/*
 * 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.catalina.filters;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.net.URI;
import java.net.URISyntaxException;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.List;
import java.util.Locale;
import java.util.Set;

import javax.servlet.FilterChain;
import javax.servlet.GenericFilter;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.apache.juli.logging.Log;
import org.apache.juli.logging.LogFactory;
import org.apache.tomcat.util.http.ResponseUtil;
import org.apache.tomcat.util.res.StringManager;

/**
 * <p>
 * A {@link javax.servlet.Filter} that enable client-side cross-origin requests
 * by implementing W3C's CORS (<b>C</b>ross-<b>O</b>rigin <b>R</b>esource
 * <b>S</b>haring) specification for resources. Each {@link HttpServletRequest}
 * request is inspected as per specification, and appropriate response headers
 * are added to {@link HttpServletResponse}.
 * </p>
 *
 * <p>
 * By default, it also sets following request attributes, that help to
 * determine the nature of the request downstream.
 * </p>
 * <ul>
 * <li><b>cors.isCorsRequest:</b> Flag to determine if the request is a CORS
 * request. Set to <code>true</code> if a CORS request; <code>false</code>
 * otherwise.</li>
 * <li><b>cors.request.origin:</b> The Origin URL, i.e. the URL of the page from
 * where the request is originated.</li>
 * <li>
 * <b>cors.request.type:</b> Type of request. Possible values:
 * <ul>
 * <li>SIMPLE: A request which is not preceded by a pre-flight request.</li>
 * <li>ACTUAL: A request which is preceded by a pre-flight request.</li>
 * <li>PRE_FLIGHT: A pre-flight request.</li>
 * <li>NOT_CORS: A normal same-origin request.</li>
 * <li>INVALID_CORS: A cross-origin request which is invalid.</li>
 * </ul>
 * </li>
 * <li><b>cors.request.headers:</b> Request headers sent as
 * 'Access-Control-Request-Headers' header, for pre-flight request.</li>
 * </ul>
 *
 * @see <a href="http://www.w3.org/TR/cors/">CORS specification</a>
 *
 */
public class CorsFilter extends GenericFilter {

    private static final long serialVersionUID = 1L;
    private static final StringManager sm = StringManager.getManager(CorsFilter.class);

    private transient Log log = LogFactory.getLog(CorsFilter.class); // must not be static


    /**
     * A {@link Collection} of origins consisting of zero or more origins that
     * are allowed access to the resource.
     */
    private final Collection<String> allowedOrigins = new HashSet<>();

    /**
     * Determines if any origin is allowed to make request.
     */
    private boolean anyOriginAllowed;

    /**
     * A {@link Collection} of methods consisting of zero or more methods that
     * are supported by the resource.
     */
    private final Collection<String> allowedHttpMethods = new HashSet<>();

    /**
     * A {@link Collection} of headers consisting of zero or more header field
     * names that are supported by the resource.
     */
    private final Collection<String> allowedHttpHeaders = new HashSet<>();

    /**
     * A {@link Collection} of exposed headers consisting of zero or more header
     * field names of headers other than the simple response headers that the
     * resource might use and can be exposed.
     */
    private final Collection<String> exposedHeaders = new HashSet<>();

    /**
     * A supports credentials flag that indicates whether the resource supports
     * user credentials in the request. It is true when the resource does and
     * false otherwise.
     */
    private boolean supportsCredentials;

    /**
     * Indicates (in seconds) how long the results of a pre-flight request can
     * be cached in a pre-flight result cache.
     */
    private long preflightMaxAge;

    /**
     * Determines if the request should be decorated or not.
     */
    private boolean decorateRequest;


    @Override
    public void doFilter(final ServletRequest servletRequest,
            final ServletResponse servletResponse, final FilterChain filterChain)
            throws IOException, ServletException {
        if (!(servletRequest instanceof HttpServletRequest) ||
                !(servletResponse instanceof HttpServletResponse)) {
            throw new ServletException(sm.getString("corsFilter.onlyHttp"));
        }

        // Safe to downcast at this point.
        HttpServletRequest request = (HttpServletRequest) servletRequest;
        HttpServletResponse response = (HttpServletResponse) servletResponse;

        // Determines the CORS request type.
        CorsFilter.CORSRequestType requestType = checkRequestType(request);

        // Adds CORS specific attributes to request.
        if (decorateRequest) {
            CorsFilter.decorateCORSProperties(request, requestType);
        }
        switch (requestType) {
        case SIMPLE:
            // Handles a Simple CORS request.
        case ACTUAL:
            // Handles an Actual CORS request.
            this.handleSimpleCORS(request, response, filterChain);
            break;
        case PRE_FLIGHT:
            // Handles a Pre-flight CORS request.
            this.handlePreflightCORS(request, response, filterChain);
            break;
        case NOT_CORS:
            // Handles a Normal request that is not a cross-origin request.
            this.handleNonCORS(request, response, filterChain);
            break;
        default:
            // Handles a CORS request that violates specification.
            this.handleInvalidCORS(request, response, filterChain);
            break;
        }
    }


