/*
 * 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 javax.servlet.jsp;

import java.io.IOException;

import javax.servlet.RequestDispatcher;
import javax.servlet.Servlet;
import javax.servlet.ServletConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpSession;
import javax.servlet.jsp.tagext.BodyContent;

/**
 * <p>
 * PageContext extends JspContext to provide useful context information for when JSP technology is used in a Servlet
 * environment.
 * <p>
 * A PageContext instance provides access to all the namespaces associated with a JSP page, provides access to several
 * page attributes, as well as a layer above the implementation details. Implicit objects are added to the pageContext
 * automatically.
 * <p>
 * The <code> PageContext </code> class is an abstract class, designed to be extended to provide implementation
 * dependent implementations thereof, by conformant JSP engine runtime environments. A PageContext instance is obtained
 * by a JSP implementation class by calling the JspFactory.getPageContext() method, and is released by calling
 * JspFactory.releasePageContext().
 * <p>
 * An example of how PageContext, JspFactory, and other classes can be used within a JSP Page Implementation object is
 * given elsewhere.
 * <p>
 * The PageContext provides a number of facilities to the page/component author and page implementor, including:
 * <ul>
 * <li>a single API to manage the various scoped namespaces
 * <li>a number of convenience API's to access various public objects
 * <li>a mechanism to obtain the JspWriter for output
 * <li>a mechanism to manage session usage by the page
 * <li>a mechanism to expose page directive attributes to the scripting environment
 * <li>mechanisms to forward or include the current request to other active components in the application
 * <li>a mechanism to handle errorpage exception processing
 * </ul>
 * <p>
 * <B>Methods Intended for Container Generated Code</B>
 * <p>
 * Some methods are intended to be used by the code generated by the container, not by code written by JSP page authors,
 * or JSP tag library authors.
 * <p>
 * The methods supporting <B>lifecycle</B> are <code>initialize()</code> and <code>release()</code>
 * <p>
 * The following methods enable the <B>management of nested</B> JspWriter streams to implement Tag Extensions:
 * <code>pushBody()</code>
 * <p>
 * <B>Methods Intended for JSP authors</B>
 * <p>
 * The following methods provide <B>convenient access</B> to implicit objects: <code>getException()</code>,
 * <code>getPage()</code> <code>getRequest()</code>, <code>getResponse()</code>, <code>getSession()</code>,
 * <code>getServletConfig()</code> and <code>getServletContext()</code>.
 * <p>
 * The following methods provide support for <B>forwarding, inclusion and error handling</B>: <code>forward()</code>,
 * <code>include()</code>, and <code>handlePageException()</code>.
 */
public abstract class PageContext extends JspContext {

    /**
     * Sole constructor. (For invocation by subclass constructors, typically implicit.)
     */
    public PageContext() {
        // NOOP by default
    }

    /**
     * Page scope: (this is the default) the named reference remains available in this PageContext until the return from
     * the current Servlet.service() invocation.
     */

    public static final int PAGE_SCOPE = 1;

    /**
     * Request scope: the named reference remains available from the ServletRequest associated with the Servlet until
     * the current request is completed.
     */

    public static final int REQUEST_SCOPE = 2;

    /**
     * Session scope (only valid if this page participates in a session): the named reference remains available from the
     * HttpSession (if any) associated with the Servlet until the HttpSession is invalidated.
     */

    public static final int SESSION_SCOPE = 3;

    /**
     * Application scope: named reference remains available in the ServletContext until it is reclaimed.
     */

    public static final int APPLICATION_SCOPE = 4;

    /**
     * Name used to store the Servlet in this PageContext's nametables.
     */

    public static final String PAGE = "javax.servlet.jsp.jspPage";

    /**
     * Name used to store this PageContext in it's own name table.
     */

    public static final String PAGECONTEXT = "javax.servlet.jsp.jspPageContext";

    /**
     * Name used to store ServletRequest in PageContext name table.
     */

    public static final String REQUEST = "javax.servlet.jsp.jspRequest";

    /**
     * Name used to store ServletResponse in PageContext name table.
     */

    public static final String RESPONSE = "javax.servlet.jsp.jspResponse";

