[MJLINK-83] Implement multiple launcher elements (#202)

* [MJLINK-83] Implement multiple launcher elements

* Add tests

src/main/java/org/apache/maven/plugins/jlink/JLinkMojo.java

@@ -139,6 +139,17 @@ public class JLinkMojo extends AbstractJLinkMojo {
     @Parameter
     private String launcher;
 
+    /**
+     * Specify one or more launchers for jlink.
+     * The command line equivalent is:
+     * <code>--launcher &lt;name&gt;=&lt;module&gt;[/&lt;mainclass&gt;]</code>.
+     * The valid values are a list of
+     * <code>&lt;name&gt;=&lt;module&gt;[/&lt;mainclass&gt;]</code> terms,
+     * separated by commas.
+     */
+    @Parameter
+    private List<String> launchers;
+
     /**
      * These JVM arguments will be appended to the {@code lib/modules} file.<br>
      * <strong>This parameter requires at least JDK 14.<br></strong>
@@ -633,7 +644,8 @@ private void ifOutputDirectoryExistsDelteIt() throws MojoExecutionException {
         }
     }
 
-    protected List<String> createJlinkArgs(Collection<String> pathsOfModules, Collection<String> modulesToAdd) {
+    protected List<String> createJlinkArgs(Collection<String> pathsOfModules, Collection<String> modulesToAdd)
+            throws MojoExecutionException {
         List<String> jlinkArgs = new ArrayList<>();
 
         if (stripDebug) {
@@ -656,8 +668,17 @@ protected List<String> createJlinkArgs(Collection<String> pathsOfModules, Collec
             jlinkArgs.add(compress);
         }
         if (launcher != null) {
-            jlinkArgs.add("--launcher");
-            jlinkArgs.add(launcher);
+            if (launchers != null) {
+                throw new MojoExecutionException("Specify either single <launcher> or multiple <launchers>, not both.");
+            } else {
+                launchers = List.of(launcher);
+            }
+        }
+        if (launchers != null) {
+            for (String item : launchers) {
+                jlinkArgs.add("--launcher");
+                jlinkArgs.add(item);
+            }
         }
         if (addOptions != null && !addOptions.isEmpty()) {
             jlinkArgs.add("--add-options");

src/test/java/org/apache/maven/plugins/jlink/MultipleLauncherTest.java

@@ -0,0 +1,114 @@
+/*
+ * 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 org.apache.maven.plugins.jlink;
+
+import java.lang.reflect.Field;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Set;
+
+import org.apache.maven.plugin.MojoExecutionException;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+public class MultipleLauncherTest {
+    @Test
+    void testSingleLauncher() throws Exception {
+        // It's OK to specify one launcher with "<launcher>"
+        // given
+        JLinkMojo mojo = new JLinkMojo();
+        Field launcher = mojo.getClass().getDeclaredField("launcher");
+        launcher.setAccessible(true);
+        launcher.set(mojo, "l=com.example.Launch");
+
+        // when
+        List<String> jlinkArgs = mojo.createJlinkArgs(List.of(), List.of());
+
+        // then
+        assertThat(launched(jlinkArgs)).contains("l=com.example.Launch");
+    }
+
+    @Test
+    void testOneMultipleLauncher() throws Exception {
+        // It's OK to specify one launcher with "<launchers>"
+        // given
+        JLinkMojo mojo = new JLinkMojo();
+        Field launchers = mojo.getClass().getDeclaredField("launchers");
+        launchers.setAccessible(true);
+        launchers.set(mojo, List.of("l=com.example.Launch"));
+
+        // when
+        List<String> jlinkArgs = mojo.createJlinkArgs(List.of(), List.of());
+
+        // then
+        assertThat(launched(jlinkArgs)).contains("l=com.example.Launch");
+    }
+
+    @Test
+    void testMultipleLaunchers() throws Exception {
+        // It's OK to specify multiple launchers with the "<launchers>" element
+        // given
+        JLinkMojo mojo = new JLinkMojo();
+        Field launcher = mojo.getClass().getDeclaredField("launchers");
+        launcher.setAccessible(true);
+        launcher.set(mojo, List.of("l1=com.example.Launch1", "l2=com.example.Launch2"));
+
+        // when
+        List<String> jlinkArgs = mojo.createJlinkArgs(List.of(), List.of());
+
+        // then
+        assertThat(launched(jlinkArgs)).contains("l1=com.example.Launch1", "l2=com.example.Launch2");
+    }
+
+    @Test
+    void testInvalidLauncherConfig() throws Exception {
+        // It's an error to specify both "<launcher>" and "<launchers>"
+        // given
+        JLinkMojo mojo = new JLinkMojo();
+        Field launcher = mojo.getClass().getDeclaredField("launcher");
+        launcher.setAccessible(true);
+        launcher.set(mojo, "l3=com.example.Launch3");
+        Field launchers = mojo.getClass().getDeclaredField("launchers");
+        launchers.setAccessible(true);
+        launchers.set(mojo, List.of("l1=com.example.Launch1", "l2=com.example.Launch2"));
+
+        // When
+        assertThatThrownBy(() -> mojo.createJlinkArgs(List.of(), List.of())).isInstanceOf(MojoExecutionException.class);
+    }
+
+    // Helper function - gather all the classes named by --launcher args
+    private static Set<String> launched(List<String> args) {
+        Set<String> classNames = new HashSet<>();
+        Iterator<String> iterator = args.iterator();
+        while (iterator.hasNext()) {
+            String arg = iterator.next();
+            if ("--launcher".equals(arg)) {
+                if (iterator.hasNext()) {
+                    classNames.add(iterator.next());
+                } else {
+                    throw new IllegalStateException("--launcher arg with no classname");
+                }
+            }
+        }
+        return classNames;
+    }
+}