HttpServletRequest.java

/*
 * Copyright (c) 1997, 2020 Oracle and/or its affiliates and others.
 * All rights reserved.
 * Copyright 2004 The Apache Software Foundation
 *
 * 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 jakarta.servlet.http;

import jakarta.servlet.RequestDispatcher;
import jakarta.servlet.ServletException;
import jakarta.servlet.ServletRequest;
import java.io.IOException;
import java.util.*;

/**
 *
 * Extends the {@link jakarta.servlet.ServletRequest} interface to provide request information for HTTP servlets.
 *
 * <p>
 * The servlet container creates an <code>HttpServletRequest</code> object and passes it as an argument to the servlet's
 * service methods (<code>doGet</code>, <code>doPost</code>, etc).
 *
 *
 * @author Various
 */
public interface HttpServletRequest extends ServletRequest {

    /**
     * String identifier for Basic authentication. Value "BASIC"
     */
    public static final String BASIC_AUTH = "BASIC";

    /**
     * String identifier for Form authentication. Value "FORM"
     */
    public static final String FORM_AUTH = "FORM";

    /**
     * String identifier for Client Certificate authentication. Value "CLIENT_CERT"
     */
    public static final String CLIENT_CERT_AUTH = "CLIENT_CERT";

    /**
     * String identifier for Digest authentication. Value "DIGEST"
     */
    public static final String DIGEST_AUTH = "DIGEST";

    /**
     * Returns the name of the authentication scheme used to protect the servlet. All servlet containers support basic, form
     * and client certificate authentication, and may additionally support digest authentication. If the servlet is not
     * authenticated <code>null</code> is returned.
     *
     * @return one of the static members BASIC_AUTH, FORM_AUTH, CLIENT_CERT_AUTH, DIGEST_AUTH (suitable for == comparison)
     * or the container-specific string indicating the authentication scheme, or <code>null</code> if the request was not
     * authenticated.
     */
    public String getAuthType();

    /**
     * Returns an array containing all of the <code>Cookie</code> objects the client sent with this request. This method
     * returns <code>null</code> if no cookies were sent.
     *
     * @return an array of all the <code>Cookies</code> included with this request, or <code>null</code> if the request has
     * no cookies
     */
    public Cookie[] getCookies();

    /**
     * Returns the value of the specified request header as a <code>long</code> value that represents a <code>Date</code>
     * object. Use this method with headers that contain dates, such as <code>If-Modified-Since</code>.
     *
     * <p>
     * The date is returned as the number of milliseconds since January 1, 1970 GMT. The header name is case insensitive.
     *
     * <p>
     * If the request did not have a header of the specified name, this method returns -1. If the header can't be converted
     * to a date, the method throws an <code>IllegalArgumentException</code>.
     *
     * @param name a <code>String</code> specifying the name of the header
     *
     * @return a <code>long</code> value representing the date specified in the header expressed as the number of
     * milliseconds since January 1, 1970 GMT, or -1 if the named header was not included with the request
     *
     * @exception IllegalArgumentException If the header value can't be converted to a date
     */
    public long getDateHeader(String name);

    /**
     * Returns the value of the specified request header as a <code>String</code>. If the request did not include a header
     * of the specified name, this method returns <code>null</code>. If there are multiple headers with the same name, this
     * method returns the first head in the request. The header name is case insensitive. You can use this method with any
     * request header.
     *
     * @param name a <code>String</code> specifying the header name
     *
     * @return a <code>String</code> containing the value of the requested header, or <code>null</code> if the request does
     * not have a header of that name
     */
    public String getHeader(String name);

    /**
     * Returns all the values of the specified request header as an <code>Enumeration</code> of <code>String</code> objects.
     *
     * <p>
     * Some headers, such as <code>Accept-Language</code> can be sent by clients as several headers each with a different
     * value rather than sending the header as a comma separated list.
     *
     * <p>
     * If the request did not include any headers of the specified name, this method returns an empty
     * <code>Enumeration</code>. The header name is case insensitive. You can use this method with any request header.
     *
     * @param name a <code>String</code> specifying the header name
     *
     * @return an <code>Enumeration</code> containing the values of the requested header. If the request does not have any
     * headers of that name return an empty enumeration. If the container does not allow access to header information,
     * return null
     */
    public Enumeration<String> getHeaders(String name);

