/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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.apache.shiro.spring.web;

import org.apache.shiro.config.Ini;
import org.apache.shiro.mgt.SecurityManager;
import org.apache.shiro.util.CollectionUtils;
import org.apache.shiro.lang.util.Nameable;
import org.apache.shiro.lang.util.StringUtils;
import org.apache.shiro.web.config.IniFilterChainResolverFactory;
import org.apache.shiro.web.config.ShiroFilterConfiguration;
import org.apache.shiro.web.filter.AccessControlFilter;
import org.apache.shiro.web.filter.InvalidRequestFilter;
import org.apache.shiro.web.filter.authc.AuthenticationFilter;
import org.apache.shiro.web.filter.authz.AuthorizationFilter;
import org.apache.shiro.web.filter.mgt.DefaultFilter;
import org.apache.shiro.web.filter.mgt.DefaultFilterChainManager;
import org.apache.shiro.web.filter.mgt.FilterChainManager;
import org.apache.shiro.web.filter.mgt.FilterChainResolver;
import org.apache.shiro.web.filter.mgt.PathMatchingFilterChainResolver;
import org.apache.shiro.web.mgt.WebSecurityManager;
import org.apache.shiro.web.servlet.AbstractShiroFilter;
import org.apache.shiro.web.servlet.OncePerRequestFilter;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.BeansException;
import org.springframework.beans.factory.BeanInitializationException;
import org.springframework.beans.factory.FactoryBean;
import org.springframework.beans.factory.config.BeanPostProcessor;

import javax.servlet.Filter;
import java.util.ArrayList;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;

/**
 * {@link org.springframework.beans.factory.FactoryBean FactoryBean} to be used in Spring-based web applications for
 * defining the master Shiro Filter.
 * <h4>Usage</h4>
 * Declare a DelegatingFilterProxy in {@code web.xml}, matching the filter name to the bean id:
 * <pre>
 * &lt;filter&gt;
 *   &lt;filter-name&gt;<b>shiroFilter</b>&lt;/filter-name&gt;
 *   &lt;filter-class&gt;org.springframework.web.filter.DelegatingFilterProxy&lt;filter-class&gt;
 *   &lt;init-param&gt;
 *    &lt;param-name&gt;targetFilterLifecycle&lt;/param-name&gt;
 *     &lt;param-value&gt;true&lt;/param-value&gt;
 *   &lt;/init-param&gt;
 * &lt;/filter&gt;
 * </pre>
 * Then, in your spring XML file that defines your web ApplicationContext:
 * <pre>
 * &lt;bean id="<b>shiroFilter</b>" class="org.apache.shiro.spring.web.ShiroFilterFactoryBean"&gt;
 *    &lt;property name="securityManager" ref="securityManager"/&gt;
 *    &lt;!-- other properties as necessary ... --&gt;
 * &lt;/bean&gt;
 * </pre>
 * <h4>Filter Auto-Discovery</h4>
 * While there is a {@link #setFilters(java.util.Map) filters} property that allows you to assign a filter beans
 * to the 'pool' of filters available when defining {@link #setFilterChainDefinitions(String) filter chains}, it is
 * optional.
 * <p/>
 * This implementation is also a {@link BeanPostProcessor} and will acquire
 * any {@link javax.servlet.Filter Filter} beans defined independently in your Spring application context.  Upon
 * discovery, they will be automatically added to the {@link #setFilters(java.util.Map) map} keyed by the bean ID.
 * That ID can then be used in the filter chain definitions, for example:
 *
 * <pre>
 * &lt;bean id="<b>myCustomFilter</b>" class="com.class.that.implements.javax.servlet.Filter"/&gt;
 * ...
 * &lt;bean id="shiroFilter" class="org.apache.shiro.spring.web.ShiroFilterFactoryBean"&gt;
 *    ...
 *    &lt;property name="filterChainDefinitions"&gt;
 *        &lt;value&gt;
 *            /some/path/** = authc, <b>myCustomFilter</b>
 *        &lt;/value&gt;
 *    &lt;/property&gt;
 * &lt;/bean&gt;
 * </pre>
 * <h4>Global Property Values</h4>
 * Most Shiro servlet Filter implementations exist for defining custom Filter
 * {@link #setFilterChainDefinitions(String) chain definitions}.  Most implementations subclass one of the
 * {@link AccessControlFilter}, {@link AuthenticationFilter}, {@link AuthorizationFilter} classes to simplify things,
 * and each of these 3 classes has configurable properties that are application-specific.
 * <p/>
 * A dilemma arises where, if you want to for example set the application's 'loginUrl' for any Filter, you don't want
 * to have to manually specify that value for <em>each</em> filter instance defined.
 * <p/>
 * To prevent configuration duplication, this implementation provides the following properties to allow you
 * to set relevant values in only one place:
 * <ul>
 * <li>{@link #setLoginUrl(String)}</li>
 * <li>{@link #setSuccessUrl(String)}</li>
 * <li>{@link #setUnauthorizedUrl(String)}</li>
 * </ul>
 *
 * Then at startup, any values specified via these 3 properties will be applied to all configured
 * Filter instances so you don't have to specify them individually on each filter instance.  To ensure your own custom
 * filters benefit from this convenience, your filter implementation should subclass one of the 3 mentioned
 * earlier.
 *
 * @see org.springframework.web.filter.DelegatingFilterProxy DelegatingFilterProxy
 * @since 1.0
 */
public class ShiroFilterFactoryBean implements FactoryBean, BeanPostProcessor {

