Implement S3051 checking the signature of the main method.

Author: tomasz-tylenda-sonarsource

java-checks-test-sources/default/src/main/java/checks/mainSignature/CompactSource.java

@@ -0,0 +1,7 @@
+void main() {
+  IO.println("compact source");
+}
+
+int main(int i) { // Noncompliant
+  return i * 2;
+}

java-checks-test-sources/default/src/main/java/checks/mainSignature/NonInstantiable.java

@@ -0,0 +1,15 @@
+package checks.mainSignature;
+
+interface NonInstantiable {
+  int multiply(int a, int b);
+
+  default void main() { // Compliant
+    System.out.println("default");
+  }
+
+  int main(int a); // Noncompliant
+
+  static void main(String[] arg) { // Compliant
+    System.out.println("yep, inside an interface");
+  }
+}

java-checks-test-sources/default/src/main/java/checks/mainSignature/PrivateMain.java

@@ -0,0 +1,9 @@
+package checks.mainSignature;
+
+public class PrivateMain {
+  private static void main(String[] args) { // Noncompliant
+  }
+
+  private void main() { // Noncompliant
+  }
+}

java-checks-test-sources/default/src/main/java/checks/mainSignature/Sample.java

@@ -0,0 +1,60 @@
+package checks.mainSignature;
+
+public class Sample {
+  public static class Traditional {
+    public static void main(String[] args) { // Compliant
+      System.out.println("traditional");
+    }
+  }
+
+  public static class NoArg {
+    public static void main() { // Compliant
+      System.out.println("no arg");
+    }
+  }
+
+  public static class Instance {
+    void main(String[] args) { // Compliant
+      System.out.println("instance");
+    }
+  }
+
+  public static class Wrong {
+    public static void main(String[] args) {
+      System.out.println(new Wrong().main(42));
+    }
+
+    int main(int x) { // Noncompliant
+      return x * x;
+    }
+
+    String main(String x, String y) { // Noncompliant
+      return "";
+    }
+  }
+
+  public static class WrongReturn {
+    public static int main(String[] args) { // Noncompliant
+      return 1;
+    }
+  }
+
+  @interface MyAnnotation {
+    String main() default ""; // Noncompliant
+  }
+
+  enum MyEnum {
+    A, B;
+
+    void main(int x) { // Noncompliant
+    }
+  }
+
+  record MyRecord(int value) {
+    void main(int x) { // Noncompliant
+    }
+
+    static void main(String[] args) {
+    }
+  }
+}

java-checks-test-sources/default/src/main/java/checks/mainSignature/Varargs.java

@@ -0,0 +1,10 @@
+package checks.mainSignature;
+
+public class Varargs {
+  void main(String... args) {
+  }
+
+  void main(int i, String ... args) { // Noncompliant
+
+  }
+}

java-checks-test-sources/default/src/main/java/checks/mainSignature/main.java

@@ -0,0 +1,10 @@
+package checks.mainSignature;
+
+// This one is really weird. Doesn't really matter what we report,
+// as long as nothing crashes.
+
+public class main {
+  public main() {}
+
+  public main(String[] args) {}
+}

java-checks/src/main/java/org/sonar/java/checks/MainMethodSignatureCheck.java

@@ -0,0 +1,78 @@
+/*
+ * SonarQube Java
+ * Copyright (C) 2012-2025 SonarSource Sàrl
+ * mailto:info AT sonarsource DOT com
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the Sonar Source-Available License Version 1, as published by SonarSource SA.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the Sonar Source-Available License for more details.
+ *
+ * You should have received a copy of the Sonar Source-Available License
+ * along with this program; if not, see https://sonarsource.com/license/ssal/
+ */
+package org.sonar.java.checks;
+
+import java.util.Collections;
+import java.util.List;
+import org.sonar.check.Rule;
+import org.sonar.java.model.ModifiersUtils;
+import org.sonar.plugins.java.api.IssuableSubscriptionVisitor;
+import org.sonar.plugins.java.api.tree.*;
+
+/**
+ * Checks that the "main" method has the correct signature for a program entry point.
+ *
+ * <p>Note, that even with a correct signature, the "main" method may not be valid entry point.
+ * For example, it may be declared in an abstract class or an interface.
+ */
+@Rule(key = "S3051")
+public class MainMethodSignatureCheck extends IssuableSubscriptionVisitor {
+
+  private static final String MESSAGE = "\"main\" method should only be used for the program entry point and should have appropriate signature.";
+
+  @Override
+  public List<Tree.Kind> nodesToVisit() {
+    return List.of(Tree.Kind.METHOD);
+  }
+
+  @Override
+  public void visitNode(Tree tree) {
+    MethodTree methodTree = (MethodTree) tree;
+    if (!"main".equals(methodTree.simpleName().name())) {
+      return;
+    }
+    if (!isValidMainSignature(methodTree)) {
+      reportIssue(methodTree.simpleName(), MESSAGE);
+    }
+  }
+
+  /**
+   * Checks if the signature is a valid "main" method signature in Java 25.
+   *
+   * @return true, false, or null if it cannot be determined
+   */
+  private static Boolean isValidMainSignature(MethodTree methodTree) {
+    // main cannot be private
+    if (ModifiersUtils.hasModifier(methodTree.modifiers(), Modifier.PRIVATE)) {
+      return false;
+    }
+
+    // return type must be void
+    TypeTree returnType = methodTree.returnType();
+    // null check just to avoid NPE
+    if (returnType == null || !returnType.symbolType().isVoid()) {
+      return false;
+    }
+
+    List<VariableTree> parameters = methodTree.parameters();
+    // no arguments or single String[] argument
+    return parameters.isEmpty() ||
+      (parameters.size() == 1
+      && parameters.get(0).type() instanceof ArrayTypeTree att
+      && "String".equals(att.type().symbolType().name()));
+  }
+}

java-checks/src/test/java/org/sonar/java/checks/MainMethodSignatureCheckTest.java