    @Override
    public void init() throws ServletException {
        parseAndStore(
                getInitParameter(PARAM_CORS_ALLOWED_ORIGINS,     DEFAULT_ALLOWED_ORIGINS),
                getInitParameter(PARAM_CORS_ALLOWED_METHODS,     DEFAULT_ALLOWED_HTTP_METHODS),
                getInitParameter(PARAM_CORS_ALLOWED_HEADERS,     DEFAULT_ALLOWED_HTTP_HEADERS),
                getInitParameter(PARAM_CORS_EXPOSED_HEADERS,     DEFAULT_EXPOSED_HEADERS),
                getInitParameter(PARAM_CORS_SUPPORT_CREDENTIALS, DEFAULT_SUPPORTS_CREDENTIALS),
                getInitParameter(PARAM_CORS_PREFLIGHT_MAXAGE,    DEFAULT_PREFLIGHT_MAXAGE),
                getInitParameter(PARAM_CORS_REQUEST_DECORATE,    DEFAULT_DECORATE_REQUEST)
        );
    }


    /**
     * This method returns the parameter's value if it exists, or defaultValue
     * if not.
     *
     * @param name          The parameter's name
     *
     * @param defaultValue  The default value to return if the parameter does
     *                      not exist
     *
     * @return The parameter's value or the default value if the parameter does
     *         not exist
     */
    private String getInitParameter(String name, String defaultValue) {

        String value = getInitParameter(name);
        if (value != null) {
            return value;
        }

        return defaultValue;
    }


    /**
     * Handles a CORS request of type {@link CORSRequestType}.SIMPLE.
     *
     * @param request The {@link HttpServletRequest} object.
     * @param response The {@link HttpServletResponse} object.
     * @param filterChain The {@link FilterChain} object.
     * @throws IOException an IO error occurred
     * @throws ServletException Servlet error propagation
     * @see <a href="http://www.w3.org/TR/cors/#resource-requests">Simple
     *      Cross-Origin Request, Actual Request, and Redirects</a>
     */
    protected void handleSimpleCORS(final HttpServletRequest request,
            final HttpServletResponse response, final FilterChain filterChain)
            throws IOException, ServletException {

        CorsFilter.CORSRequestType requestType = checkRequestType(request);
        if (!(requestType == CorsFilter.CORSRequestType.SIMPLE ||
                requestType == CorsFilter.CORSRequestType.ACTUAL)) {
            throw new IllegalArgumentException(
                    sm.getString("corsFilter.wrongType2",
                            CorsFilter.CORSRequestType.SIMPLE,
                            CorsFilter.CORSRequestType.ACTUAL));
        }

        final String origin = request.getHeader(CorsFilter.REQUEST_HEADER_ORIGIN);
        final String method = request.getMethod();

        // Section 6.1.2
        if (!isOriginAllowed(origin)) {
            handleInvalidCORS(request, response, filterChain);
            return;
        }

        if (!allowedHttpMethods.contains(method)) {
            handleInvalidCORS(request, response, filterChain);
            return;
        }

        addStandardHeaders(request, response);

        // Forward the request down the filter chain.
        filterChain.doFilter(request, response);
    }


    /**
     * Handles CORS pre-flight request.
     *
     * @param request The {@link HttpServletRequest} object.
     * @param response The {@link HttpServletResponse} object.
     * @param filterChain The {@link FilterChain} object.
     * @throws IOException an IO error occurred
     * @throws ServletException Servlet error propagation
     */
    protected void handlePreflightCORS(final HttpServletRequest request,
            final HttpServletResponse response, final FilterChain filterChain)
            throws IOException, ServletException {

        CORSRequestType requestType = checkRequestType(request);
        if (requestType != CORSRequestType.PRE_FLIGHT) {
            throw new IllegalArgumentException(sm.getString("corsFilter.wrongType1",
                    CORSRequestType.PRE_FLIGHT.name().toLowerCase(Locale.ENGLISH)));
        }

        final String origin = request.getHeader(CorsFilter.REQUEST_HEADER_ORIGIN);

        // Section 6.2.2
        if (!isOriginAllowed(origin)) {
            handleInvalidCORS(request, response, filterChain);
            return;
        }

        // Section 6.2.3
        String accessControlRequestMethod = request.getHeader(
                CorsFilter.REQUEST_HEADER_ACCESS_CONTROL_REQUEST_METHOD);
        if (accessControlRequestMethod == null) {
            handleInvalidCORS(request, response, filterChain);
            return;
        } else {
            accessControlRequestMethod = accessControlRequestMethod.trim();
        }

