/*
 * Copyright 2002-2018 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
 *
 *      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.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 {

	@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 : new Object[0]);
	}


	/**
	 * 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)) {
				Class<?>[] paramTypes = candidate.getParameterTypes();
				if (paramTypes.length == argCount) {
					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;
	}

}
