Allow to not use --module-version for the Java compiler

src/it/GH-331-no-module-version/invoker.properties

@@ -0,0 +1,18 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+# 
+#   http://www.apache.org/licenses/LICENSE-2.0
+# 
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+invoker.java.version = 9+

src/it/GH-331-no-module-version/pom.xml

@@ -0,0 +1,45 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+  ~ Licensed to the Apache Software Foundation (ASF) under one
+  ~ or more contributor license agreements.  See the NOTICE file
+  ~ distributed with this work for additional information
+  ~ regarding copyright ownership.  The ASF licenses this file
+  ~ to you under the Apache License, Version 2.0 (the
+  ~ "License"); you may not use this file except in compliance
+  ~ with the License.  You may obtain a copy of the License at
+  ~
+  ~   http://www.apache.org/licenses/LICENSE-2.0
+  ~
+  ~ Unless required by applicable law or agreed to in writing,
+  ~ software distributed under the License is distributed on an
+  ~ "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+  ~ KIND, either express or implied.  See the License for the
+  ~ specific language governing permissions and limitations
+  ~ under the License.
+  -->
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+
+  <groupId>org.apache.maven.plugins.compiler.it</groupId>
+  <artifactId>GH-331-no-module-version</artifactId>
+  <version>1.0-SNAPSHOT</version>
+
+  <url>https://github.com/apache/maven-compiler-plugin/pull/331</url>
+
+  <properties>
+    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+    <maven.compiler.release>9</maven.compiler.release>
+    <maven.compiler.useModuleVersion>false</maven.compiler.useModuleVersion>
+  </properties>
+
+  <build>
+    <plugins>
+      <plugin>
+        <artifactId>maven-compiler-plugin</artifactId>
+        <version>@project.version@</version>
+      </plugin>
+    </plugins>
+  </build>
+</project>

src/it/GH-331-no-module-version/src/main/java/com/foo/MyClass.java

@@ -0,0 +1,21 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package com.foo;
+
+public class MyClass {}

src/it/GH-331-no-module-version/src/main/java/module-info.java

@@ -0,0 +1,22 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+module M.N {
+    exports com.foo;
+}

src/it/GH-331-no-module-version/verify.groovy

@@ -0,0 +1,25 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+def log = new File(basedir, 'build.log').text
+
+assert log.count(' --module-version') == 0
+
+def descriptor = java.lang.module.ModuleFinder.of(basedir.toPath().resolve('target/classes')).find('M.N').get().descriptor()
+assert descriptor.version().empty
+assert 'M.N' == descriptor.toNameAndVersion()

src/main/java/org/apache/maven/plugin/compiler/CompilerMojo.java

@@ -172,6 +172,16 @@ public class CompilerMojo extends AbstractCompilerMojo {
     @Parameter(property = "maven.compiler.moduleVersion", defaultValue = "${project.version}")
     private String moduleVersion;
 
+    /**
+     * Use the {@code --module-version} argument for the Java compiler.
+     * This is ignored if not applicable, e.g., in non-modular projects.
+     *
+     * @see <a href="https://docs.oracle.com/en/java/javase/17/docs/specs/man/javac.html#option-module-version">javac --module-version</a>
+     * @since 3.15.0
+     */
+    @Parameter(property = "maven.compiler.useModuleVersion", defaultValue = "true")
+    private boolean useModuleVersion;
+
     final LocationManager locationManager = new LocationManager();
 
     private List<String> classpathElements;
@@ -311,9 +321,10 @@ protected void preparePaths(Set<File> sourceFiles) {
                     modulepathElements.add(file.getPath());
                 }
 
-                compilerArgs.add("--module-version");
-                compilerArgs.add(moduleVersion);
-
+                if (useModuleVersion) {
+                    compilerArgs.add("--module-version");
+                    compilerArgs.add(moduleVersion);
+                }
             } catch (IOException e) {
                 getLog().warn(e.getMessage());
             }