diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index cdd6f37..2bc93d4 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -6,11 +6,9 @@ on:
 jobs:
   ci:
     runs-on: ubuntu-latest
-    env:
-      GRADLE_OPTS: -Dorg.gradle.java.installations.fromEnv=JAVA_HOME_11_x64,JAVA_HOME_17_x64
     steps:
       - name: Checkout repo
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
         with:
           fetch-depth: 0
 
@@ -21,6 +19,7 @@ jobs:
           java-version: |
             11
             17
+            21
 
       - name: Validate Gradle Wrapper
         uses: gradle/wrapper-validation-action@v1
diff --git a/.github/workflows/release.yaml b/.github/workflows/release.yaml
index 7ea073e..998bd22 100644
--- a/.github/workflows/release.yaml
+++ b/.github/workflows/release.yaml
@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout repo
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
         with:
           fetch-depth: 0
       - name: Setup Java
@@ -17,6 +17,7 @@ jobs:
           java-version: |
             11
             17
+            21
 
       - name: Setup Gradle
         uses: gradle/gradle-build-action@v2
diff --git a/build.gradle.kts b/build.gradle.kts
index 5cacf62..e05f14f 100644
--- a/build.gradle.kts
+++ b/build.gradle.kts
@@ -29,7 +29,7 @@ dependencies {
 
   // testing
   compatTestImplementation(gradleTestKit())
-  compatTestImplementation("org.spockframework:spock-core:2.0-groovy-3.0")
+  compatTestImplementation("org.spockframework:spock-core:2.3-groovy-3.0")
 }
 
 tasks.named<Jar>("jar") {
@@ -59,6 +59,14 @@ stutter {
       compatibleRange("7.3")
     }
   }
+  val java21 by matrices.creating {
+    javaToolchain {
+      languageVersion.set(JavaLanguageVersion.of(21))
+    }
+    gradleVersions {
+      compatibleRange("8.4")
+    }
+  }
 }
 
 tasks.named("check") {
diff --git a/gradle.lockfile b/gradle.lockfile
index 0ba5df9..cb65017 100644
--- a/gradle.lockfile
+++ b/gradle.lockfile
@@ -1,25 +1,17 @@
 # This is a Gradle generated file for dependency locking.
 # Manual edits can break the build and are not advised.
 # This file is expected to be part of source control.
-cglib:cglib-nodep:3.3.0=compatTestCompileClasspath,compatTestRuntimeClasspath
 com.googlecode.javaewah:JavaEWAH:1.1.13=compatTestCompileClasspath,compatTestRuntimeClasspath,compileClasspath,runtimeClasspath,testCompileClasspath,testRuntimeClasspath
-net.bytebuddy:byte-buddy:1.11.0=compatTestCompileClasspath,compatTestRuntimeClasspath
 org.ajoberstar.grgit:grgit-core:5.2.0=compatTestCompileClasspath,compatTestRuntimeClasspath,compileClasspath,runtimeClasspath,testCompileClasspath,testRuntimeClasspath
 org.ajoberstar.grgit:grgit-gradle:5.2.0=compileClasspath,runtimeClasspath,testCompileClasspath,testRuntimeClasspath
-org.apiguardian:apiguardian-api:1.1.0=compatTestCompileClasspath,compatTestRuntimeClasspath
-org.assertj:assertj-core:3.16.1=compatTestCompileClasspath,compatTestRuntimeClasspath
+org.apiguardian:apiguardian-api:1.1.2=compatTestCompileClasspath
 org.codehaus.groovy:groovy:3.0.17=compatTestCompileClasspath,compatTestRuntimeClasspath,compileClasspath,runtimeClasspath,testCompileClasspath,testRuntimeClasspath
 org.eclipse.jgit:org.eclipse.jgit:6.5.0.202303070854-r=compatTestCompileClasspath,compatTestRuntimeClasspath,compileClasspath,runtimeClasspath,testCompileClasspath,testRuntimeClasspath
 org.hamcrest:hamcrest:2.2=compatTestCompileClasspath,compatTestRuntimeClasspath
-org.jetbrains:annotations:20.1.0=compatTestCompileClasspath,compatTestRuntimeClasspath
-org.junit.platform:junit-platform-commons:1.7.2=compatTestCompileClasspath,compatTestRuntimeClasspath
-org.junit.platform:junit-platform-engine:1.7.2=compatTestCompileClasspath,compatTestRuntimeClasspath
-org.junit.platform:junit-platform-launcher:1.7.2=compatTestCompileClasspath,compatTestRuntimeClasspath
-org.junit.platform:junit-platform-testkit:1.7.2=compatTestCompileClasspath,compatTestRuntimeClasspath
-org.junit:junit-bom:5.7.2=compatTestCompileClasspath,compatTestRuntimeClasspath
-org.objenesis:objenesis:3.2=compatTestCompileClasspath,compatTestRuntimeClasspath
+org.junit.platform:junit-platform-commons:1.9.0=compatTestCompileClasspath,compatTestRuntimeClasspath
+org.junit.platform:junit-platform-engine:1.9.0=compatTestCompileClasspath,compatTestRuntimeClasspath
+org.junit:junit-bom:5.9.0=compatTestCompileClasspath,compatTestRuntimeClasspath
 org.opentest4j:opentest4j:1.2.0=compatTestCompileClasspath,compatTestRuntimeClasspath
-org.ow2.asm:asm:9.1=compatTestCompileClasspath,compatTestRuntimeClasspath
 org.slf4j:slf4j-api:1.7.30=compatTestCompileClasspath,compatTestRuntimeClasspath,compileClasspath,runtimeClasspath,testCompileClasspath,testRuntimeClasspath
-org.spockframework:spock-core:2.0-groovy-3.0=compatTestCompileClasspath,compatTestRuntimeClasspath
+org.spockframework:spock-core:2.3-groovy-3.0=compatTestCompileClasspath,compatTestRuntimeClasspath
 empty=annotationProcessor,compatTestAnnotationProcessor,signatures,testAnnotationProcessor
diff --git a/gradle/wrapper/gradle-wrapper.jar b/gradle/wrapper/gradle-wrapper.jar
index c1962a7..7f93135 100644
Binary files a/gradle/wrapper/gradle-wrapper.jar and b/gradle/wrapper/gradle-wrapper.jar differ
diff --git a/gradle/wrapper/gradle-wrapper.properties b/gradle/wrapper/gradle-wrapper.properties
index 37aef8d..3fa8f86 100644
--- a/gradle/wrapper/gradle-wrapper.properties
+++ b/gradle/wrapper/gradle-wrapper.properties
@@ -1,6 +1,7 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-8.1.1-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-8.4-bin.zip
 networkTimeout=10000
+validateDistributionUrl=true
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
diff --git a/gradlew b/gradlew
index aeb74cb..1aa94a4 100755
--- a/gradlew
+++ b/gradlew
@@ -83,7 +83,8 @@ done
 # This is normally unused
 # shellcheck disable=SC2034
 APP_BASE_NAME=${0##*/}
-APP_HOME=$( cd "${APP_HOME:-./}" && pwd -P ) || exit
+# Discard cd standard output in case $CDPATH is set (https://github.com/gradle/gradle/issues/25036)
+APP_HOME=$( cd "${APP_HOME:-./}" > /dev/null && pwd -P ) || exit
 
