Add OpenRewrite recipe for JEP 512 instance main methods

src/main/java/org/openrewrite/java/migrate/lang/MigrateMainMethodToInstanceMain.java

@@ -0,0 +1,79 @@
+/*
+ * Copyright 2025 the original author or authors.
+ * <p>
+ * Licensed under the Moderne Source Available License (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * <p>
+ * https://docs.moderne.io/licensing/moderne-source-available-license
+ * <p>
+ * 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.openrewrite.java.migrate.lang;
+
+import org.openrewrite.ExecutionContext;
+import org.openrewrite.Preconditions;
+import org.openrewrite.Recipe;
+import org.openrewrite.TreeVisitor;
+import org.openrewrite.java.JavaIsoVisitor;
+import org.openrewrite.java.search.UsesJavaVersion;
+import org.openrewrite.java.tree.J;
+import org.openrewrite.java.tree.JavaType;
+import org.openrewrite.java.tree.TypeUtils;
+import org.openrewrite.staticanalysis.VariableReferences;
+
+import static java.util.Collections.emptyList;
+
+public class MigrateMainMethodToInstanceMain extends Recipe {
+    @Override
+    public String getDisplayName() {
+        return "Migrate `public static void main(String[] args)` to instance `void main()`";
+    }
+
+    @Override
+    public String getDescription() {
+        return "Migrate `public static void main(String[] args)` method to instance `void main()` method when the `args` parameter is unused, as supported by JEP 512 in Java 25+.";
+    }
+
+    @Override
+    public TreeVisitor<?, ExecutionContext> getVisitor() {
+        return Preconditions.check(new UsesJavaVersion<>(25), new JavaIsoVisitor<ExecutionContext>() {
+            @Override
+            public J.MethodDeclaration visitMethodDeclaration(J.MethodDeclaration method, ExecutionContext ctx) {
+                J.MethodDeclaration md = super.visitMethodDeclaration(method, ctx);
+
+                // Check if this is a main method: public static void main(String[] args)
+                if (!"main".equals(md.getSimpleName()) ||
+                        md.getReturnTypeExpression() == null ||
+                        md.getReturnTypeExpression().getType() != JavaType.Primitive.Void ||
+                        !md.hasModifier(J.Modifier.Type.Public) ||
+                        !md.hasModifier(J.Modifier.Type.Static) ||
+                        md.getParameters().size() != 1 ||
+                        !(md.getParameters().get(0) instanceof J.VariableDeclarations) ||
+                        md.getBody() == null) {
+                    return md;
+                }
+
+                // Check if parameter is String[] type
+                J.VariableDeclarations param = (J.VariableDeclarations) md.getParameters().get(0);
+                JavaType paramType = param.getType();
+                if (!TypeUtils.isOfClassType(paramType, "java.lang.String") || !(paramType instanceof JavaType.Array)) {
+                    return md;
+                }
+
+                // Remove the parameter if unused
+                J.Identifier variableName = param.getVariables().get(0).getName();
+                if (VariableReferences.findRhsReferences(md.getBody(), variableName).isEmpty()) {
+                    md = md.withParameters(emptyList());
+                }
+                return md.withReturnTypeExpression(md.getReturnTypeExpression().withPrefix(md.getModifiers().get(0).getPrefix()))
+                        .withModifiers(emptyList());
+            }
+        });
+    }
+
+}

src/main/resources/META-INF/rewrite/java-version-25.yml

@@ -31,6 +31,7 @@ recipeList:
   - org.openrewrite.github.SetupJavaUpgradeJavaVersion:
       minimumJavaMajorVersion: 25
   - org.openrewrite.java.migrate.io.ReplaceSystemOutWithIOPrint
+  - org.openrewrite.java.migrate.lang.MigrateMainMethodToInstanceMain
   - org.openrewrite.java.migrate.lang.MigrateProcessWaitForDuration
   - org.openrewrite.java.migrate.lang.ReplaceUnusedVariablesWithUnderscore
   - org.openrewrite.java.migrate.util.MigrateInflaterDeflaterToClose

src/test/java/org/openrewrite/java/migrate/lang/MigrateMainMethodToInstanceMainTest.java