        // Section 6.2.4
        String accessControlRequestHeadersHeader = request.getHeader(
                CorsFilter.REQUEST_HEADER_ACCESS_CONTROL_REQUEST_HEADERS);
        List<String> accessControlRequestHeaders = new LinkedList<>();
        if (accessControlRequestHeadersHeader != null &&
                !accessControlRequestHeadersHeader.trim().isEmpty()) {
            String[] headers = accessControlRequestHeadersHeader.trim().split(",");
            for (String header : headers) {
                accessControlRequestHeaders.add(header.trim().toLowerCase(Locale.ENGLISH));
            }
        }

        // Section 6.2.5
        if (!allowedHttpMethods.contains(accessControlRequestMethod)) {
            handleInvalidCORS(request, response, filterChain);
            return;
        }

        // Section 6.2.6
        if (!accessControlRequestHeaders.isEmpty()) {
            for (String header : accessControlRequestHeaders) {
                if (!allowedHttpHeaders.contains(header)) {
                    handleInvalidCORS(request, response, filterChain);
                    return;
                }
            }
        }

        addStandardHeaders(request, response);

        // Do not forward the request down the filter chain.
    }


    /**
     * Handles a request, that's not a CORS request, but is a valid request i.e.
     * it is not a cross-origin request. This implementation, just forwards the
     * request down the filter chain.
     *
     * @param request The {@link HttpServletRequest} object.
     * @param response The {@link HttpServletResponse} object.
     * @param filterChain The {@link FilterChain} object.
     * @throws IOException an IO error occurred
     * @throws ServletException Servlet error propagation
     */
    private void handleNonCORS(final HttpServletRequest request,
            final HttpServletResponse response, final FilterChain filterChain)
            throws IOException, ServletException {

        addStandardHeaders(request, response);

        // Let request pass.
        filterChain.doFilter(request, response);
    }


    /**
     * Handles a CORS request that violates specification.
     *
     * @param request The {@link HttpServletRequest} object.
     * @param response The {@link HttpServletResponse} object.
     * @param filterChain The {@link FilterChain} object.
     */
    private void handleInvalidCORS(final HttpServletRequest request,
            final HttpServletResponse response, final FilterChain filterChain) {
        String origin = request.getHeader(CorsFilter.REQUEST_HEADER_ORIGIN);
        String method = request.getMethod();
        String accessControlRequestHeaders = request.getHeader(
                REQUEST_HEADER_ACCESS_CONTROL_REQUEST_HEADERS);

        response.setContentType("text/plain");
        response.setStatus(HttpServletResponse.SC_FORBIDDEN);
        response.resetBuffer();

        if (log.isDebugEnabled()) {
            // Debug so no need for i18n
            StringBuilder message = new StringBuilder("Invalid CORS request; Origin=");
            message.append(origin);
            message.append(";Method=");
            message.append(method);
            if (accessControlRequestHeaders != null) {
                message.append(";Access-Control-Request-Headers=");
                message.append(accessControlRequestHeaders);
            }
            log.debug(message.toString());
        }
    }


    /*
     * Sets a standard set of headers to reduce response variation which in turn
     * is intended to aid caching.
     */
    private void addStandardHeaders(final HttpServletRequest request,
            final HttpServletResponse response) {

        final String method = request.getMethod();
        final String origin = request.getHeader(CorsFilter.REQUEST_HEADER_ORIGIN);

        if (!anyOriginAllowed) {
            // If only specific origins are allowed, the response will vary by
            // origin
            ResponseUtil.addVaryFieldName(response, CorsFilter.REQUEST_HEADER_ORIGIN);
        }

        // CORS requests (SIMPLE, ACTUAL, PRE_FLIGHT) set the following headers
        // although non-CORS requests do not need to. The headers are always set
        // as a) they do no harm in the non-CORS case and b) it allows the same
        // response to be cached for CORS and non-CORS requests.

        // Add a single Access-Control-Allow-Origin header.
        if (anyOriginAllowed) {
            // If any origin is allowed, return header with '*'.
            response.addHeader(CorsFilter.RESPONSE_HEADER_ACCESS_CONTROL_ALLOW_ORIGIN, "*");
        } else {
            // Add a single Access-Control-Allow-Origin header, with the value
            // of the Origin header as value.
            response.addHeader(CorsFilter.RESPONSE_HEADER_ACCESS_CONTROL_ALLOW_ORIGIN, origin);
        }

        // If the resource supports credentials, add a single
        // Access-Control-Allow-Credentials header with the case-sensitive
        // string "true" as value.
        if (supportsCredentials) {
            response.addHeader(CorsFilter.RESPONSE_HEADER_ACCESS_CONTROL_ALLOW_CREDENTIALS, "true");
        }

