/*
 * Copyright 2002-2019 the original author or authors.
 *
 * Licensed 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
 *
 *      https://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.springframework.util;

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

import org.springframework.lang.Nullable;

/**
 * Helper class that allows for specifying a method to invoke in a declarative
 * fashion, be it static or non-static.
 *
 * <p>Usage: Specify "targetClass"/"targetMethod" or "targetObject"/"targetMethod",
 * optionally specify arguments, prepare the invoker. Afterwards, you may
 * invoke the method any number of times, obtaining the invocation result.
 *
 * @author Colin Sampaleanu
 * @author Juergen Hoeller
 * @since 19.02.2004
 * @see #prepare
 * @see #invoke
 */
public class MethodInvoker {

        private static final Object[] EMPTY_ARGUMENTS = new Object[0];


        @Nullable
        protected Class<?> targetClass;

        @Nullable
        private Object targetObject;

        @Nullable
        private String targetMethod;

        @Nullable
        private String staticMethod;

        @Nullable
        private Object[] arguments;

        /** The method we will call. */
        @Nullable
        private Method methodObject;


        /**
         * Set the target class on which to call the target method.
         * Only necessary when the target method is static; else,
         * a target object needs to be specified anyway.
         * @see #setTargetObject
         * @see #setTargetMethod
         */
        public void setTargetClass(@Nullable Class<?> targetClass) {
                this.targetClass = targetClass;
        }

        /**
         * Return the target class on which to call the target method.
         */
        @Nullable
        public Class<?> getTargetClass() {
                return this.targetClass;
        }

        /**
         * Set the target object on which to call the target method.
         * Only necessary when the target method is not static;
         * else, a target class is sufficient.
         * @see #setTargetClass
         * @see #setTargetMethod
         */
        public void setTargetObject(@Nullable Object targetObject) {
                this.targetObject = targetObject;
                if (targetObject != null) {
                        this.targetClass = targetObject.getClass();
                }
        }

        /**
         * Return the target object on which to call the target method.
         */
        @Nullable
        public Object getTargetObject() {
                return this.targetObject;
        }

        /**
         * Set the name of the method to be invoked.
         * Refers to either a static method or a non-static method,
         * depending on a target object being set.
         * @see #setTargetClass
         * @see #setTargetObject
         */
        public void setTargetMethod(@Nullable String targetMethod) {
                this.targetMethod = targetMethod;
        }

        /**
         * Return the name of the method to be invoked.
         */
        @Nullable
        public String getTargetMethod() {
                return this.targetMethod;
        }

        /**
         * Set a fully qualified static method name to invoke,
         * e.g. "example.MyExampleClass.myExampleMethod".
         * Convenient alternative to specifying targetClass and targetMethod.
         * @see #setTargetClass
         * @see #setTargetMethod
         */
        public void setStaticMethod(String staticMethod) {
                this.staticMethod = staticMethod;
        }

        /**
         * Set arguments for the method invocation. If this property is not set,
         * or the Object array is of length 0, a method with no arguments is assumed.
         */
        public void setArguments(Object... arguments) {
                this.arguments = arguments;
        }

        /**
         * Return the arguments for the method invocation.
         */
        public Object[] getArguments() {
                return (this.arguments != null ? this.arguments : EMPTY_ARGUMENTS);
        }


        /**
         * Prepare the specified method.
         * The method can be invoked any number of times afterwards.
         * @see #getPreparedMethod
         * @see #invoke
         */
        public void prepare() throws ClassNotFoundException, NoSuchMethodException {
                if (this.staticMethod != null) {
                        int lastDotIndex = this.staticMethod.lastIndexOf('.');
                        if (lastDotIndex == -1 || lastDotIndex == this.staticMethod.length()) {
                                throw new IllegalArgumentException(
                                                "staticMethod must be a fully qualified class plus method name: " +
                                                "e.g. 'example.MyExampleClass.myExampleMethod'");
                        }
                        String className = this.staticMethod.substring(0, lastDotIndex);
                        String methodName = this.staticMethod.substring(lastDotIndex + 1);
                        this.targetClass = resolveClassName(className);
                        this.targetMethod = methodName;
                }

                Class<?> targetClass = getTargetClass();
                String targetMethod = getTargetMethod();
                Assert.notNull(targetClass, "Either 'targetClass' or 'targetObject' is required");
                Assert.notNull(targetMethod, "Property 'targetMethod' is required");

                Object[] arguments = getArguments();
                Class<?>[] argTypes = new Class<?>[arguments.length];
                for (int i = 0; i < arguments.length; ++i) {
                        argTypes[i] = (arguments[i] != null ? arguments[i].getClass() : Object.class);
                }

                // Try to get the exact method first.
                try {
                        this.methodObject = targetClass.getMethod(targetMethod, argTypes);
                }
                catch (NoSuchMethodException ex) {
                        // Just rethrow exception if we can't get any match.
                        this.methodObject = findMatchingMethod();
                        if (this.methodObject == null) {
                                throw ex;
                        }
                }
        }