    private static transient final Logger log = LoggerFactory.getLogger(ShiroFilterFactoryBean.class);

    private SecurityManager securityManager;

    private Map<String, Filter> filters;

    private List<String> globalFilters;

    private Map<String, String> filterChainDefinitionMap; //urlPathExpression_to_comma-delimited-filter-chain-definition

    private String loginUrl;
    private String successUrl;
    private String unauthorizedUrl;

    private AbstractShiroFilter instance;

    private ShiroFilterConfiguration filterConfiguration;

    public ShiroFilterFactoryBean() {
        this.filters = new LinkedHashMap<String, Filter>();
        this.globalFilters = new ArrayList<>();
        this.globalFilters.add(DefaultFilter.invalidRequest.name());
        this.filterChainDefinitionMap = new LinkedHashMap<String, String>(); //order matters!
        this.filterConfiguration = new ShiroFilterConfiguration();
    }

    /**
     * Gets the application {@code SecurityManager} instance to be used by the constructed Shiro Filter.  This is a
     * required property - failure to set it will throw an initialization exception.
     *
     * @return the application {@code SecurityManager} instance to be used by the constructed Shiro Filter.
     */
    public SecurityManager getSecurityManager() {
        return securityManager;
    }

    /**
     * Sets the application {@code SecurityManager} instance to be used by the constructed Shiro Filter.  This is a
     * required property - failure to set it will throw an initialization exception.
     *
     * @param securityManager the application {@code SecurityManager} instance to be used by the constructed Shiro Filter.
     */
    public void setSecurityManager(SecurityManager securityManager) {
        this.securityManager = securityManager;
    }

    /**
     * Gets the application {@code ShiroFilterConfiguration} instance to be used by the constructed Shiro Filter.
     *
     * @return the application {@code ShiroFilterConfiguration} instance to be used by the constructed Shiro Filter.
     */
    public ShiroFilterConfiguration getShiroFilterConfiguration() {
        return filterConfiguration;
    }

    /**
     * Sets the application {@code ShiroFilterConfiguration} instance to be used by the constructed Shiro Filter.
     *
     * @param filterConfiguration the application {@code SecurityManager} instance to be used by the constructed Shiro Filter.
     */
    public void setShiroFilterConfiguration(ShiroFilterConfiguration filterConfiguration) {
        this.filterConfiguration = filterConfiguration;
    }

    /**
     * Returns the application's login URL to be assigned to all acquired Filters that subclass
     * {@link AccessControlFilter} or {@code null} if no value should be assigned globally. The default value
     * is {@code null}.
     *
     * @return the application's login URL to be assigned to all acquired Filters that subclass
     *         {@link AccessControlFilter} or {@code null} if no value should be assigned globally.
     * @see #setLoginUrl
     */
    public String getLoginUrl() {
        return loginUrl;
    }