    /**
     * Returns an enumeration of all the header names this request contains. If the request has no headers, this method
     * returns an empty enumeration.
     *
     * <p>
     * Some servlet containers do not allow servlets to access headers using this method, in which case this method returns
     * <code>null</code>
     *
     * @return an enumeration of all the header names sent with this request; if the request has no headers, an empty
     * enumeration; if the servlet container does not allow servlets to use this method, <code>null</code>
     */
    public Enumeration<String> getHeaderNames();

    /**
     * Returns the value of the specified request header as an <code>int</code>. If the request does not have a header of
     * the specified name, this method returns -1. If the header cannot be converted to an integer, this method throws a
     * <code>NumberFormatException</code>.
     *
     * <p>
     * The header name is case insensitive.
     *
     * @param name a <code>String</code> specifying the name of a request header
     *
     * @return an integer expressing the value of the request header or -1 if the request doesn't have a header of this name
     *
     * @exception NumberFormatException If the header value can't be converted to an <code>int</code>
     */
    public int getIntHeader(String name);

    /**
     * Return the HttpServletMapping of the request.
     * <p>
     * The mapping returned depends on the current {@link jakarta.servlet.DispatcherType} as obtained from
     * {@link #getDispatcherType()}:
     * <dl>
     * <dt>{@link jakarta.servlet.DispatcherType#REQUEST}, {@link jakarta.servlet.DispatcherType#ASYNC},
     * {@link jakarta.servlet.DispatcherType#ERROR}</dt>
     * <dd>Return the mapping for the target of the dispatch i.e. the mapping for the current
     * {@link jakarta.servlet.Servlet}.</dd>
     *
     * <dt>{@link jakarta.servlet.DispatcherType#INCLUDE}</dt>
     * <dd>Return the mapping as prior to the current dispatch. i.e the mapping returned is unchanged by a call to</dd>
     * {@link RequestDispatcher#include(ServletRequest, ServletResponse)}.
     *
     * <dt>{@link jakarta.servlet.DispatcherType#FORWARD}</dt>
     * <dd>Return the mapping for the target of the dispatch i.e. the mapping for the current
     * {@link jakarta.servlet.Servlet}, unless the {@link jakarta.servlet.RequestDispatcher} was obtained via
     * {@link jakarta.servlet.ServletContext#getNamedDispatcher(String)}, in which case return the mapping as prior to the
     * current dispatch. i.e the mapping returned is changed during a call to
     * {@link RequestDispatcher#forward(ServletRequest, ServletResponse)} only if the dispatcher is not a named
     * dispatcher.</dd>
     * </dl>
     * </p>
     * <p>
     * For example:
     * <ul>
     * <li>For a sequence Servlet1&nbsp;--include--&gt;&nbsp;Servlet2&nbsp;--include--&gt;&nbsp;Servlet3, a call to this
     * method in Servlet3 will return the mapping for Servlet1.</li>
     * <li>For a sequence Servlet1&nbsp;--async--&gt;&nbsp;Servlet2&nbsp;--named-forward--&gt;&nbsp;Servlet3, a call to this
     * method in Servlet3 will return the mapping for Servlet2.</li>
     * </ul>
     * </p>
     * <p>
     * The returned object is immutable. Servlet 4.0 compliant implementations must override this method.
     * </p>
     * 
     * @implSpec The default implementation returns a {@code
     * HttpServletMapping} that returns the empty string for the match value, pattern and servlet name and {@code null} for
     * the match type.
     *
     * @return An instance of {@code HttpServletMapping} describing the manner in which the current request was invoked.
     * 
     * @since 4.0
     */
    default public HttpServletMapping getHttpServletMapping() {
        return new HttpServletMapping() {
            @Override
            public String getMatchValue() {
                return "";
            }

            @Override
            public String getPattern() {
                return "";
            }

            @Override
            public String getServletName() {
                return "";
            }

            @Override
            public MappingMatch getMappingMatch() {
                return null;
            }

            @Override
            public String toString() {
                return "MappingImpl{" + "matchValue=" + getMatchValue() + ", pattern=" + getPattern() + ", servletName="
                        + getServletName() + ", mappingMatch=" + getMappingMatch() + "} HttpServletRequest {"
                        + HttpServletRequest.this.toString() + '}';
            }

        };
    }

    /**
     * Returns the name of the HTTP method with which this request was made, for example, GET, POST, or PUT.
     *
     * @return a <code>String</code> specifying the name of the method with which this request was made
     */
    public String getMethod();