        // If the list of exposed headers is not empty add one or more
        // Access-Control-Expose-Headers headers, with as values the header
        // field names given in the list of exposed headers.
        if ((exposedHeaders != null) && (exposedHeaders.size() > 0)) {
            String exposedHeadersString = join(exposedHeaders, ",");
            response.addHeader(CorsFilter.RESPONSE_HEADER_ACCESS_CONTROL_EXPOSE_HEADERS,
                    exposedHeadersString);
        }

        if ("OPTIONS".equals(method)) {
            // For an OPTIONS request, the response will vary based on the
            // value or absence of the following headers. Hence they need be be
            // included in the Vary header.
            ResponseUtil.addVaryFieldName(
                    response, CorsFilter.REQUEST_HEADER_ACCESS_CONTROL_REQUEST_METHOD);
            ResponseUtil.addVaryFieldName(
                    response, CorsFilter.REQUEST_HEADER_ACCESS_CONTROL_REQUEST_HEADERS);

            // CORS PRE_FLIGHT (OPTIONS) requests set the following headers although
            // non-CORS OPTIONS requests do not need to. The headers are always set
            // as a) they do no harm in the non-CORS case and b) it allows the same
            // response to be cached for CORS and non-CORS requests.

            if (preflightMaxAge > 0) {
                response.addHeader(
                        CorsFilter.RESPONSE_HEADER_ACCESS_CONTROL_MAX_AGE,
                        String.valueOf(preflightMaxAge));
            }

            if  ((allowedHttpMethods != null) && (!allowedHttpMethods.isEmpty())) {
                response.addHeader(
                        CorsFilter.RESPONSE_HEADER_ACCESS_CONTROL_ALLOW_METHODS,
                        join(allowedHttpMethods, ","));
            }

            if ((allowedHttpHeaders != null) && (!allowedHttpHeaders.isEmpty())) {
                response.addHeader(
                        CorsFilter.RESPONSE_HEADER_ACCESS_CONTROL_ALLOW_HEADERS,
                        join(allowedHttpHeaders, ","));
            }
        }
    }


    /**
     * Decorates the {@link HttpServletRequest}, with CORS attributes.
     * <ul>
     * <li><b>cors.isCorsRequest:</b> Flag to determine if request is a CORS
     * request. Set to <code>true</code> if CORS request; <code>false</code>
     * otherwise.</li>
     * <li><b>cors.request.origin:</b> The Origin URL.</li>
     * <li><b>cors.request.type:</b> Type of request. Values:
     * <code>simple</code> or <code>preflight</code> or <code>not_cors</code> or
     * <code>invalid_cors</code></li>
     * <li><b>cors.request.headers:</b> Request headers sent as
     * 'Access-Control-Request-Headers' header, for pre-flight request.</li>
     * </ul>
     *
     * @param request The {@link HttpServletRequest} object.
     * @param corsRequestType The {@link CORSRequestType} object.
     */
    protected static void decorateCORSProperties(
            final HttpServletRequest request,
            final CORSRequestType corsRequestType) {
        if (request == null) {
            throw new IllegalArgumentException(sm.getString("corsFilter.nullRequest"));
        }

        if (corsRequestType == null) {
            throw new IllegalArgumentException(sm.getString("corsFilter.nullRequestType"));
        }

        switch (corsRequestType) {
        case SIMPLE:
        case ACTUAL:
            request.setAttribute(
                    CorsFilter.HTTP_REQUEST_ATTRIBUTE_IS_CORS_REQUEST, Boolean.TRUE);
            request.setAttribute(CorsFilter.HTTP_REQUEST_ATTRIBUTE_ORIGIN,
                    request.getHeader(CorsFilter.REQUEST_HEADER_ORIGIN));
            request.setAttribute(
                    CorsFilter.HTTP_REQUEST_ATTRIBUTE_REQUEST_TYPE,
                    corsRequestType.name().toLowerCase(Locale.ENGLISH));
            break;
        case PRE_FLIGHT:
            request.setAttribute(
                    CorsFilter.HTTP_REQUEST_ATTRIBUTE_IS_CORS_REQUEST,
                    Boolean.TRUE);
            request.setAttribute(CorsFilter.HTTP_REQUEST_ATTRIBUTE_ORIGIN,
                    request.getHeader(CorsFilter.REQUEST_HEADER_ORIGIN));
            request.setAttribute(
                    CorsFilter.HTTP_REQUEST_ATTRIBUTE_REQUEST_TYPE,
                    corsRequestType.name().toLowerCase(Locale.ENGLISH));
            String headers = request.getHeader(
                    REQUEST_HEADER_ACCESS_CONTROL_REQUEST_HEADERS);
            if (headers == null) {
                headers = "";
            }
            request.setAttribute(
                    CorsFilter.HTTP_REQUEST_ATTRIBUTE_REQUEST_HEADERS, headers);
            break;
        case NOT_CORS:
            request.setAttribute(
                    CorsFilter.HTTP_REQUEST_ATTRIBUTE_IS_CORS_REQUEST,
                    Boolean.FALSE);
            break;
        default:
            // Don't set any attributes
            break;
        }
    }


