Java In Detail

Java in detail

By default ([java] version = "auto") the server runs on the Java version that its Minecraft version asks for. Craft Conductor looks for exactly that version, in this order:

1. that version in the server's [java.versions],
2. the shared Java folder (<home>/.craft-conductor/java/, one copy per version for every server on this computer; /data/.craft-conductor/java/ in Docker),
3. [java] default (your system java), if it's exactly that version,
4. Java already installed on this computer: JAVA_HOME and the usual install folders (Eclipse Adoptium, Microsoft, Oracle, Zulu, Corretto, BellSoft on Windows; /Library/Java/JavaVirtualMachines and Homebrew on macOS; /usr/lib/jvm, /usr/java, /opt/java on Linux). Each is run once with -XshowSettings:properties -version (nothing else) to read its version and architecture, and the answer is remembered by the file's size and date. A build for another architecture (an x64 Java on an Apple silicon Mac) is skipped. The one picked is written into [java.versions], so the server's choice stays explicit,
5. a download of Eclipse Temurin into the shared folder, checksum-checked (turn this off with auto_install = false).

A newer major version is never picked by itself (some loaders and older mods break on one): the Java page offers it, or set [java] version = 25.

Command	What it does
craft-conductor java list	the shared Java (and which servers use it), Java found on this computer, and which one the server uses
craft-conductor java install 21 17	download Temurin runtimes into the shared folder
craft-conductor java update	bring the shared Java to its newest patch releases
craft-conductor java use 21	always run on Java 21 (must be at least what Minecraft needs)
craft-conductor java use auto	go back to following the Minecraft version
craft-conductor java remove 17	delete a shared version (refused while a server uses it)

How the server is started

java -Xms… -Xmx… [Aikar's flags] -Dfile.encoding=UTF-8 -Dstdout.encoding=UTF-8 -Dstderr.encoding=UTF-8 -Dstdin.encoding=UTF-8 -Dsun.stdout.encoding=UTF-8 -Dsun.stderr.encoding=UTF-8 [jvm_args] … and the server's own launch arguments.

The -D…encoding settings make the server's console speak UTF-8 both ways, on every Java, so a command or a Broadcast in any language reaches the server as typed, and the Console and logs/latest.log show what it says (otherwise, on Windows, Java 17 and older and Paper's console write in the computer's own code page). Anything in [server] jvm_args comes after them, so a -Dfile.encoding of your own still wins.

The shared folder

.craft-conductor/java/
  21/jdk-21.0.4+7/        a release (craft-conductor-java.json says what it is)
  21/jdk-21.0.5+11/       a newer patch release, installed next to it
  servers/                which server last started on which Java (one small file each)
  checked.json            what each Java found on the computer turned out to be

• An update installs the new release next to the old one; servers move to it at their next start.
• Each server start leaves its process id in <release>/.in-use/. An older release is deleted only when no process listed there is running (and, on Windows, when no program has its files open), and not within five minutes of being replaced (a server may be starting on it that moment).
• Deleting a server never deletes a Java another server uses. A shared version that only the deleted server used is removed with it.
• Backups, snapshots and server exports never include Java.

Servers from before 0.23 kept their own copy in <server>/.craft-conductor/java/. Nothing moves it: it's no longer used, and can be deleted by hand.

When a Minecraft upgrade needs a newer Java, it's found or downloaded during staging, before the server is stopped.

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