/*
 * Copyright 2002-2017 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.web.method.annotation;

import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

import org.springframework.core.ExceptionDepthComparator;
import org.springframework.core.MethodIntrospector;
import org.springframework.core.annotation.AnnotationUtils;
import org.springframework.util.ClassUtils;
import org.springframework.util.ReflectionUtils.MethodFilter;
import org.springframework.web.bind.annotation.ExceptionHandler;

/**
 * Discovers {@linkplain ExceptionHandler @ExceptionHandler} methods in a given class,
 * including all of its superclasses, and helps to resolve a given {@link Exception}
 * to the exception types supported by a given {@link Method}.
 *
 * @author Rossen Stoyanchev
 * @author Juergen Hoeller
 * @since 3.1
 */
public class ExceptionHandlerMethodResolver {

        /**
         * A filter for selecting {@code @ExceptionHandler} methods.
         */
        public static final MethodFilter EXCEPTION_HANDLER_METHODS = new MethodFilter() {
                @Override
                public boolean matches(Method method) {
                        return (AnnotationUtils.findAnnotation(method, ExceptionHandler.class) != null);
                }
        };

        /**
         * Arbitrary {@link Method} reference, indicating no method found in the cache.
         */
        private static final Method NO_METHOD_FOUND = ClassUtils.getMethodIfAvailable(System.class, "currentTimeMillis");


        private final Map<Class<? extends Throwable>, Method> mappedMethods =
                        new ConcurrentHashMap<Class<? extends Throwable>, Method>(16);

        private final Map<Class<? extends Throwable>, Method> exceptionLookupCache =
                        new ConcurrentHashMap<Class<? extends Throwable>, Method>(16);


        /**
         * A constructor that finds {@link ExceptionHandler} methods in the given type.
         * @param handlerType the type to introspect
         */
        public ExceptionHandlerMethodResolver(Class<?> handlerType) {
                for (Method method : MethodIntrospector.selectMethods(handlerType, EXCEPTION_HANDLER_METHODS)) {
                        for (Class<? extends Throwable> exceptionType : detectExceptionMappings(method)) {
                                addExceptionMapping(exceptionType, method);
                        }
                }
        }


        /**
         * Extract exception mappings from the {@code @ExceptionHandler} annotation first,
         * and then as a fallback from the method signature itself.
         */
        @SuppressWarnings("unchecked")
        private List<Class<? extends Throwable>> detectExceptionMappings(Method method) {
                List<Class<? extends Throwable>> result = new ArrayList<Class<? extends Throwable>>();
                detectAnnotationExceptionMappings(method, result);
                if (result.isEmpty()) {
                        for (Class<?> paramType : method.getParameterTypes()) {
                                if (Throwable.class.isAssignableFrom(paramType)) {
                                        result.add((Class<? extends Throwable>) paramType);
                                }
                        }
                }
                if (result.isEmpty()) {
                        throw new IllegalStateException("No exception types mapped to " + method);
                }
                return result;
        }

        protected void detectAnnotationExceptionMappings(Method method, List<Class<? extends Throwable>> result) {
                ExceptionHandler ann = AnnotationUtils.findAnnotation(method, ExceptionHandler.class);
                result.addAll(Arrays.asList(ann.value()));
        }

        private void addExceptionMapping(Class<? extends Throwable> exceptionType, Method method) {
                Method oldMethod = this.mappedMethods.put(exceptionType, method);
                if (oldMethod != null && !oldMethod.equals(method)) {
                        throw new IllegalStateException("Ambiguous @ExceptionHandler method mapped for [" +
                                        exceptionType + "]: {" + oldMethod + ", " + method + "}");
                }
        }

        /**
         * Whether the contained type has any exception mappings.
         */
        public boolean hasExceptionMappings() {
                return !this.mappedMethods.isEmpty();
        }

        /**
         * Find a {@link Method} to handle the given exception.
         * Use {@link ExceptionDepthComparator} if more than one match is found.
         * @param exception the exception
         * @return a Method to handle the exception, or {@code null} if none found
         */
        public Method resolveMethod(Exception exception) {
                Method method = resolveMethodByExceptionType(exception.getClass());
                if (method == null) {
                        Throwable cause = exception.getCause();
                        if (cause != null) {
                                method = resolveMethodByExceptionType(cause.getClass());
                        }
                }
                return method;
        }

        /**
         * Find a {@link Method} to handle the given exception type. This can be
         * useful if an {@link Exception} instance is not available (e.g. for tools).
         * @param exceptionType the exception type
         * @return a Method to handle the exception, or {@code null} if none found
         */
        public Method resolveMethodByExceptionType(Class<? extends Throwable> exceptionType) {
                Method method = this.exceptionLookupCache.get(exceptionType);
                if (method == null) {
                        method = getMappedMethod(exceptionType);
                        this.exceptionLookupCache.put(exceptionType, (method != null ? method : NO_METHOD_FOUND));
                }
                return (method != NO_METHOD_FOUND ? method : null);
        }

        /**
         * Return the {@link Method} mapped to the given exception type, or {@code null} if none.
         */
        private Method getMappedMethod(Class<? extends Throwable> exceptionType) {
                List<Class<? extends Throwable>> matches = new ArrayList<Class<? extends Throwable>>();
                for (Class<? extends Throwable> mappedException : this.mappedMethods.keySet()) {
                        if (mappedException.isAssignableFrom(exceptionType)) {
                                matches.add(mappedException);
                        }
                }
                if (!matches.isEmpty()) {
                        Collections.sort(matches, new ExceptionDepthComparator(exceptionType));
                        return this.mappedMethods.get(matches.get(0));
                }
                else {
                        return null;
                }
        }

}