    /**
     * Joins elements of {@link Set} into a string, where each element is
     * separated by the provided separator.
     *
     * @param elements The {@link Set} containing elements to join together.
     * @param joinSeparator The character to be used for separating elements.
     * @return The joined {@link String}; <code>null</code> if elements
     *         {@link Set} is null.
     */
    protected static String join(final Collection<String> elements,
            final String joinSeparator) {
        String separator = ",";
        if (elements == null) {
            return null;
        }
        if (joinSeparator != null) {
            separator = joinSeparator;
        }
        StringBuilder buffer = new StringBuilder();
        boolean isFirst = true;
        for (String element : elements) {
            if (!isFirst) {
                buffer.append(separator);
            } else {
                isFirst = false;
            }

            if (element != null) {
                buffer.append(element);
            }
        }

        return buffer.toString();
    }


    /**
     * Determines the request type.
     *
     * @param request The HTTP Servlet request
     * @return the CORS type
     */
    protected CORSRequestType checkRequestType(final HttpServletRequest request) {
        CORSRequestType requestType = CORSRequestType.INVALID_CORS;
        if (request == null) {
            throw new IllegalArgumentException(
                    sm.getString("corsFilter.nullRequest"));
        }
        String originHeader = request.getHeader(REQUEST_HEADER_ORIGIN);
        // Section 6.1.1 and Section 6.2.1
        if (originHeader != null) {
            if (originHeader.isEmpty()) {
                requestType = CORSRequestType.INVALID_CORS;
            } else if (!isValidOrigin(originHeader)) {
                requestType = CORSRequestType.INVALID_CORS;
            } else if (isLocalOrigin(request, originHeader)) {
                return CORSRequestType.NOT_CORS;
            } else {
                String method = request.getMethod();
                if (method != null) {
                    if ("OPTIONS".equals(method)) {
                        String accessControlRequestMethodHeader =
                                request.getHeader(
                                        REQUEST_HEADER_ACCESS_CONTROL_REQUEST_METHOD);
                        if (accessControlRequestMethodHeader != null &&
                                !accessControlRequestMethodHeader.isEmpty()) {
                            requestType = CORSRequestType.PRE_FLIGHT;
                        } else if (accessControlRequestMethodHeader != null &&
                                accessControlRequestMethodHeader.isEmpty()) {
                            requestType = CORSRequestType.INVALID_CORS;
                        } else {
                            requestType = CORSRequestType.ACTUAL;
                        }
                    } else if ("GET".equals(method) || "HEAD".equals(method)) {
                        requestType = CORSRequestType.SIMPLE;
                    } else if ("POST".equals(method)) {
                        String mediaType = getMediaType(request.getContentType());
                        if (mediaType != null) {
                            if (SIMPLE_HTTP_REQUEST_CONTENT_TYPE_VALUES
                                    .contains(mediaType)) {
                                requestType = CORSRequestType.SIMPLE;
                            } else {
                                requestType = CORSRequestType.ACTUAL;
                            }
                        }
                    } else {
                        requestType = CORSRequestType.ACTUAL;
                    }
                }
            }
        } else {
            requestType = CORSRequestType.NOT_CORS;
        }

        return requestType;
    }


    private boolean isLocalOrigin(HttpServletRequest request, String origin) {

        // Build scheme://host:port from request
        StringBuilder target = new StringBuilder();
        String scheme = request.getScheme();
        if (scheme == null) {
            return false;
        } else {
            scheme = scheme.toLowerCase(Locale.ENGLISH);
        }
        target.append(scheme);
        target.append("://");

        String host = request.getServerName();
        if (host == null) {
            return false;
        }
        target.append(host);

        int port = request.getServerPort();
        if ("http".equals(scheme) && port != 80 ||
                "https".equals(scheme) && port != 443) {
            target.append(':');
            target.append(port);
        }

        return origin.equalsIgnoreCase(target.toString());
    }


    /**
     * Return the lower case, trimmed value of the media type from the content
     * type.
     */
    private String getMediaType(String contentType) {
        if (contentType == null) {
            return null;
        }
        String result = contentType.toLowerCase(Locale.ENGLISH);
        int firstSemiColonIndex = result.indexOf(';');
        if (firstSemiColonIndex > -1) {
            result = result.substring(0, firstSemiColonIndex);
        }
        result = result.trim();
        return result;
    }

    /**
     * Checks if the Origin is allowed to make a CORS request.
     *
     * @param origin
     *            The Origin.
     * @return <code>true</code> if origin is allowed; <code>false</code>
     *         otherwise.
     */
    private boolean isOriginAllowed(final String origin) {
        if (anyOriginAllowed) {
            return true;
        }

        // If 'Origin' header is a case-sensitive match of any of allowed
        // origins, then return true, else return false.
        return allowedOrigins.contains(origin);
    }