    /**
     * Returns any extra path information associated with the URL the client sent when it made this request. The extra path
     * information follows the servlet path but precedes the query string and will start with a "/" character.
     *
     * <p>
     * This method returns <code>null</code> if there was no extra path information.
     *
     * @return a <code>String</code>, decoded by the web container, specifying extra path information that comes after the
     * servlet path but before the query string in the request URL; or <code>null</code> if the URL does not have any extra
     * path information
     */
    public String getPathInfo();

    /**
     * Returns any extra path information after the servlet name but before the query string, and translates it to a real
     * path.
     *
     * <p>
     * If the URL does not have any extra path information, this method returns <code>null</code> or the servlet container
     * cannot translate the virtual path to a real path for any reason (such as when the web application is executed from an
     * archive).
     *
     * The web container does not decode this string.
     *
     * @return a <code>String</code> specifying the real path, or <code>null</code> if the URL does not have any extra path
     * information
     */
    public String getPathTranslated();

    /**
     * Instantiates a new instance of {@link PushBuilder} for issuing server push responses from the current request. This
     * method returns null if the current connection does not support server push, or server push has been disabled by the
     * client via a {@code SETTINGS_ENABLE_PUSH} settings frame value of {@code 0} (zero).
     *
     * @implSpec The default implementation returns null.
     *
     * @return a {@link PushBuilder} for issuing server push responses from the current request, or null if push is not
     * supported
     *
     * @since Servlet 4.0
     */
    default public PushBuilder newPushBuilder() {
        return null;
    }

    /**
     * Returns the portion of the request URI that indicates the context of the request. The context path always comes first
     * in a request URI. The path starts with a "/" character but does not end with a "/" character. For servlets in the
     * default (root) context, this method returns "". The container does not decode this string.
     *
     * <p>
     * It is possible that a servlet container may match a context by more than one context path. In such cases this method
     * will return the actual context path used by the request and it may differ from the path returned by the
     * {@link jakarta.servlet.ServletContext#getContextPath()} method. The context path returned by
     * {@link jakarta.servlet.ServletContext#getContextPath()} should be considered as the prime or preferred context path
     * of the application.
     *
     * @return a <code>String</code> specifying the portion of the request URI that indicates the context of the request
     *
     * @see jakarta.servlet.ServletContext#getContextPath()
     */
    public String getContextPath();

    /**
     * Returns the query string that is contained in the request URL after the path. This method returns <code>null</code>
     * if the URL does not have a query string.
     *
     * @return a <code>String</code> containing the query string or <code>null</code> if the URL contains no query string.
     * The value is not decoded by the container.
     */
    public String getQueryString();

    /**
     * Returns the login of the user making this request, if the user has been authenticated, or <code>null</code> if the
     * user has not been authenticated. Whether the user name is sent with each subsequent request depends on the browser
     * and type of authentication.
     *
     * @return a <code>String</code> specifying the login of the user making this request, or <code>null</code> if the user
     * login is not known
     */
    public String getRemoteUser();

    /**
     * Returns a boolean indicating whether the authenticated user is included in the specified logical "role". Roles and
     * role membership can be defined using deployment descriptors. If the user has not been authenticated, the method
     * returns <code>false</code>.
     *
     * <p>
     * The role name "*" should never be used as an argument in calling <code>isUserInRole</code>. Any call to
     * <code>isUserInRole</code> with "*" must return false. If the role-name of the security-role to be tested is "**", and
     * the application has NOT declared an application security-role with role-name "**", <code>isUserInRole</code> must
     * only return true if the user has been authenticated; that is, only when {@link #getRemoteUser} and
     * {@link #getUserPrincipal} would both return a non-null value. Otherwise, the container must check the user for
     * membership in the application role.
     *
     * @param role a <code>String</code> specifying the name of the role
     *
     * @return a <code>boolean</code> indicating whether the user making this request belongs to a given role;
     * <code>false</code> if the user has not been authenticated
     */
    public boolean isUserInRole(String role);

    /**
     * Returns a <code>java.security.Principal</code> object containing the name of the current authenticated user. If the
     * user has not been authenticated, the method returns <code>null</code>.
     *
     * @return a <code>java.security.Principal</code> containing the name of the user making this request; <code>null</code>
     * if the user has not been authenticated
     */
    public java.security.Principal getUserPrincipal();

