments share this list and are ultimately passed together to the Java command.

Mermaid flowchart

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3. Build the classpath

Process

• Create an empty class_paths list.
• Iterate through the manifest's libraries array. For each library, convert its name field into the corresponding file path.
• Add the converted path only if it is not already in class_paths, avoiding duplicates.
• Finally, append the path of the game's main JAR, {version}.jar, to the classpath.

Maven coordinate to path rules

The maven_name_to_path conversion algorithm:

1. Extract the file extension
   • If the coordinate contains @, use the part after it as the extension, such as @zip, and remove the @ suffix from the coordinate.
   • Without @, use "jar" as the default extension.
2. Parse coordinate segments
   • Split the remaining string on :.
   • Valid forms contain three segments (groupId:artifactId:version) or four segments (groupId:artifactId:version:classifier).
3. Generate the path
   • Four segments → groupId-path/artifactId/version/artifactId-version-classifier.suffix
   • Three segments → groupId-path/artifactId/version/artifactId-version.suffix
   • Any other form → return an empty string.
4. Convert the group path
   • Replace dots . in groupId with slashes / to produce the directory hierarchy.

Examples:

• "org.apache.commons:commons-lang3:3.12.0" → "org/apache/commons/commons-lang3/commons-lang3-3.12.0.jar"
• "com.example:my-lib:2.1.0:beta@zip" → "com/example/my-lib/2.1.0/my-lib-2.1.0-beta.zip"

⚠️ Path recommendation: Prefer absolute paths. If you use relative paths, make sure the working directory is correct.

Mermaid flowchart

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4. Join the strings

Join JVM arguments

Join the jvm_args list with spaces:

jvm_arg = " ".join(jvm_args)

Join the classpath

Join class_paths with the platform separator, then append the game's main JAR:

delimiter = ";" if os.name == "nt" else ":"   # Windows uses semicolons; other platforms use colons
class_path = delimiter.join(class_paths)
# Append the game main JAR (assuming its path is version_jar_path)
class_path += delimiter + version_jar_path

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5. Replace placeholders

After joining the arguments, replace JVM placeholders with their actual values. Common placeholders are listed below.

Placeholder	Meaning
${library_directory}	Actual path to .minecraft/libraries
${assets_root}	Actual path to .minecraft/assets
${assets_index_name}	Asset-index value, such as 1.16
${natives_directory}	Local native-library directory, commonly versions/{version}/natives
${game_directory}	Game runtime directory; versions/{version} with isolation, otherwise versions
${launcher_name}	Launcher name; originally reserved by the official launcher and has no practical effect
${launcher_version}	Launcher version; same as above
${version_type}	Version type, the manifest's type
${auth_player_name}	Player name; only English letters, digits, and underscores are allowed
${user_type}	Account type: offline Legacy or Microsoft sign-in
${auth_uuid}	Account UUID; the form without hyphens is recommended
${auth_access_token}	Sign-in token; for offline mode, any value such as "None" may be used
${version_name}	Version name, which is also the folder name
${classpath}	Special placeholder; replace it with the classpath plus mainClass

${classpath} replacement example

jvm_arg = jvm_arg.replace("${classpath}", f"{class_path} {manifest['mainClass']}")

💡 Tip: Quoting each argument can prevent problems with spaces in paths. Do not quote the classpath and main class together.

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6. Add the Java executable and heap-memory options

The final launch command also needs:

• The complete path to the Java executable, such as /path/to/java
• Heap-memory settings, -Xms and -Xmx

Example:

/path/to/java -Xms2G -Xmx2G ... # followed by jvm_arg

📝 Tip: Launch arguments are usually long. Consider saving the complete command in a script, such as .sh or .bat, before running it.

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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