    /**
     * Parses each param-value and populates configuration variables. If a param
     * is provided, it overrides the default.
     *
     * @param allowedOrigins
     *            A {@link String} of comma separated origins.
     * @param allowedHttpMethods
     *            A {@link String} of comma separated HTTP methods.
     * @param allowedHttpHeaders
     *            A {@link String} of comma separated HTTP headers.
     * @param exposedHeaders
     *            A {@link String} of comma separated headers that needs to be
     *            exposed.
     * @param supportsCredentials
     *            "true" if support credentials needs to be enabled.
     * @param preflightMaxAge
     *            The amount of seconds the user agent is allowed to cache the
     *            result of the pre-flight request.
     * @throws ServletException
     */
    private void parseAndStore(final String allowedOrigins,
            final String allowedHttpMethods, final String allowedHttpHeaders,
            final String exposedHeaders, final String supportsCredentials,
            final String preflightMaxAge, final String decorateRequest)
                    throws ServletException {

        if (allowedOrigins.trim().equals("*")) {
            this.anyOriginAllowed = true;
        } else {
            this.anyOriginAllowed = false;
            Set<String> setAllowedOrigins = parseStringToSet(allowedOrigins);
            this.allowedOrigins.clear();
            this.allowedOrigins.addAll(setAllowedOrigins);
        }

        Set<String> setAllowedHttpMethods = parseStringToSet(allowedHttpMethods);
        this.allowedHttpMethods.clear();
        this.allowedHttpMethods.addAll(setAllowedHttpMethods);

        Set<String> setAllowedHttpHeaders = parseStringToSet(allowedHttpHeaders);
        Set<String> lowerCaseHeaders = new HashSet<>();
        for (String header : setAllowedHttpHeaders) {
            String lowerCase = header.toLowerCase(Locale.ENGLISH);
            lowerCaseHeaders.add(lowerCase);
        }
        this.allowedHttpHeaders.clear();
        this.allowedHttpHeaders.addAll(lowerCaseHeaders);

        Set<String> setExposedHeaders = parseStringToSet(exposedHeaders);
        this.exposedHeaders.clear();
        this.exposedHeaders.addAll(setExposedHeaders);

        // For any value other then 'true' this will be false.
        this.supportsCredentials = Boolean.parseBoolean(supportsCredentials);

        if (this.supportsCredentials && this.anyOriginAllowed) {
            throw new ServletException(sm.getString("corsFilter.invalidSupportsCredentials"));
        }

        try {
            if (!preflightMaxAge.isEmpty()) {
                this.preflightMaxAge = Long.parseLong(preflightMaxAge);
            } else {
                this.preflightMaxAge = 0L;
            }
        } catch (NumberFormatException e) {
            throw new ServletException(
                    sm.getString("corsFilter.invalidPreflightMaxAge"), e);
        }

        // For any value other then 'true' this will be false.
        this.decorateRequest = Boolean.parseBoolean(decorateRequest);
    }

    /**
     * Takes a comma separated list and returns a Set<String>.
     *
     * @param data
     *            A comma separated list of strings.
     * @return Set<String>
     */
    private Set<String> parseStringToSet(final String data) {
        String[] splits;

        if (data != null && data.length() > 0) {
            splits = data.split(",");
        } else {
            splits = new String[] {};
        }

        Set<String> set = new HashSet<>();
        if (splits.length > 0) {
            for (String split : splits) {
                set.add(split.trim());
            }
        }

        return set;
    }


    /**
     * Checks if a given origin is valid or not. Criteria:
     * <ul>
     * <li>If an encoded character is present in origin, it's not valid.</li>
     * <li>If origin is "null", it's valid.</li>
     * <li>Origin should be a valid {@link URI}</li>
     * </ul>
     *
     * @param origin The origin URI
     * @return <code>true</code> if the origin was valid
     * @see <a href="http://tools.ietf.org/html/rfc952">RFC952</a>
     */
    protected static boolean isValidOrigin(String origin) {
        // Checks for encoded characters. Helps prevent CRLF injection.
        if (origin.contains("%")) {
            return false;
        }

        // "null" is a valid origin
        if ("null".equals(origin)) {
            return true;
        }

        // RFC6454, section 4. "If uri-scheme is file, the implementation MAY
        // return an implementation-defined value.". No limits are placed on
        // that value so treat all file URIs as valid origins.
        if (origin.startsWith("file://")) {
            return true;
        }

        URI originURI;
        try {
            originURI = new URI(origin);
        } catch (URISyntaxException e) {
            return false;
        }
        // If scheme for URI is null, return false. Return true otherwise.
        return originURI.getScheme() != null;

    }


    /**
     * Determines if any origin is allowed to make CORS request.
     *
     * @return <code>true</code> if it's enabled; false otherwise.
     */
    public boolean isAnyOriginAllowed() {
        return anyOriginAllowed;
    }


    /**
     * Obtain the headers to expose.
     *
     * @return the headers that should be exposed by browser.
     */
    public Collection<String> getExposedHeaders() {
        return exposedHeaders;
    }