    /**
     * Returns the session ID specified by the client. This may not be the same as the ID of the current valid session for
     * this request. If the client did not specify a session ID, this method returns <code>null</code>.
     *
     * @return a <code>String</code> specifying the session ID, or <code>null</code> if the request did not specify a
     * session ID
     *
     * @see #isRequestedSessionIdValid
     */
    public String getRequestedSessionId();

    /**
     * Returns the part of this request's URL from the protocol name up to the query string in the first line of the HTTP
     * request. The web container does not decode this String. For example:
     *
     * <table summary="Examples of Returned Values">
     * <tr align=left>
     * <th>First line of HTTP request</th>
     * <th>Returned Value</th>
     * <tr>
     * <td>POST /some/path.html HTTP/1.1
     * <td>
     * <td>/some/path.html
     * <tr>
     * <td>GET http://foo.bar/a.html HTTP/1.0
     * <td>
     * <td>/a.html
     * <tr>
     * <td>HEAD /xyz?a=b HTTP/1.1
     * <td>
     * <td>/xyz
     * </table>
     *
     * <p>
     * To reconstruct an URL with a scheme and host, use {@link HttpUtils#getRequestURL}.
     *
     * @return a <code>String</code> containing the part of the URL from the protocol name up to the query string
     *
     * @see HttpUtils#getRequestURL
     */
    public String getRequestURI();

    /**
     * Reconstructs the URL the client used to make the request. The returned URL contains a protocol, server name, port
     * number, and server path, but it does not include query string parameters.
     *
     * <p>
     * If this request has been forwarded using {@link jakarta.servlet.RequestDispatcher#forward}, the server path in the
     * reconstructed URL must reflect the path used to obtain the RequestDispatcher, and not the server path specified by
     * the client.
     *
     * <p>
     * Because this method returns a <code>StringBuffer</code>, not a string, you can modify the URL easily, for example, to
     * append query parameters.
     *
     * <p>
     * This method is useful for creating redirect messages and for reporting errors.
     *
     * @return a <code>StringBuffer</code> object containing the reconstructed URL
     */
    public StringBuffer getRequestURL();

    /**
     * Returns the part of this request's URL that calls the servlet. This path starts with a "/" character and includes
     * either the servlet name or a path to the servlet, but does not include any extra path information or a query string.
     *
     * <p>
     * This method will return an empty string ("") if the servlet used to process this request was matched using the "/*"
     * pattern.
     *
     * @return a <code>String</code> containing the name or path of the servlet being called, as specified in the request
     * URL, decoded, or an empty string if the servlet used to process the request is matched using the "/*" pattern.
     */
    public String getServletPath();

    /**
     * Returns the current <code>HttpSession</code> associated with this request or, if there is no current session and
     * <code>create</code> is true, returns a new session.
     *
     * <p>
     * If <code>create</code> is <code>false</code> and the request has no valid <code>HttpSession</code>, this method
     * returns <code>null</code>.
     *
     * <p>
     * To make sure the session is properly maintained, you must call this method before the response is committed. If the
     * container is using cookies to maintain session integrity and is asked to create a new session when the response is
     * committed, an IllegalStateException is thrown.
     *
     * @param create <code>true</code> to create a new session for this request if necessary; <code>false</code> to return
     * <code>null</code> if there's no current session
     *
     * @return the <code>HttpSession</code> associated with this request or <code>null</code> if <code>create</code> is
     * <code>false</code> and the request has no valid session
     *
     * @see #getSession()
     */
    public HttpSession getSession(boolean create);

    /**
     * Returns the current session associated with this request, or if the request does not have a session, creates one.
     *
     * @return the <code>HttpSession</code> associated with this request
     *
     * @see #getSession(boolean)
     */
    public HttpSession getSession();

    /**
     * Change the session id of the current session associated with this request and return the new session id.
     *
     * @return the new session id
     *
     * @throws IllegalStateException if there is no session associated with the request
     *
     * @since Servlet 3.1
     */
    public String changeSessionId();

    /**
     * Checks whether the requested session ID is still valid.
     *
     * <p>
     * If the client did not specify any session ID, this method returns <code>false</code>.
     *
     * @return <code>true</code> if this request has an id for a valid session in the current session context;
     * <code>false</code> otherwise
     *
     * @see #getRequestedSessionId
     * @see #getSession
     * @see HttpSessionContext
     */
    public boolean isRequestedSessionIdValid();