    /**
     * Sets the application's login URL to be assigned to all acquired Filters that subclass
     * {@link AccessControlFilter}.  This is a convenience mechanism: for all configured {@link #setFilters filters},
     * as well for any default ones ({@code authc}, {@code user}, etc.), this value will be passed on to each Filter
     * via the {@link AccessControlFilter#setLoginUrl(String)} method<b>*</b>.  This eliminates the need to
     * configure the 'loginUrl' property manually on each filter instance, and instead that can be configured once
     * via this attribute.
     * <p/>
     * <b>*</b>If a filter already has already been explicitly configured with a value, it will
     * <em>not</em> receive this value. Individual filter configuration overrides this global convenience property.
     *
     * @param loginUrl the application's login URL to apply to as a convenience to all discovered
     *                 {@link AccessControlFilter} instances.
     * @see AccessControlFilter#setLoginUrl(String)
     */
    public void setLoginUrl(String loginUrl) {
        this.loginUrl = loginUrl;
    }

    /**
     * Returns the application's after-login success URL to be assigned to all acquired Filters that subclass
     * {@link AuthenticationFilter} or {@code null} if no value should be assigned globally. The default value
     * is {@code null}.
     *
     * @return the application's after-login success URL to be assigned to all acquired Filters that subclass
     *         {@link AuthenticationFilter} or {@code null} if no value should be assigned globally.
     * @see #setSuccessUrl
     */
    public String getSuccessUrl() {
        return successUrl;
    }

    /**
     * Sets the application's after-login success URL to be assigned to all acquired Filters that subclass
     * {@link AuthenticationFilter}.  This is a convenience mechanism: for all configured {@link #setFilters filters},
     * as well for any default ones ({@code authc}, {@code user}, etc.), this value will be passed on to each Filter
     * via the {@link AuthenticationFilter#setSuccessUrl(String)} method<b>*</b>.  This eliminates the need to
     * configure the 'successUrl' property manually on each filter instance, and instead that can be configured once
     * via this attribute.
     * <p/>
     * <b>*</b>If a filter already has already been explicitly configured with a value, it will
     * <em>not</em> receive this value. Individual filter configuration overrides this global convenience property.
     *
     * @param successUrl the application's after-login success URL to apply to as a convenience to all discovered
     *                   {@link AccessControlFilter} instances.
     * @see AuthenticationFilter#setSuccessUrl(String)
     */
    public void setSuccessUrl(String successUrl) {
        this.successUrl = successUrl;
    }

    /**
     * Returns the application's after-login success URL to be assigned to all acquired Filters that subclass
     * {@link AuthenticationFilter} or {@code null} if no value should be assigned globally. The default value
     * is {@code null}.
     *
     * @return the application's after-login success URL to be assigned to all acquired Filters that subclass
     *         {@link AuthenticationFilter} or {@code null} if no value should be assigned globally.
     * @see #setSuccessUrl
     */
    public String getUnauthorizedUrl() {
        return unauthorizedUrl;
    }

    /**
     * Sets the application's 'unauthorized' URL to be assigned to all acquired Filters that subclass
     * {@link AuthorizationFilter}.  This is a convenience mechanism: for all configured {@link #setFilters filters},
     * as well for any default ones ({@code roles}, {@code perms}, etc.), this value will be passed on to each Filter
     * via the {@link AuthorizationFilter#setUnauthorizedUrl(String)} method<b>*</b>.  This eliminates the need to
     * configure the 'unauthorizedUrl' property manually on each filter instance, and instead that can be configured once
     * via this attribute.
     * <p/>
     * <b>*</b>If a filter already has already been explicitly configured with a value, it will
     * <em>not</em> receive this value. Individual filter configuration overrides this global convenience property.
     *
     * @param unauthorizedUrl the application's 'unauthorized' URL to apply to as a convenience to all discovered
     *                        {@link AuthorizationFilter} instances.
     * @see AuthorizationFilter#setUnauthorizedUrl(String)
     */
    public void setUnauthorizedUrl(String unauthorizedUrl) {
        this.unauthorizedUrl = unauthorizedUrl;
    }

    /**
     * Returns the filterName-to-Filter map of filters available for reference when defining filter chain definitions.
     * All filter chain definitions will reference filters by the names in this map (i.e. the keys).
     *
     * @return the filterName-to-Filter map of filters available for reference when defining filter chain definitions.
     */
    public Map<String, Filter> getFilters() {
        return filters;
    }