    /**
     * Determines is supports credentials is enabled.
     *
     * @return <code>true</code> if the use of credentials is supported
     *         otherwise <code>false</code>
     */
    public boolean isSupportsCredentials() {
        return supportsCredentials;
    }


    /**
     * Returns the preflight response cache time in seconds.
     *
     * @return Time to cache in seconds.
     */
    public long getPreflightMaxAge() {
        return preflightMaxAge;
    }


    /**
     * Returns the {@link Set} of allowed origins that are allowed to make
     * requests.
     *
     * @return {@link Set}
     */
    public Collection<String> getAllowedOrigins() {
        return allowedOrigins;
    }


    /**
     * Returns a {@link Set} of HTTP methods that are allowed to make requests.
     *
     * @return {@link Set}
     */
    public Collection<String> getAllowedHttpMethods() {
        return allowedHttpMethods;
    }


    /**
     * Returns a {@link Set} of headers support by resource.
     *
     * @return {@link Set}
     */
    public Collection<String> getAllowedHttpHeaders() {
        return allowedHttpHeaders;
    }


    /*
     * Log objects are not Serializable but this Filter is because it extends
     * GenericFilter. Tomcat won't serialize a Filter but in case something else
     * does...
     */
    private void readObject(ObjectInputStream ois) throws ClassNotFoundException, IOException {
        ois.defaultReadObject();
        log = LogFactory.getLog(CorsFilter.class);
    }


    // -------------------------------------------------- CORS Response Headers

    /**
     * The Access-Control-Allow-Origin header indicates whether a resource can
     * be shared based by returning the value of the Origin request header in
     * the response.
     */
    public static final String RESPONSE_HEADER_ACCESS_CONTROL_ALLOW_ORIGIN =
            "Access-Control-Allow-Origin";

    /**
     * The Access-Control-Allow-Credentials header indicates whether the
     * response to request can be exposed when the omit credentials flag is
     * unset. When part of the response to a preflight request it indicates that
     * the actual request can include user credentials.
     */
    public static final String RESPONSE_HEADER_ACCESS_CONTROL_ALLOW_CREDENTIALS =
            "Access-Control-Allow-Credentials";

    /**
     * The Access-Control-Expose-Headers header indicates which headers are safe
     * to expose to the API of a CORS API specification
     */
    public static final String RESPONSE_HEADER_ACCESS_CONTROL_EXPOSE_HEADERS =
            "Access-Control-Expose-Headers";

    /**
     * The Access-Control-Max-Age header indicates how long the results of a
     * preflight request can be cached in a preflight result cache.
     */
    public static final String RESPONSE_HEADER_ACCESS_CONTROL_MAX_AGE =
            "Access-Control-Max-Age";

    /**
     * The Access-Control-Allow-Methods header indicates, as part of the
     * response to a preflight request, which methods can be used during the
     * actual request.
     */
    public static final String RESPONSE_HEADER_ACCESS_CONTROL_ALLOW_METHODS =
            "Access-Control-Allow-Methods";

    /**
     * The Access-Control-Allow-Headers header indicates, as part of the
     * response to a preflight request, which header field names can be used
     * during the actual request.
     */
    public static final String RESPONSE_HEADER_ACCESS_CONTROL_ALLOW_HEADERS =
            "Access-Control-Allow-Headers";

    // -------------------------------------------------- CORS Request Headers

    /**
     * The Vary header indicates allows disabling proxy caching by indicating
     * the the response depends on the origin.
     *
     * @deprecated Unused. Will be removed in Tomcat 10
     */
    @Deprecated
    public static final String REQUEST_HEADER_VARY = "Vary";

    /**
     * The Origin header indicates where the cross-origin request or preflight
     * request originates from.
     */
    public static final String REQUEST_HEADER_ORIGIN = "Origin";

    /**
     * The Access-Control-Request-Method header indicates which method will be
     * used in the actual request as part of the preflight request.
     */
    public static final String REQUEST_HEADER_ACCESS_CONTROL_REQUEST_METHOD =
            "Access-Control-Request-Method";

    /**
     * The Access-Control-Request-Headers header indicates which headers will be
     * used in the actual request as part of the preflight request.
     */
    public static final String REQUEST_HEADER_ACCESS_CONTROL_REQUEST_HEADERS =
            "Access-Control-Request-Headers";

    // ----------------------------------------------------- Request attributes
    /**
     * The prefix to a CORS request attribute.
     */
    public static final String HTTP_REQUEST_ATTRIBUTE_PREFIX = "cors.";

    /**
     * Attribute that contains the origin of the request.
     */
    public static final String HTTP_REQUEST_ATTRIBUTE_ORIGIN =
            HTTP_REQUEST_ATTRIBUTE_PREFIX + "request.origin";

    /**
     * Boolean value, suggesting if the request is a CORS request or not.
     */
    public static final String HTTP_REQUEST_ATTRIBUTE_IS_CORS_REQUEST =
            HTTP_REQUEST_ATTRIBUTE_PREFIX + "isCorsRequest";