        /**
         * Resolve the given class name into a Class.
         * <p>The default implementations uses {@code ClassUtils.forName},
         * using the thread context class loader.
         * @param className the class name to resolve
         * @return the resolved Class
         * @throws ClassNotFoundException if the class name was invalid
         */
        protected Class<?> resolveClassName(String className) throws ClassNotFoundException {
                return ClassUtils.forName(className, ClassUtils.getDefaultClassLoader());
        }

        /**
         * Find a matching method with the specified name for the specified arguments.
         * @return a matching method, or {@code null} if none
         * @see #getTargetClass()
         * @see #getTargetMethod()
         * @see #getArguments()
         */
        @Nullable
        protected Method findMatchingMethod() {
                String targetMethod = getTargetMethod();
                Object[] arguments = getArguments();
                int argCount = arguments.length;

                Class<?> targetClass = getTargetClass();
                Assert.state(targetClass != null, "No target class set");
                Method[] candidates = ReflectionUtils.getAllDeclaredMethods(targetClass);
                int minTypeDiffWeight = Integer.MAX_VALUE;
                Method matchingMethod = null;

                for (Method candidate : candidates) {
                        if (candidate.getName().equals(targetMethod)) {
                                if (candidate.getParameterCount() == argCount) {
                                        Class<?>[] paramTypes = candidate.getParameterTypes();
                                        int typeDiffWeight = getTypeDifferenceWeight(paramTypes, arguments);
                                        if (typeDiffWeight < minTypeDiffWeight) {
                                                minTypeDiffWeight = typeDiffWeight;
                                                matchingMethod = candidate;
                                        }
                                }
                        }
                }

                return matchingMethod;
        }

        /**
         * Return the prepared Method object that will be invoked.
         * <p>Can for example be used to determine the return type.
         * @return the prepared Method object (never {@code null})
         * @throws IllegalStateException if the invoker hasn't been prepared yet
         * @see #prepare
         * @see #invoke
         */
        public Method getPreparedMethod() throws IllegalStateException {
                if (this.methodObject == null) {
                        throw new IllegalStateException("prepare() must be called prior to invoke() on MethodInvoker");
                }
                return this.methodObject;
        }

        /**
         * Return whether this invoker has been prepared already,
         * i.e. whether it allows access to {@link #getPreparedMethod()} already.
         */
        public boolean isPrepared() {
                return (this.methodObject != null);
        }

        /**
         * Invoke the specified method.
         * <p>The invoker needs to have been prepared before.
         * @return the object (possibly null) returned by the method invocation,
         * or {@code null} if the method has a void return type
         * @throws InvocationTargetException if the target method threw an exception
         * @throws IllegalAccessException if the target method couldn't be accessed
         * @see #prepare
         */
        @Nullable
        public Object invoke() throws InvocationTargetException, IllegalAccessException {
                // In the static case, target will simply be {@code null}.
                Object targetObject = getTargetObject();
                Method preparedMethod = getPreparedMethod();
                if (targetObject == null && !Modifier.isStatic(preparedMethod.getModifiers())) {
                        throw new IllegalArgumentException("Target method must not be non-static without a target");
                }
                ReflectionUtils.makeAccessible(preparedMethod);
                return preparedMethod.invoke(targetObject, getArguments());
        }


        /**
         * Algorithm that judges the match between the declared parameter types of a candidate method
         * and a specific list of arguments that this method is supposed to be invoked with.
         * <p>Determines a weight that represents the class hierarchy difference between types and
         * arguments. A direct match, i.e. type Integer -> arg of class Integer, does not increase
         * the result - all direct matches means weight 0. A match between type Object and arg of
         * class Integer would increase the weight by 2, due to the superclass 2 steps up in the
         * hierarchy (i.e. Object) being the last one that still matches the required type Object.
         * Type Number and class Integer would increase the weight by 1 accordingly, due to the
         * superclass 1 step up the hierarchy (i.e. Number) still matching the required type Number.
         * Therefore, with an arg of type Integer, a constructor (Integer) would be preferred to a
         * constructor (Number) which would in turn be preferred to a constructor (Object).
         * All argument weights get accumulated.
         * <p>Note: This is the algorithm used by MethodInvoker itself and also the algorithm
         * used for constructor and factory method selection in Spring's bean container (in case
         * of lenient constructor resolution which is the default for regular bean definitions).
         * @param paramTypes the parameter types to match
         * @param args the arguments to match
         * @return the accumulated weight for all arguments
         */
        public static int getTypeDifferenceWeight(Class<?>[] paramTypes, Object[] args) {
                int result = 0;
                for (int i = 0; i < paramTypes.length; i++) {
                        if (!ClassUtils.isAssignableValue(paramTypes[i], args[i])) {
                                return Integer.MAX_VALUE;
                        }
                        if (args[i] != null) {
                                Class<?> paramType = paramTypes[i];
                                Class<?> superClass = args[i].getClass().getSuperclass();
                                while (superClass != null) {
                                        if (paramType.equals(superClass)) {
                                                result = result + 2;
                                                superClass = null;
                                        }
                                        else if (ClassUtils.isAssignable(paramType, superClass)) {
                                                result = result + 2;
                                                superClass = superClass.getSuperclass();
                                        }
                                        else {
                                                superClass = null;
                                        }
                                }
                                if (paramType.isInterface()) {
                                        result = result + 1;
                                }
                        }
                }
                return result;
        }

}