@@ -0,0 +1,76 @@
+/*
+ * SonarQube Java
+ * Copyright (C) 2012-2025 SonarSource Sàrl
+ * mailto:info AT sonarsource DOT com
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the Sonar Source-Available License Version 1, as published by SonarSource SA.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the Sonar Source-Available License for more details.
+ *
+ * You should have received a copy of the Sonar Source-Available License
+ * along with this program; if not, see https://sonarsource.com/license/ssal/
+ */
+package org.sonar.java.checks;
+
+import org.junit.jupiter.api.Test;
+import org.sonar.java.checks.verifier.CheckVerifier;
+
+import static org.sonar.java.checks.verifier.TestUtils.mainCodeSourcesPath;
+
+class MainMethodSignatureCheckTest {
+
+  private static final MainMethodSignatureCheck CHECK = new MainMethodSignatureCheck();
+
+  @Test
+  void test() {
+    CheckVerifier.newVerifier()
+      .onFile(mainCodeSourcesPath("checks/mainSignature/Sample.java"))
+      .withCheck(CHECK)
+      .verifyIssues();
+  }
+
+  @Test
+  void nonInstantiable() {
+    CheckVerifier.newVerifier()
+      .onFile(mainCodeSourcesPath("checks/mainSignature/NonInstantiable.java"))
+      .withCheck(CHECK)
+      .verifyIssues();
+  }
+
+  @Test
+  void privateMain() {
+    CheckVerifier.newVerifier()
+      .onFile(mainCodeSourcesPath("checks/mainSignature/PrivateMain.java"))
+      .withCheck(CHECK)
+      .verifyIssues();
+  }
+
+  @Test
+  void compactSource() {
+    CheckVerifier.newVerifier()
+      .onFile(mainCodeSourcesPath("checks/mainSignature/CompactSource.java"))
+      .withCheck(CHECK)
+      .verifyIssues();
+  }
+
+  @Test
+  void varargs() {
+    CheckVerifier.newVerifier()
+      .onFile(mainCodeSourcesPath("checks/mainSignature/Varargs.java"))
+      .withCheck(CHECK)
+      .verifyIssues();
+  }
+
+  @Test
+  void constructor() {
+    // The check will not run, because it does not visit constructor nodes.
+    CheckVerifier.newVerifier()
+      .onFile(mainCodeSourcesPath("checks/mainSignature/main.java"))
+      .withCheck(CHECK)
+      .verifyNoIssues();
+  }
+}

sonar-java-plugin/src/main/resources/org/sonar/l10n/java/rules/java/S3051.html

@@ -0,0 +1,40 @@
+<h2>Why is this an issue?</h2>
+<p>A method named <code>main</code> with an invalid signature for a program entry point is confusing. It suggests the method is intended to be an entry point, but the JVM will not recognize it as such.</p>
+<p>Valid signatures for a program entry point are:</p>
+<ul>
+  <li><code>public static void main(String[] args)</code> - traditional signature</li>
+  <li><code>static void main(String[] args)</code> - static main without public modifier (Java 21+)</li>
+  <li><code>void main(String[] args)</code> - instance main method (Java 21+)</li>
+  <li><code>void main()</code> - main method without parameters (Java 21+)</li>
+  <li><code>static void main()</code> - static main without parameters (Java 21+)</li>
+</ul>
+<h3>Noncompliant code example</h3>
+<pre>
+class App {
+  // Wrong return type
+  int main(String[] args) { // Noncompliant
+    return 0;
+  }
+
+  // Wrong parameter type
+  void main(int[] args) { // Noncompliant
+  }
+
+  // Too many parameters
+  void main(String[] args, int extra) { // Noncompliant
+  }
+}
+</pre>
+<h3>Compliant solution</h3>
+<pre>
+class App {
+  public static void main(String[] args) { // Compliant
+  }
+
+  // Or using Java 21+ flexible main methods:
+  void main() { // Compliant
+  }
+}
+</pre>
+<h3>Exceptions</h3>
+<p>Methods named <code>main</code> in interfaces, abstract classes, and annotations are ignored, as these cannot be used as program entry points.</p>

sonar-java-plugin/src/main/resources/org/sonar/l10n/java/rules/java/S3051.json

@@ -0,0 +1,23 @@
+{
+  "title": "\"main\" method should have the correct signature",
+  "type": "CODE_SMELL",
+  "code": {
+    "impacts": {
+      "MAINTAINABILITY": "LOW"
+    },
+    "attribute": "CLEAR"
+  },
+  "status": "ready",
+  "remediation": {
+    "func": "Constant/Issue",
+    "constantCost": "5min"
+  },
+  "tags": [
+    "confusing"
+  ],
+  "defaultSeverity": "Minor",
+  "ruleSpecification": "RSPEC-3051",
+  "sqKey": "S3051",
+  "scope": "Main",
+  "quickfix": "unknown"
+}