Add a build script that picks a JDK Gradle can run on

A plain ./gradlew build fails outright on a machine whose default java
is newer than 23, which is now an ordinary thing for a distro to ship.
Gradle 8.12 cannot run there, and what it says about it is that it
could not create task ':test' because type T is not present - nothing
about Java, and nothing to act on.

build.sh picks a JDK between 17 and 23 and builds with that. JAVA_HOME
wins if it already points at a usable one, otherwise the openjdk paths
are tried in turn; if there is nothing to pick it says so and names the
range, rather than leaving the Gradle error to be decoded. Arguments
pass through, so it is a wrapper around the wrapper and not a second
way to describe the build.

It reports the jar by reading mod_id and mod_version out of
gradle.properties instead of listing build/libs, which keeps old
versions sitting in that directory from being mistaken for the build
that just ran.

The toolchain is untouched by any of this - the mod still compiles
against Java 21, whatever Gradle itself is running on.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -238,11 +238,24 @@ new piece is wanted rather than a repaint of an existing one.
 ## Building
 
 ```sh
-./gradlew build
+./build.sh
 ```
 
-Needs nothing but NeoForge - Truly Modular is a runtime dependency only, since
+The jar lands in `build/libs/`. Needs nothing but NeoForge - Truly Modular is a
-this mod talks to it purely through datapack JSON.
+runtime dependency only, since this mod talks to it purely through datapack
+JSON.
+
+`./gradlew build` does the same thing, and is fine if the java on your path is
+one Gradle can run on. Gradle 8.12 cannot run on a JDK newer than 23, which a
+distro's default java may well be by now, and the failure when it is says
+`Could not create task ':test'` and `Type T not present` without ever mentioning
+Java. `build.sh` exists to pick a JDK between 17 and 23 - honouring `JAVA_HOME`
+if it already points at one - and to say what is wrong in as many words when
+there is none to pick. Extra arguments are passed straight through, so
+`./build.sh clean build` and `./build.sh --info` work.
+
+None of this touches what the mod is compiled against: the toolchain in
+`build.gradle` is Java 21 either way, and Gradle provisions it if it is missing.
 
 ## Regenerating materials
 

build.sh

@@ -0,0 +1,56 @@
+#!/usr/bin/env bash
+# Builds the mod jar into build/libs/.
+#
+# Gradle 8.12 cannot run on a JDK newer than 23, and a distro's default java is
+# often newer than that, so this picks a JDK Gradle can run on rather than
+# leaving you to read the "Type T not present" failure that comes of not doing
+# so. The build itself still compiles against Java 21 either way - that is the
+# toolchain in build.gradle, and Gradle provisions it if it is missing.
+set -euo pipefail
+
+cd "$(dirname "$0")"
+
+# Major version of the java at $1/bin/java, or nothing if it will not run.
+java_major() {
+    local out
+    out=$("$1/bin/java" -version 2>&1) || return 0
+    sed -n 's/^[^"]*"\([0-9]*\).*/\1/p' <<<"$out" | head -1
+}
+
+usable() {
+    local major
+    [ -x "$1/bin/java" ] || return 1
+    major=$(java_major "$1")
+    [ -n "$major" ] && [ "$major" -ge 17 ] && [ "$major" -le 23 ]
+}
+
+if [ -z "${JAVA_HOME:-}" ] || ! usable "$JAVA_HOME"; then
+    for candidate in /usr/lib/jvm/java-{21,17}-openjdk /usr/lib/jvm/java-{21,17}-openjdk-amd64; do
+        if usable "$candidate"; then
+            export JAVA_HOME="$candidate"
+            break
+        fi
+    done
+fi
+
+if [ -z "${JAVA_HOME:-}" ] || ! usable "$JAVA_HOME"; then
+    echo "build.sh: no JDK between 17 and 23 found - Gradle 8.12 cannot run on anything newer." >&2
+    echo "Install one (e.g. pacman -S jdk17-openjdk) or point JAVA_HOME at it." >&2
+    exit 1
+fi
+
+echo "build.sh: using JDK $(java_major "$JAVA_HOME") at $JAVA_HOME"
+./gradlew build "$@"
+
+# Name the jar this run produced. Old versions linger in build/libs/, so listing
+# the whole directory would be ambiguous about which one is current.
+prop() { sed -n "s/^$1=//p" gradle.properties; }
+jar="build/libs/$(prop mod_id)-$(prop mod_version).jar"
+
+echo
+if [ -f "$jar" ]; then
+    echo "Built: $jar"
+else
+    echo "build.sh: expected $jar, but it is not there." >&2
+    exit 1
+fi