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/**
 * Copyright (C) 2015 The Gravitee team (http://gravitee.io)
 *
 * 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 io.gravitee.el.spel.context;

import io.gravitee.common.util.EnvironmentUtils;
import io.gravitee.el.spel.SpelTemplateEngine;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.core.env.ConfigurableEnvironment;
import org.springframework.core.env.Environment;
import org.springframework.expression.spel.support.ReflectiveMethodResolver;
import org.springframework.lang.Nullable;
import org.springframework.util.ClassUtils;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.lang.reflect.Method;
import java.util.*;
import java.util.concurrent.ConcurrentHashMap;
import java.util.stream.Collectors;
import java.util.stream.Stream;

/**
 * The {@link SecuredMethodResolver} is a thread-safe singleton class which can be used to replace the default {@link ReflectiveMethodResolver} used by the {@link org.springframework.expression.spel.support.StandardEvaluationContext}.
 * 
 * This method resolver is particularly useful to restrict usage to a subset of methods in an EL expression.
 * 

 * By default, the whitelist methods are loaded from whitelist file located in the classpath.
 * The list can be either replaced either completed specifying a 'el.whitelist.list' configuration (array).
 *
 * @author Jeoffrey HAEYAERT (jeoffrey.haeyaert at graviteesource.com)
 * @author GraviteeSource Team
 */
public class SecuredMethodResolver extends ReflectiveMethodResolver {

    private static final Logger logger = LoggerFactory.getLogger(SecuredMethodResolver.class);
    private static final Method[] EMPTY = new Method[0];
    public static final String WHITELIST_MODE = "append";
    public static final String EL_WHITELIST_MODE_KEY = "el.whitelist.mode";
    public static final String EL_WHITELIST_LIST_KEY = "el.whitelist.list";
    public static final String WHITELIST_METHOD_PREFIX = "method ";
    public static final String WHITELIST_CLASS_PREFIX = "class ";

    private static SecuredMethodResolver INSTANCE;
    private final static Map, Method[]> methodsByType = new ConcurrentHashMap<>();
    private final static Map, Method[]> methodsByTypeAndSuperTypes = new ConcurrentHashMap<>();

    /**
     * Initialize the method resolver loading all whitelisted methods from environment configuration and / or built-in whitelist.
     * Once initialized, instance of {@link SecuredMethodResolver} can be retrieved using {@link SecuredMethodResolver#INSTANCE}.
     *
     * @param environment an optional environment, if null, only built-in whitelist will be initialized.
     */
    public static void initialize(@Nullable Environment environment) {

        loadWhitelistMethods(environment);

        // Force instance creation if not already done.
        getInstance();
    }

    static SecuredMethodResolver getInstance() {

        if (INSTANCE == null) {
            INSTANCE = new SecuredMethodResolver();
        }

        return INSTANCE;
    }

    private SecuredMethodResolver() {
    }

    @Override
    protected Method[] getMethods(Class type) {

        if (methodsByTypeAndSuperTypes.containsKey(type)) {
            return methodsByTypeAndSuperTypes.get(type);
        }

        Method[] methods = methodsByType.getOrDefault(type, EMPTY);

        if (type.getSuperclass() != null) {
            methods = Stream.concat(Arrays.stream(methods), Arrays.stream(getMethods(type.getSuperclass()))).toArray(Method[]::new);
        }

        for (Class anInterface : type.getInterfaces()) {
            methods = Stream.concat(Arrays.stream(methods), Arrays.stream(getMethods(anInterface))).toArray(Method[]::new);
        }

        methodsByTypeAndSuperTypes.put(type, methods);

        return methods;
    }

    private static void loadWhitelistMethods(Environment environment) {

        List methods = new ArrayList<>();
        boolean loadBuiltInWhitelist = true;

        // Load whitelist from configuration.
        if (environment != null) {
            // Built-in whitelist will not be loaded if mode is not 'append' (ie: set to 'replace').
            loadBuiltInWhitelist = WHITELIST_MODE.equals(environment.getProperty(EL_WHITELIST_MODE_KEY, WHITELIST_MODE));

            Collection