    /**
     * Sets the filterName-to-Filter map of filters available for reference when creating
     * {@link #setFilterChainDefinitionMap(java.util.Map) filter chain definitions}.
     * <p/>
     * <b>Note:</b> This property is optional:  this {@code FactoryBean} implementation will discover all beans in the
     * web application context that implement the {@link Filter} interface and automatically add them to this filter
     * map under their bean name.
     * <p/>
     * For example, just defining this bean in a web Spring XML application context:
     * <pre>
     * &lt;bean id=&quot;myFilter&quot; class=&quot;com.class.that.implements.javax.servlet.Filter&quot;&gt;
     * ...
     * &lt;/bean&gt;</pre>
     * Will automatically place that bean into this Filters map under the key '<b>myFilter</b>'.
     *
     * @param filters the optional filterName-to-Filter map of filters available for reference when creating
     *                {@link #setFilterChainDefinitionMap (java.util.Map) filter chain definitions}.
     */
    public void setFilters(Map<String, Filter> filters) {
        this.filters = filters;
    }

    /**
     * Returns the chainName-to-chainDefinition map of chain definitions to use for creating filter chains intercepted
     * by the Shiro Filter.  Each map entry should conform to the format defined by the
     * {@link FilterChainManager#createChain(String, String)} JavaDoc, where the map key is the chain name (e.g. URL
     * path expression) and the map value is the comma-delimited string chain definition.
     *
     * @return he chainName-to-chainDefinition map of chain definitions to use for creating filter chains intercepted
     *         by the Shiro Filter.
     */
    public Map<String, String> getFilterChainDefinitionMap() {
        return filterChainDefinitionMap;
    }

    /**
     * Sets the chainName-to-chainDefinition map of chain definitions to use for creating filter chains intercepted
     * by the Shiro Filter.  Each map entry should conform to the format defined by the
     * {@link FilterChainManager#createChain(String, String)} JavaDoc, where the map key is the chain name (e.g. URL
     * path expression) and the map value is the comma-delimited string chain definition.
     *
     * @param filterChainDefinitionMap the chainName-to-chainDefinition map of chain definitions to use for creating
     *                                 filter chains intercepted by the Shiro Filter.
     */
    public void setFilterChainDefinitionMap(Map<String, String> filterChainDefinitionMap) {
        this.filterChainDefinitionMap = filterChainDefinitionMap;
    }

    /**
     * A convenience method that sets the {@link #setFilterChainDefinitionMap(java.util.Map) filterChainDefinitionMap}
     * property by accepting a {@link java.util.Properties Properties}-compatible string (multi-line key/value pairs).
     * Each key/value pair must conform to the format defined by the
     * {@link FilterChainManager#createChain(String,String)} JavaDoc - each property key is an ant URL
     * path expression and the value is the comma-delimited chain definition.
     *
     * @param definitions a {@link java.util.Properties Properties}-compatible string (multi-line key/value pairs)
     *                    where each key/value pair represents a single urlPathExpression-commaDelimitedChainDefinition.
     */
    public void setFilterChainDefinitions(String definitions) {
        Ini ini = new Ini();
        ini.load(definitions);
        //did they explicitly state a 'urls' section?  Not necessary, but just in case:
        Ini.Section section = ini.getSection(IniFilterChainResolverFactory.URLS);
        if (CollectionUtils.isEmpty(section)) {
            //no urls section.  Since this _is_ a urls chain definition property, just assume the
            //default section contains only the definitions:
            section = ini.getSection(Ini.DEFAULT_SECTION_NAME);
        }
        setFilterChainDefinitionMap(section);
    }

    /**
     * Sets the list of filters that will be executed against every request.  Defaults to the {@link InvalidRequestFilter} which will block known invalid request attacks.
     * @param globalFilters the list of filters to execute before specific path filters.
     */
    public void setGlobalFilters(List<String> globalFilters) {
        this.globalFilters = globalFilters;
    }

    /**
     * Lazily creates and returns a {@link AbstractShiroFilter} concrete instance via the
     * {@link #createInstance} method.
     *
     * @return the application's Shiro Filter instance used to filter incoming web requests.
     * @throws Exception if there is a problem creating the {@code Filter} instance.
     */
    public Object getObject() throws Exception {
        if (instance == null) {
            instance = createInstance();
        }
        return instance;
    }

