src/java.base/share/classes/jdk/internal/misc/MethodFinder.java

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
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package jdk.internal.misc;

import java.lang.reflect.Method;
import java.lang.reflect.Modifier;

import jdk.internal.access.JavaLangAccess;
import jdk.internal.access.SharedSecrets;

/**
 * A collection of static methods that return specific method objects of interest.
 */
public class MethodFinder {

    private static final JavaLangAccess JLA = SharedSecrets.getJavaLangAccess();

    private MethodFinder() {
        throw new AssertionError("private constructor");
    }

    /**
     * Return the first method that meets the requirements of an application main method
     * {@jls 12.1.4}. The method must:
     * <ul>
     * <li>be declared in this class's hierarchy</li>
     * <li>have the name "main"</li>
     * <li>have a single argument of type {@code String[]}, {@code String...} or no argument</li>
     * <li>have the return type of void</li>
     * <li>be public, protected or package private</li>
     *</ul>
     *
     * The method returned would be used by a launcher to initiate the execution of an
     * application.
     *
     * Searching continues until a main method is found or the search is exhausted. The
     * primary search occurs in two phases, once for a main method with a {@code
     * String[]} or {@code String...} argument and failing that, once for a main method
     * with a no arguments. The search itself uses recursion to first look at methods
     * in this class, then default methods in this class's interface hierarchy and
     * then repeating these steps with the class's super class.
     *
     * @apiNote The method returned may be declared in this class, a super class
     * or as a default method of an interface that the class or super class
     * implements.
     * <p>It is not possible to declare a static main method and instance main
     * method with the same signature in the same class. {@jls 8.4.2} states that
     * "It is a compile-time error to declare two methods with override-equivalent
     * signatures in a class."
     * <p>{@link SecurityException SecurityExceptions} can halt
     * the search. In this case, a null is returned.
     *
     * @return the main method if a method found or null if no method is found
     *
     * @jls 8.2 Class Members
     * @jls 8.4 Method Declarations
     * @jls 8.4.2 Method Signature
     * @jls 12.1.4 Invoke a main method
     */
    public static Method findMainMethod(Class<?> cls) {
        Method mainMethod = JLA.findMethod(cls, true, "main", String[].class);

        if (mainMethod == null) {
            //if not public method, try to lookup a non-public one
            mainMethod = JLA.findMethod(cls, false, "main", String[].class);
        }

        if (mainMethod == null || !isValidMainMethod(mainMethod)) {
            mainMethod = JLA.findMethod(cls, false, "main");
        }

        if (mainMethod == null || !isValidMainMethod(mainMethod)) {
            return null;
        }

        return mainMethod;
    }

    private static boolean isValidMainMethod(Method mainMethodCandidate) {
        return mainMethodCandidate.getReturnType() == void.class &&
               !Modifier.isPrivate(mainMethodCandidate.getModifiers());

    }

}