    /**
     * Type of CORS request, of type {@link CORSRequestType}.
     */
    public static final String HTTP_REQUEST_ATTRIBUTE_REQUEST_TYPE =
            HTTP_REQUEST_ATTRIBUTE_PREFIX + "request.type";

    /**
     * Request headers sent as 'Access-Control-Request-Headers' header, for
     * pre-flight request.
     */
    public static final String HTTP_REQUEST_ATTRIBUTE_REQUEST_HEADERS =
            HTTP_REQUEST_ATTRIBUTE_PREFIX + "request.headers";

    // -------------------------------------------------------------- Constants
    /**
     * Enumerates varies types of CORS requests. Also, provides utility methods
     * to determine the request type.
     */
    protected enum CORSRequestType {
        /**
         * A simple HTTP request, i.e. it shouldn't be pre-flighted.
         */
        SIMPLE,
        /**
         * A HTTP request that needs to be pre-flighted.
         */
        ACTUAL,
        /**
         * A pre-flight CORS request, to get meta information, before a
         * non-simple HTTP request is sent.
         */
        PRE_FLIGHT,
        /**
         * Not a CORS request, but a normal request.
         */
        NOT_CORS,
        /**
         * An invalid CORS request, i.e. it qualifies to be a CORS request, but
         * fails to be a valid one.
         */
        INVALID_CORS
    }

    /**
     * {@link Collection} of media type values for the Content-Type header that
     * will be treated as 'simple'. Note media-type values are compared ignoring
     * parameters and in a case-insensitive manner.
     *
     * @see  <a href="http://www.w3.org/TR/cors/#terminology"
     *       >http://www.w3.org/TR/cors/#terminology</a>
     */
    public static final Collection<String> SIMPLE_HTTP_REQUEST_CONTENT_TYPE_VALUES =
            Collections.unmodifiableSet(new HashSet<>(Arrays.asList(
                    "application/x-www-form-urlencoded", "multipart/form-data", "text/plain")));

    // ------------------------------------------------ Configuration Defaults
    /**
     * By default, no origins are allowed to make requests.
     */
    public static final String DEFAULT_ALLOWED_ORIGINS = "";

    /**
     * By default, following methods are supported: GET, POST, HEAD and OPTIONS.
     */
    public static final String DEFAULT_ALLOWED_HTTP_METHODS =
            "GET,POST,HEAD,OPTIONS";

    /**
     * By default, time duration to cache pre-flight response is 30 mins.
     */
    public static final String DEFAULT_PREFLIGHT_MAXAGE = "1800";

    /**
     * By default, support credentials is disabled.
     */
    public static final String DEFAULT_SUPPORTS_CREDENTIALS = "false";

    /**
     * By default, following headers are supported:
     * Origin,Accept,X-Requested-With, Content-Type,
     * Access-Control-Request-Method, and Access-Control-Request-Headers.
     */
    public static final String DEFAULT_ALLOWED_HTTP_HEADERS =
            "Origin,Accept,X-Requested-With,Content-Type," +
            "Access-Control-Request-Method,Access-Control-Request-Headers";

    /**
     * By default, none of the headers are exposed in response.
     */
    public static final String DEFAULT_EXPOSED_HEADERS = "";

    /**
     * By default, request is decorated with CORS attributes.
     */
    public static final String DEFAULT_DECORATE_REQUEST = "true";

    // ----------------------------------------Filter Config Init param-name(s)
    /**
     * Key to retrieve allowed origins from {@link javax.servlet.FilterConfig}.
     */
    public static final String PARAM_CORS_ALLOWED_ORIGINS =
            "cors.allowed.origins";

    /**
     * Key to retrieve support credentials from
     * {@link javax.servlet.FilterConfig}.
     */
    public static final String PARAM_CORS_SUPPORT_CREDENTIALS =
            "cors.support.credentials";

    /**
     * Key to retrieve exposed headers from {@link javax.servlet.FilterConfig}.
     */
    public static final String PARAM_CORS_EXPOSED_HEADERS =
            "cors.exposed.headers";

    /**
     * Key to retrieve allowed headers from {@link javax.servlet.FilterConfig}.
     */
    public static final String PARAM_CORS_ALLOWED_HEADERS =
            "cors.allowed.headers";

    /**
     * Key to retrieve allowed methods from {@link javax.servlet.FilterConfig}.
     */
    public static final String PARAM_CORS_ALLOWED_METHODS =
            "cors.allowed.methods";

    /**
     * Key to retrieve preflight max age from
     * {@link javax.servlet.FilterConfig}.
     */
    public static final String PARAM_CORS_PREFLIGHT_MAXAGE =
            "cors.preflight.maxage";

    /**
     * Key to determine if request should be decorated.
     */
    public static final String PARAM_CORS_REQUEST_DECORATE =
            "cors.request.decorate";
}