    /**
     * Returns <code>{@link org.apache.shiro.web.servlet.AbstractShiroFilter}.class</code>
     *
     * @return <code>{@link org.apache.shiro.web.servlet.AbstractShiroFilter}.class</code>
     */
    public Class getObjectType() {
        return SpringShiroFilter.class;
    }

    /**
     * Returns {@code true} always.  There is almost always only ever 1 Shiro {@code Filter} per web application.
     *
     * @return {@code true} always.  There is almost always only ever 1 Shiro {@code Filter} per web application.
     */
    public boolean isSingleton() {
        return true;
    }

    protected FilterChainManager createFilterChainManager() {

        DefaultFilterChainManager manager = new DefaultFilterChainManager();
        Map<String, Filter> defaultFilters = manager.getFilters();
        //apply global settings if necessary:
        for (Filter filter : defaultFilters.values()) {
            applyGlobalPropertiesIfNecessary(filter);
        }

        //Apply the acquired and/or configured filters:
        Map<String, Filter> filters = getFilters();
        if (!CollectionUtils.isEmpty(filters)) {
            for (Map.Entry<String, Filter> entry : filters.entrySet()) {
                String name = entry.getKey();
                Filter filter = entry.getValue();
                applyGlobalPropertiesIfNecessary(filter);
                if (filter instanceof Nameable) {
                    ((Nameable) filter).setName(name);
                }
                //'init' argument is false, since Spring-configured filters should be initialized
                //in Spring (i.e. 'init-method=blah') or implement InitializingBean:
                manager.addFilter(name, filter, false);
            }
        }

        // set the global filters
        manager.setGlobalFilters(this.globalFilters);

        //build up the chains:
        Map<String, String> chains = getFilterChainDefinitionMap();
        if (!CollectionUtils.isEmpty(chains)) {
            for (Map.Entry<String, String> entry : chains.entrySet()) {
                String url = entry.getKey();
                String chainDefinition = entry.getValue();
                manager.createChain(url, chainDefinition);
            }
        }

        // create the default chain, to match anything the path matching would have missed
        manager.createDefaultChain("/**"); // TODO this assumes ANT path matching, which might be OK here

        return manager;
    }

    /**
     * This implementation:
     * <ol>
     * <li>Ensures the required {@link #setSecurityManager(org.apache.shiro.mgt.SecurityManager) securityManager}
     * property has been set</li>
     * <li>{@link #createFilterChainManager() Creates} a {@link FilterChainManager} instance that reflects the
     * configured {@link #setFilters(java.util.Map) filters} and
     * {@link #setFilterChainDefinitionMap(java.util.Map) filter chain definitions}</li>
     * <li>Wraps the FilterChainManager with a suitable
     * {@link org.apache.shiro.web.filter.mgt.FilterChainResolver FilterChainResolver} since the Shiro Filter
     * implementations do not know of {@code FilterChainManager}s</li>
     * <li>Sets both the {@code SecurityManager} and {@code FilterChainResolver} instances on a new Shiro Filter
     * instance and returns that filter instance.</li>
     * </ol>
     *
     * @return a new Shiro Filter reflecting any configured filters and filter chain definitions.
     * @throws Exception if there is a problem creating the AbstractShiroFilter instance.
     */
    protected AbstractShiroFilter createInstance() throws Exception {

        log.debug("Creating Shiro Filter instance.");

        SecurityManager securityManager = getSecurityManager();
        if (securityManager == null) {
            String msg = "SecurityManager property must be set.";
            throw new BeanInitializationException(msg);
        }

        if (!(securityManager instanceof WebSecurityManager)) {
            String msg = "The security manager does not implement the WebSecurityManager interface.";
            throw new BeanInitializationException(msg);
        }

        FilterChainManager manager = createFilterChainManager();

        //Expose the constructed FilterChainManager by first wrapping it in a
        // FilterChainResolver implementation. The AbstractShiroFilter implementations
        // do not know about FilterChainManagers - only resolvers:
        PathMatchingFilterChainResolver chainResolver = new PathMatchingFilterChainResolver();
        chainResolver.setFilterChainManager(manager);