    /**
     * <p>
     * Checks whether the requested session ID was conveyed to the server as an HTTP cookie.
     * </p>
     *
     * @return <code>true</code> if the session ID was conveyed to the server an an HTTP cookie; otherwise,
     * <code>false</code>
     *
     * @see #getSession
     */
    public boolean isRequestedSessionIdFromCookie();

    /**
     * <p>
     * Checks whether the requested session ID was conveyed to the server as part of the request URL.
     * </p>
     *
     * @return <code>true</code> if the session ID was conveyed to the server as part of a URL; otherwise,
     * <code>false</code>
     *
     * @see #getSession
     */
    public boolean isRequestedSessionIdFromURL();

    /**
     * @deprecated As of Version 2.1 of the Java Servlet API, use {@link #isRequestedSessionIdFromURL} instead.
     *
     * @return <code>true</code> if the session ID was conveyed to the server as part of a URL; otherwise,
     * <code>false</code>
     */
    @Deprecated
    public boolean isRequestedSessionIdFromUrl();

    /**
     * Use the container login mechanism configured for the <code>ServletContext</code> to authenticate the user making this
     * request.
     *
     * <p>
     * This method may modify and commit the argument <code>HttpServletResponse</code>.
     *
     * @param response The <code>HttpServletResponse</code> associated with this <code>HttpServletRequest</code>
     *
     * @return <code>true</code> when non-null values were or have been established as the values returned by
     * <code>getUserPrincipal</code>, <code>getRemoteUser</code>, and <code>getAuthType</code>. Return <code>false</code> if
     * authentication is incomplete and the underlying login mechanism has committed, in the response, the message (e.g.,
     * challenge) and HTTP status code to be returned to the user.
     *
     * @throws IOException if an input or output error occurred while reading from this request or writing to the given
     * response
     *
     * @throws IllegalStateException if the login mechanism attempted to modify the response and it was already committed
     *
     * @throws ServletException if the authentication failed and the caller is responsible for handling the error (i.e., the
     * underlying login mechanism did NOT establish the message and HTTP status code to be returned to the user)
     *
     * @since Servlet 3.0
     */
    public boolean authenticate(HttpServletResponse response) throws IOException, ServletException;

    /**
     * Validate the provided username and password in the password validation realm used by the web container login
     * mechanism configured for the <code>ServletContext</code>.
     *
     * <p>
     * This method returns without throwing a <code>ServletException</code> when the login mechanism configured for the
     * <code>ServletContext</code> supports username password validation, and when, at the time of the call to login, the
     * identity of the caller of the request had not been established (i.e, all of <code>getUserPrincipal</code>,
     * <code>getRemoteUser</code>, and <code>getAuthType</code> return null), and when validation of the provided
     * credentials is successful. Otherwise, this method throws a <code>ServletException</code> as described below.
     *
     * <p>
     * When this method returns without throwing an exception, it must have established non-null values as the values
     * returned by <code>getUserPrincipal</code>, <code>getRemoteUser</code>, and <code>getAuthType</code>.
     *
     * @param username The <code>String</code> value corresponding to the login identifier of the user.
     *
     * @param password The password <code>String</code> corresponding to the identified user.
     *
     * @exception ServletException if the configured login mechanism does not support username password authentication, or
     * if a non-null caller identity had already been established (prior to the call to login), or if validation of the
     * provided username and password fails.
     *
     * @since Servlet 3.0
     */
    public void login(String username, String password) throws ServletException;

    /**
     * Establish <code>null</code> as the value returned when <code>getUserPrincipal</code>, <code>getRemoteUser</code>, and
     * <code>getAuthType</code> is called on the request.
     *
     * @exception ServletException if logout fails
     *
     * @since Servlet 3.0
     */
    public void logout() throws ServletException;

