Add multi-release jar with module-info.java

Also apply minor changes to Gradle setup - most notably, hardcode values
taken from configuration properties that are not required to be
configurable.

Author: damianmalczewski

build.gradle.kts

@@ -2,6 +2,7 @@ import com.diffplug.spotless.LineEnding
 
 plugins {
     id("internal.java-library-convention")
+    id("internal.mrjar-module-info-convention")
     id("internal.publishing-convention")
     alias(libs.plugins.nmcp)
     alias(libs.plugins.spotless)
@@ -78,24 +79,3 @@ spotless {
         lineEndings = LineEnding.UNIX
     }
 }
-
-// This module targets Java 8 for its main sources to maintain compatibility with older runtime environments used by
-// dependent systems.
-//
-// Unit tests, however, are executed on Java 17 because JUnit 6 requires Java 17 or newer. The Gradle toolchain
-// configuration ensures that only the test compilation and execution use Java 17, while the main code remains compiled
-// for Java 8.
-//
-// In short:
-//   - src/main -> Java 8 (for compatibility)
-//   - src/test -> Java 17 (required by JUnit 6)
-
-// JUnit 6 requires at Java 17+, main keeps Java 8.
-tasks.named<JavaCompile>("compileTestJava") {
-    javaCompiler = javaToolchains.compilerFor { languageVersion = JavaLanguageVersion.of(17) }
-}
-
-// JUnit 6 requires at Java 17+, main keeps Java 8.
-tasks.withType<Test>().configureEach {
-    javaLauncher = javaToolchains.launcherFor { languageVersion = JavaLanguageVersion.of(17) }
-}

buildSrc/src/main/kotlin/internal.common-convention.gradle.kts

@@ -1,9 +1,3 @@
-group = property("internal.group") as String
-
-//
-// Not assigning version as it is assigned in CI/CD using -Pversion=X.Y.Z parameter.
-//
-
 repositories {
     mavenCentral()
 }

buildSrc/src/main/kotlin/internal.java-library-convention.gradle.kts

@@ -7,7 +7,7 @@ plugins {
 }
 
 java {
-    toolchain.languageVersion = providers.gradleProperty("internal.java.version").map { JavaLanguageVersion.of(it) }
+    toolchain.languageVersion = JavaLanguageVersion.of(8)
     withSourcesJar()
     withJavadocJar()
 }
@@ -45,3 +45,24 @@ tasks.withType<Test>().configureEach {
     systemProperty("user.language", "en")
     systemProperty("user.country", "US")
 }
+
+// This library targets Java 8 for its main sources to maintain compatibility with older runtime environments used by
+// dependent systems.
+//
+// Unit tests, however, are executed on Java 17 because JUnit 6 requires Java 17 or newer. The Gradle toolchain
+// configuration ensures that only the test compilation and execution use Java 17, while the main code remains compiled
+// for Java 8.
+//
+// In short:
+//   - src/main -> Java 8 (for compatibility)
+//   - src/test -> Java 17 (required by JUnit 6)
+
+// JUnit 6 requires at Java 17+, main keeps Java 8.
+tasks.named<JavaCompile>("compileTestJava") {
+    javaCompiler = javaToolchains.compilerFor { languageVersion = JavaLanguageVersion.of(17) }
+}
+
+// JUnit 6 requires at Java 17+, main keeps Java 8.
+tasks.withType<Test>().configureEach {
+    javaLauncher = javaToolchains.launcherFor { languageVersion = JavaLanguageVersion.of(17) }
+}

buildSrc/src/main/kotlin/internal.mrjar-module-info-convention.gradle.kts

@@ -0,0 +1,35 @@
+plugins {
+    id("internal.common-convention")
+    id("java-library")
+}
+
+// This convention plugin adds compilation of module-info.java with Java 9 transforms output into a multi-release JAR
+// for supporting modules if used by Java 9+.
+
+val sourceSets = extensions.getByType<SourceSetContainer>()
+
+val main9SourceSet = sourceSets.create("main9") {
+    java.srcDirs("src/main9/java", "src/main/java")
+}
+
+configurations.named(main9SourceSet.compileClasspathConfigurationName) {
+    extendsFrom(configurations.named("compileClasspath").get())
+}
+configurations.named(main9SourceSet.runtimeClasspathConfigurationName) {
+    extendsFrom(configurations.named("runtimeClasspath").get())
+}
+
+tasks.named<JavaCompile>("compileMain9Java") {
+    javaCompiler = javaToolchains.compilerFor { languageVersion = JavaLanguageVersion.of(9) }
+}
+
+tasks.named<Jar>("jar") {
+    into("META-INF/versions/9") {
+        from(main9SourceSet.output) {
+            include("module-info.class")
+        }
+    }
+    manifest {
+        attributes["Multi-Release"] = "true"
+    }
+}

gradle.properties

@@ -3,9 +3,10 @@ org.gradle.configuration-cache=true
 org.gradle.parallel=true
 org.gradle.workers.max=4
 #
+# Project settings
+group=io.github.problem4j
+#
 # Project internal settings
-internal.java.version=8
-internal.group=io.github.problem4j
 internal.pom.url=https://github.com/problem4j/problem4j-core
 internal.pom.inception-year=2025
 internal.pom.licenses.0.name=MIT License

src/main/java/io/github/problem4j/core/package-info.java

@@ -20,7 +20,7 @@
  */
 
 /**
- * Core classes and interfaces for the Problem4J library, implementing <a
+ * Core classes and interfaces for the Problem4J set of libraries, implementing <a
  * href="https://tools.ietf.org/html/rfc7807">RFC 7807: Problem Details for HTTP APIs</a>.
  *
  * <p>RFC 7807 was later extended in <a href="https://tools.ietf.org/html/rfc9457">RFC 9457</a>,

src/main9/java/module-info.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright (c) 2025-2026 Problem4J Team & Contributors
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+import org.jspecify.annotations.NullMarked;
+
+/**
+ * Core classes and interfaces for the Problem4J set of libraries, implementing <a
+ * href="https://tools.ietf.org/html/rfc7807">RFC 7807: Problem Details for HTTP APIs</a>.
+ *
+ * <p>RFC 7807 was later extended in <a href="https://tools.ietf.org/html/rfc9457">RFC 9457</a>,
+ * however core concepts remain the same.
+ *
+ * <p>This package provides the foundational types for modeling "Problem Details" objects, which are
+ * used to convey machine-readable error details in HTTP responses.
+ *
+ * <p>Key components include:
+ *
+ * <ul>
+ *   <li>{@code Problem}: The central immutable model representing a problem detail.
+ *   <li>{@code ProblemBuilder}: A fluent builder for creating {@code Problem} instances.
+ *   <li>{@code ProblemException}: A runtime exception that wraps a {@code Problem}, allowing it to
+ *       be thrown and handled by structured error handlers.
+ *   <li>{@code ProblemMapping}: An annotation for configuring how exceptions should be mapped to
+ *       {@code Problem} instances, supporting message interpolation and automatic mapping.
+ *   <li>{@code ProblemMapper}: A utility for mapping exceptions to {@code Problem} instances,
+ *       supporting annotation-based configuration for automatic mapping.
+ * </ul>
+ *
+ * <p>This library is framework-agnostic and intended to be used as a basis for higher-level
+ * integrations (e.g., with Spring Boot, Micronaut, or other web frameworks).
+ */
+@NullMarked
+module io.github.problem4j.core {
+  requires static org.jspecify;
+
+  exports io.github.problem4j.core;
+}