How to modify or use another Java

Launcher assigns identifiers for each Java that it installs (java-runtime-alpha, jre-legacy etc).

You can find (and modify) an identifier in the version file (e.g. versions/1.17/1.17.json) in the field located at $.javaVersion.component.

You may want to modify the installed JRE. By default if any JRE file is modified or tampered with, launcher will download it again. Look at the guide below to learn how to prevent the launcher from doing so.

Guide

  1. We don't know beforehand what Java version is required to launch specific Minecraft version. That's why first you need to install Minecraft version you want to work with.
  2. Open Java settings
  3. Select “Recommended”
  4. Click “Open the folder”
  5. JRE folder should open
  6. Create a file with the name “override” (the file extension is ignored)

All done! Legacy Launcher will no longer modify or update this Java, leaving this to you.

You don't need to modify .version file. Launcher will determine JRE version by querying it directly launching the executable file.