    /**
     * Gets all the {@link Part} components of this request, provided that it is of type <code>multipart/form-data</code>.
     *
     * <p>
     * If this request is of type <code>multipart/form-data</code>, but does not contain any <code>Part</code> components,
     * the returned <code>Collection</code> will be empty.
     *
     * <p>
     * Any changes to the returned <code>Collection</code> must not affect this <code>HttpServletRequest</code>.
     *
     * @return a (possibly empty) <code>Collection</code> of the <code>Part</code> components of this request
     *
     * @throws IOException if an I/O error occurred during the retrieval of the {@link Part} components of this request
     *
     * @throws ServletException if this request is not of type <code>multipart/form-data</code>
     *
     * @throws IllegalStateException if the request body is larger than <code>maxRequestSize</code>, or any
     * <code>Part</code> in the request is larger than <code>maxFileSize</code>, or there is no
     * <code>@MultipartConfig</code> or <code>multipart-config</code> in deployment descriptors
     *
     * @see jakarta.servlet.annotation.MultipartConfig#maxFileSize
     * @see jakarta.servlet.annotation.MultipartConfig#maxRequestSize
     *
     * @since Servlet 3.0
     */
    public Collection<Part> getParts() throws IOException, ServletException;

    /**
     * Gets the {@link Part} with the given name.
     *
     * @param name the name of the requested <code>Part</code>
     *
     * @return The <code>Part</code> with the given name, or <code>null</code> if this request is of type
     * <code>multipart/form-data</code>, but does not contain the requested <code>Part</code>
     *
     * @throws IOException if an I/O error occurred during the retrieval of the requested <code>Part</code>
     * @throws ServletException if this request is not of type <code>multipart/form-data</code>
     * @throws IllegalStateException if the request body is larger than <code>maxRequestSize</code>, or any
     * <code>Part</code> in the request is larger than <code>maxFileSize</code>, or there is no
     * <code>@MultipartConfig</code> or <code>multipart-config</code> in deployment descriptors
     *
     * @see jakarta.servlet.annotation.MultipartConfig#maxFileSize
     * @see jakarta.servlet.annotation.MultipartConfig#maxRequestSize
     *
     * @since Servlet 3.0
     */
    public Part getPart(String name) throws IOException, ServletException;

    /**
     * Creates an instance of <code>HttpUpgradeHandler</code> for a given class and uses it for the http protocol upgrade
     * processing.
     *
     * @param <T> The {@code Class}, which extends {@link HttpUpgradeHandler}, of the {@code handlerClass}.
     * 
     * @param handlerClass The <code>HttpUpgradeHandler</code> class used for the upgrade.
     *
     * @return an instance of the <code>HttpUpgradeHandler</code>
     *
     * @exception IOException if an I/O error occurred during the upgrade
     * @exception ServletException if the given <code>handlerClass</code> fails to be instantiated
     *
     * @see jakarta.servlet.http.HttpUpgradeHandler
     * @see jakarta.servlet.http.WebConnection
     *
     * @since Servlet 3.1
     */
    public <T extends HttpUpgradeHandler> T upgrade(Class<T> handlerClass) throws IOException, ServletException;

    /**
     * Get the request trailer fields.
     *
     * <p>
     * The returned map is not backed by the {@code HttpServletRequest} object, so changes in the returned map are not
     * reflected in the {@code HttpServletRequest} object, and vice-versa.
     * </p>
     * 
     * <p>
     * {@link #isTrailerFieldsReady()} should be called first to determine if it is safe to call this method without causing
     * an exception.
     * </p>
     *
     * @implSpec The default implementation returns an empty map.
     * 
     * @return A map of trailer fields in which all the keys are in lowercase, regardless of the case they had at the
     * protocol level. If there are no trailer fields, yet {@link #isTrailerFieldsReady} is returning true, the empty map is
     * returned.
     *
     * @throws IllegalStateException if {@link #isTrailerFieldsReady()} is false
     *
     * @since Servlet 4.0
     */
    default public Map<String, String> getTrailerFields() {
        return Collections.emptyMap();
    }

    /**
     * Return a boolean indicating whether trailer fields are ready to read using {@link #getTrailerFields}.
     *
     * This methods returns true immediately if it is known that there is no trailer in the request, for instance, the
     * underlying protocol (such as HTTP 1.0) does not supports the trailer fields, or the request is not in chunked
     * encoding in HTTP 1.1. And the method also returns true if both of the following conditions are satisfied:
     * <ol type="a">
     * <li>the application has read all the request data and an EOF indication has been returned from the {@link #getReader}
     * or {@link #getInputStream}.
     * <li>all the trailer fields sent by the client have been received. Note that it is possible that the client has sent
     * no trailer fields.
     * </ol>
     *
     * @implSpec The default implementation returns false.
     *
     * @return a boolean whether trailer fields are ready to read
     *
     * @since Servlet 4.0
     */
    default public boolean isTrailerFieldsReady() {
        return true;
    }
}