        //Now create a concrete ShiroFilter instance and apply the acquired SecurityManager and built
        //FilterChainResolver.  It doesn't matter that the instance is an anonymous inner class
        //here - we're just using it because it is a concrete AbstractShiroFilter instance that accepts
        //injection of the SecurityManager and FilterChainResolver:
        return new SpringShiroFilter((WebSecurityManager) securityManager, chainResolver, getShiroFilterConfiguration());
    }

    private void applyLoginUrlIfNecessary(Filter filter) {
        String loginUrl = getLoginUrl();
        if (StringUtils.hasText(loginUrl) && (filter instanceof AccessControlFilter)) {
            AccessControlFilter acFilter = (AccessControlFilter) filter;
            //only apply the login url if they haven't explicitly configured one already:
            String existingLoginUrl = acFilter.getLoginUrl();
            if (AccessControlFilter.DEFAULT_LOGIN_URL.equals(existingLoginUrl)) {
                acFilter.setLoginUrl(loginUrl);
            }
        }
    }

    private void applySuccessUrlIfNecessary(Filter filter) {
        String successUrl = getSuccessUrl();
        if (StringUtils.hasText(successUrl) && (filter instanceof AuthenticationFilter)) {
            AuthenticationFilter authcFilter = (AuthenticationFilter) filter;
            //only apply the successUrl if they haven't explicitly configured one already:
            String existingSuccessUrl = authcFilter.getSuccessUrl();
            if (AuthenticationFilter.DEFAULT_SUCCESS_URL.equals(existingSuccessUrl)) {
                authcFilter.setSuccessUrl(successUrl);
            }
        }
    }

    private void applyUnauthorizedUrlIfNecessary(Filter filter) {
        String unauthorizedUrl = getUnauthorizedUrl();
        if (StringUtils.hasText(unauthorizedUrl) && (filter instanceof AuthorizationFilter)) {
            AuthorizationFilter authzFilter = (AuthorizationFilter) filter;
            //only apply the unauthorizedUrl if they haven't explicitly configured one already:
            String existingUnauthorizedUrl = authzFilter.getUnauthorizedUrl();
            if (existingUnauthorizedUrl == null) {
                authzFilter.setUnauthorizedUrl(unauthorizedUrl);
            }
        }
    }

    private void applyGlobalPropertiesIfNecessary(Filter filter) {
        applyLoginUrlIfNecessary(filter);
        applySuccessUrlIfNecessary(filter);
        applyUnauthorizedUrlIfNecessary(filter);

        if (filter instanceof OncePerRequestFilter) {
            ((OncePerRequestFilter) filter).setFilterOncePerRequest(filterConfiguration.isFilterOncePerRequest());
        }
    }

    /**
     * Inspects a bean, and if it implements the {@link Filter} interface, automatically adds that filter
     * instance to the internal {@link #setFilters(java.util.Map) filters map} that will be referenced
     * later during filter chain construction.
     */
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        if (bean instanceof Filter) {
            log.debug("Found filter chain candidate filter '{}'", beanName);
            Filter filter = (Filter) bean;
            applyGlobalPropertiesIfNecessary(filter);
            getFilters().put(beanName, filter);
        } else {
            log.trace("Ignoring non-Filter bean '{}'", beanName);
        }
        return bean;
    }

    /**
     * Does nothing - only exists to satisfy the BeanPostProcessor interface and immediately returns the
     * {@code bean} argument.
     */
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        return bean;
    }

    /**
     * Ordinarily the {@code AbstractShiroFilter} must be subclassed to additionally perform configuration
     * and initialization behavior.  Because this {@code FactoryBean} implementation manually builds the
     * {@link AbstractShiroFilter}'s
     * {@link AbstractShiroFilter#setSecurityManager(org.apache.shiro.web.mgt.WebSecurityManager) securityManager} and
     * {@link AbstractShiroFilter#setFilterChainResolver(org.apache.shiro.web.filter.mgt.FilterChainResolver) filterChainResolver}
     * properties, the only thing left to do is set those properties explicitly.  We do that in a simple
     * concrete subclass in the constructor.
     */
    private static final class SpringShiroFilter extends AbstractShiroFilter {

        protected SpringShiroFilter(WebSecurityManager webSecurityManager, FilterChainResolver resolver, ShiroFilterConfiguration filterConfiguration) {
            super();
            if (webSecurityManager == null) {
                throw new IllegalArgumentException("WebSecurityManager property cannot be null.");
            }
            setSecurityManager(webSecurityManager);
            setShiroFilterConfiguration(filterConfiguration);

            if (resolver != null) {
                setFilterChainResolver(resolver);
            }
        }
    }
}
