Provide better support for toolchains for JVM projects

src/main/java/org/javamodularity/moduleplugin/extensions/DefaultModularityExtension.java

@@ -5,7 +5,10 @@
 import org.gradle.api.Project;
 import org.gradle.api.Task;
 import org.gradle.api.plugins.JavaPlugin;
+import org.gradle.api.plugins.JavaPluginExtension;
 import org.gradle.api.tasks.compile.JavaCompile;
+import org.gradle.jvm.toolchain.JavaToolchainSpec;
+import org.gradle.util.GradleVersion;
 import org.javamodularity.moduleplugin.JavaProjectHelper;
 import org.javamodularity.moduleplugin.tasks.ClasspathFile;
 
@@ -74,6 +77,17 @@ private void configureMixedJavaRelease(int mainJavaRelease, int moduleInfoJavaRe
     // TODO: Remove this method when Gradle supports it natively:  https://github.com/gradle/gradle/issues/2510
     private void setJavaRelease(JavaCompile javaCompile, int javaRelease) {
         String currentJavaVersion = JavaVersion.current().toString();
+        if (toolchainIsSupported()) {
+            JavaToolchainSpec toolchain = project.getExtensions().getByType(JavaPluginExtension.class).getToolchain();
+            if (toolchain != null) {
+                // If toolchain is enabled, the version of java compiler is NOT same to the version of JVM running Gradle
+                // so we need to get the version of toolchain explicitly as follows
+                String toolchainVersion = toolchain.getLanguageVersion().map(Object::toString).getOrNull();
+                if (toolchainVersion != null) {
+                    currentJavaVersion = toolchainVersion;
+                }
+            }
+        }
         if (!javaCompile.getSourceCompatibility().equals(currentJavaVersion)) {
             throw new IllegalStateException("sourceCompatibility should not be set together with --release option");
         }
@@ -86,8 +100,29 @@ private void setJavaRelease(JavaCompile javaCompile, int javaRelease) {
             throw new IllegalStateException("--release option is already set in compiler args");
         }
 
-        compilerArgs.add("--release");
-        compilerArgs.add(String.valueOf(javaRelease));
+        if (releaseOptionIsSupported()) {
+            // using the `convention(Integer)` method instead of the `set(Integer)` method, to let users set overwrite explicitly
+            javaCompile.getOptions().getRelease().convention(javaRelease);
+        } else {
+            compilerArgs.add("--release");
+            compilerArgs.add(String.valueOf(javaRelease));
+        }
+    }
+
+    /**
+     * @see <a href="https://github.com/gradle/gradle/issues/2510#issuecomment-657436188">The comment on GitHub issue that says {@code --release} option is added in Gradle 6.6</a>
+     * @return true if the version of Gradle is 6.6 or later
+     */
+    private boolean releaseOptionIsSupported() {
+        return GradleVersion.current().compareTo(GradleVersion.version("6.6")) >= 0;
+    }
+
+    /**
+     * @see <a href="https://docs.gradle.org/6.7/javadoc/org/gradle/api/plugins/JavaPluginExtension.html#getToolchain--">The Javadoc that says {@code JavaPluginExtension.getToolchain()} is added in Gradle 6.7</a>
+     * @return true if the version of Gradle is 6.7 or later
+     */
+    private boolean toolchainIsSupported() {
+        return GradleVersion.current().compareTo(GradleVersion.version("6.7")) >= 0;
     }
 
     private JavaProjectHelper helper() {

test-project/greeter.toolchain/build.gradle

@@ -0,0 +1,18 @@
+//region NO-OP (DSL testing)
+
+// one of the supported Java version (prefer LTS version)
+def toolchainVer = 11
+
+java {
+    toolchain {
+        languageVersion.set(JavaLanguageVersion.of(toolchainVer))
+    }
+}
+
+modularity {
+    // This version should be less than `toolchainVer` because:
+    // * if the version is same to `toolchainVer` we cannot reproduce the existing problem
+    // * if the version is greather than `toolchainVer` javac cannot create class files
+    standardJavaRelease(9)
+}
+//endregion

test-project/greeter.toolchain/src/main/java/examples/greeter/api/Greeter.java

@@ -0,0 +1,5 @@
+package examples.greeter.api;
+
+public interface Greeter {
+    String hello();
+}

test-project/greeter.toolchain/src/main/java/module-info.java

@@ -0,0 +1,3 @@
+module greeter.api {
+    exports examples.greeter.api;
+}

test-project/settings.gradle

@@ -15,3 +15,7 @@ include 'greeter.runner'
 
 include 'greeter.javaexec'
 include 'greeter.startscripts'
+
+if(GradleVersion.current().compareTo(GradleVersion.version("6.6")) >= 0) {
+    include 'greeter.toolchain'
+}