 # Use the maximum available, or set MAX_FD != -1 to use that value.
 MAX_FD=maximum
@@ -130,10 +131,13 @@ location of your Java installation."
     fi
 else
     JAVACMD=java
-    which java >/dev/null 2>&1 || die "ERROR: JAVA_HOME is not set and no 'java' command could be found in your PATH.
+    if ! command -v java >/dev/null 2>&1
+    then
+        die "ERROR: JAVA_HOME is not set and no 'java' command could be found in your PATH.
 
 Please set the JAVA_HOME variable in your environment to match the
 location of your Java installation."
+    fi
 fi
 
 # Increase the maximum file descriptors if we can.
@@ -141,7 +145,7 @@ if ! "$cygwin" && ! "$darwin" && ! "$nonstop" ; then
     case $MAX_FD in #(
       max*)
         # In POSIX sh, ulimit -H is undefined. That's why the result is checked to see if it worked.
-        # shellcheck disable=SC3045
+        # shellcheck disable=SC2039,SC3045
         MAX_FD=$( ulimit -H -n ) ||
             warn "Could not query maximum file descriptor limit"
     esac
@@ -149,7 +153,7 @@ if ! "$cygwin" && ! "$darwin" && ! "$nonstop" ; then
       '' | soft) :;; #(
       *)
         # In POSIX sh, ulimit -n is undefined. That's why the result is checked to see if it worked.
-        # shellcheck disable=SC3045
+        # shellcheck disable=SC2039,SC3045
         ulimit -n "$MAX_FD" ||
             warn "Could not set maximum file descriptor limit to $MAX_FD"
     esac
@@ -198,11 +202,11 @@ fi
 # Add default JVM options here. You can also use JAVA_OPTS and GRADLE_OPTS to pass JVM options to this script.
 DEFAULT_JVM_OPTS='"-Xmx64m" "-Xms64m"'
 
-# Collect all arguments for the java command;
-#   * $DEFAULT_JVM_OPTS, $JAVA_OPTS, and $GRADLE_OPTS can contain fragments of
-#     shell script including quotes and variable substitutions, so put them in
-#     double quotes to make sure that they get re-expanded; and
-#   * put everything else in single quotes, so that it's not re-expanded.
+# Collect all arguments for the java command:
+#   * DEFAULT_JVM_OPTS, JAVA_OPTS, JAVA_OPTS, and optsEnvironmentVar are not allowed to contain shell fragments,
+#     and any embedded shellness will be escaped.
+#   * For example: A user cannot expect ${Hostname} to be expanded, as it is an environment variable and will be
+#     treated as '${Hostname}' itself on the command line.
 
 set -- \
         "-Dorg.gradle.appname=$APP_BASE_NAME" \
diff --git a/settings.gradle.kts b/settings.gradle.kts
index 32bed36..e881127 100644
--- a/settings.gradle.kts
+++ b/settings.gradle.kts
@@ -5,7 +5,7 @@ pluginManagement {
     id("org.ajoberstar.reckon.settings") version "0.18.0"
     id("org.ajoberstar.stutter") version "0.7.2"
 
-    id("com.diffplug.spotless") version "6.18.0"
+    id("com.diffplug.spotless") version "6.22.0"
   }
 
   repositories {
diff --git a/stutter.lockfile b/stutter.lockfile
index 7f70800..6989ea6 100644
--- a/stutter.lockfile
+++ b/stutter.lockfile
@@ -1,3 +1,4 @@
 # DO NOT MODIFY: Generated by Stutter plugin.
-java11=7.0.2,7.6.1,8.0.2,8.1.1
-java17=7.3.3,7.6.1,8.0.2,8.1.1
+java11=7.0.2,7.6.3,8.0.2,8.4
+java17=7.3.3,7.6.3,8.0.2,8.4
+java21=8.4