@@ -0,0 +1,291 @@
+/*
+ * Copyright 2025 the original author or authors.
+ * <p>
+ * Licensed under the Moderne Source Available License (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * <p>
+ * https://docs.moderne.io/licensing/moderne-source-available-license
+ * <p>
+ * 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.openrewrite.java.migrate.lang;
+
+import org.junit.jupiter.api.Test;
+import org.openrewrite.DocumentExample;
+import org.openrewrite.test.RecipeSpec;
+import org.openrewrite.test.RewriteTest;
+
+import static org.openrewrite.java.Assertions.java;
+import static org.openrewrite.java.Assertions.javaVersion;
+
+@SuppressWarnings("ConfusingMainMethod")
+class MigrateMainMethodToInstanceMainTest implements RewriteTest {
+
+    @Override
+    public void defaults(RecipeSpec spec) {
+        spec.recipe(new MigrateMainMethodToInstanceMain())
+          .allSources(s -> s.markers(javaVersion(25)));
+    }
+
+    @DocumentExample
+    @Test
+    void migrateMainMethodWithUnusedArgs() {
+        //language=java
+        rewriteRun(
+          java(
+            """
+              class Application {
+                  public static void main(String[] args) {
+                      System.out.println("Hello, World!");
+                  }
+              }
+              """,
+            """
+              class Application {
+                  void main() {
+                      System.out.println("Hello, World!");
+                  }
+              }
+              """
+          )
+        );
+    }
+
+    @Test
+    void retainArgsWhenUsed() {
+        //language=java
+        rewriteRun(
+          java(
+            """
+              class Application {
+                  public static void main(String[] args) {
+                      if (args.length > 0) {
+                          System.out.println("Args provided: " + args[0]);
+                      }
+                  }
+              }
+              """,
+            """
+              class Application {
+                  void main(String[] args) {
+                      if (args.length > 0) {
+                          System.out.println("Args provided: " + args[0]);
+                      }
+                  }
+              }
+              """
+          )
+        );
+    }
+
+    @Test
+    void retainArgsWhenUsedInMethodCall() {
+        //language=java
+        rewriteRun(
+          java(
+            """
+              class Application {
+                  public static void main(String[] args) {
+                      processArgs(args);
+                  }
+
+                  private static void processArgs(String[] args) {
+                      // Process arguments
+                  }
+              }
+              """,
+            """
+              class Application {
+                  void main(String[] args) {
+                      processArgs(args);
+                  }
+
+                  private static void processArgs(String[] args) {
+                      // Process arguments
+                  }
+              }
+              """
+          )
+        );
+    }
+
+    @Test
+    void migrateMainMethodWithEmptyBody() {
+        //language=java
+        rewriteRun(
+          java(
+            """
+              class Application {
+                  public static void main(String[] args) {
+                  }
+              }
+              """,
+            """
+              class Application {
+                  void main() {
+                  }
+              }
+              """
+          )
+        );
+    }
+
+    @Test
+    void doNotMigrateNonMainMethod() {
+        //language=java
+        rewriteRun(
+          java(
+            """
+              class Application {
+                  public static void notMain(String[] args) {
+                      System.out.println("Not a main method");
+                  }
+              }
+              """
+          )
+        );
+    }
+
+    @Test
+    void doNotMigrateMainWithDifferentSignature() {
+        //language=java
+        rewriteRun(
+          java(
+            """
+              class Application {
+                  public static int main(String[] args) {
+                      return 0;
+                  }
+              }
+              """
+          )
+        );
+    }
+
+    @Test
+    void doNotMigratePrivateMain() {
+        //language=java
+        rewriteRun(
+          java(
+            """
+              class Application {
+                  private static void main(String[] args) {
+                      System.out.println("Private main");
+                  }
+              }
+              """
+          )
+        );
+    }
+
+    @Test
+    void doNotMigrateInstanceMain() {
+        //language=java
+        rewriteRun(
+          java(
+            """
+              class Application {
+                  public void main(String[] args) {
+                      System.out.println("Already instance main");
+                  }
+              }
+              """
+          )
+        );
+    }
+
+    @Test
+    void migrateMainWithComplexUnusedArgs() {
+        //language=java
+        rewriteRun(
+          java(
+            """
+              class Application {
+                  public static void main(String[] arguments) {
+                      System.out.println("Starting application...");
+                      new Application().run();
+                  }
+
+                  void run() {
+                      System.out.println("Running...");
+                  }
+              }
+              """,
+            """
+              class Application {
+                  void main() {
+                      System.out.println("Starting application...");
+                      new Application().run();
+                  }
+
+                  void run() {
+                      System.out.println("Running...");
+                  }
+              }
+              """
+          )
+        );
+    }
+
+    @Test
+    void doNotMigrateMainWithMultipleParameters() {
+        //language=java
+        rewriteRun(
+          java(
+            """
+              class Application {
+                  public static void main(String first, String[] args) {
+                      System.out.println("Invalid main method");
+                  }
+              }
+              """
+          )
+        );
+    }
+
+    @Test
+    void doNotMigrateMainWithNoParameters() {
+        //language=java
+        rewriteRun(
+          java(
+            """
+              class Application {
+                  public static void main() {
+                      System.out.println("No parameters");
+                  }
+              }
+              """
+          )
+        );
+    }
+
+    @Test
+    void migrateMainWithAnnotations() {
+        //language=java
+        rewriteRun(
+          java(
+            """
+              class Application {
+                  @SuppressWarnings("unused")
+                  public static void main(String[] args) {
+                      System.out.println("Hello!");
+                  }
+              }
+              """,
+            """
+              class Application {
+                  @SuppressWarnings("unused")
+                  void main() {
+                      System.out.println("Hello!");
+                  }
+              }
+              """
+          )
+        );
+    }
+}