    /**
     * Name used to store ServletConfig in PageContext name table.
     */

    public static final String CONFIG = "javax.servlet.jsp.jspConfig";

    /**
     * Name used to store HttpSession in PageContext name table.
     */

    public static final String SESSION = "javax.servlet.jsp.jspSession";
    /**
     * Name used to store current JspWriter in PageContext name table.
     */

    public static final String OUT = "javax.servlet.jsp.jspOut";

    /**
     * Name used to store ServletContext in PageContext name table.
     */

    public static final String APPLICATION = "javax.servlet.jsp.jspApplication";

    /**
     * Name used to store uncaught exception in ServletRequest attribute list and PageContext name table.
     */

    public static final String EXCEPTION = "javax.servlet.jsp.jspException";

    /**
     * <p>
     * The initialize method is called to initialize an uninitialized PageContext so that it may be used by a JSP
     * Implementation class to service an incoming request and response within it's _jspService() method.
     * <p>
     * This method is typically called from JspFactory.getPageContext() in order to initialize state.
     * <p>
     * This method is required to create an initial JspWriter, and associate the "out" name in page scope with this
     * newly created object.
     * <p>
     * This method should not be used by page or tag library authors.
     *
     * @param servlet      The Servlet that is associated with this PageContext
     * @param request      The currently pending request for this Servlet
     * @param response     The currently pending response for this Servlet
     * @param errorPageURL The value of the errorpage attribute from the page directive or null
     * @param needsSession The value of the session attribute from the page directive
     * @param bufferSize   The value of the buffer attribute from the page directive
     * @param autoFlush    The value of the autoflush attribute from the page directive
     *
     * @throws IOException              during creation of JspWriter
     * @throws IllegalStateException    if out not correctly initialized
     * @throws IllegalArgumentException If one of the given parameters is invalid
     */

    public abstract void initialize(Servlet servlet, ServletRequest request, ServletResponse response,
            String errorPageURL, boolean needsSession, int bufferSize, boolean autoFlush)
            throws IOException, IllegalStateException, IllegalArgumentException;

    /**
     * <p>
     * This method shall "reset" the internal state of a PageContext, releasing all internal references, and preparing
     * the PageContext for potential reuse by a later invocation of initialize(). This method is typically called from
     * JspFactory.releasePageContext().
     * <p>
     * Subclasses shall envelope this method.
     * <p>
     * This method should not be used by page or tag library authors.
     */

    public abstract void release();

    /**
     * The current value of the session object (an HttpSession).
     *
     * @return the HttpSession for this PageContext or null
     */

    public abstract HttpSession getSession();

    /**
     * The current value of the page object (In a Servlet environment, this is an instance of javax.servlet.Servlet).
     *
     * @return the Page implementation class instance associated with this PageContext
     */

    public abstract Object getPage();


    /**
     * The current value of the request object (a ServletRequest).
     *
     * @return The ServletRequest for this PageContext
     */

    public abstract ServletRequest getRequest();

    /**
     * The current value of the response object (a ServletResponse).
     *
     * @return the ServletResponse for this PageContext
     */

    public abstract ServletResponse getResponse();

    /**
     * The current value of the exception object (an Exception).
     *
     * @return any exception passed to this as an errorpage
     */

    public abstract Exception getException();

    /**
     * The ServletConfig instance.
     *
     * @return the ServletConfig for this PageContext
     */

    public abstract ServletConfig getServletConfig();

    /**
     * The ServletContext instance.
     *
     * @return the ServletContext for this PageContext
     */

    public abstract ServletContext getServletContext();

    /**
     * <p>
     * This method is used to re-direct, or "forward" the current ServletRequest and ServletResponse to another active
     * component in the application.
     * </p>
     * <p>
     * If the <I> relativeUrlPath </I> begins with a "/" then the URL specified is calculated relative to the DOCROOT of
     * the <code> ServletContext </code> for this JSP. If the path does not begin with a "/" then the URL specified is
     * calculated relative to the URL of the request that was mapped to the calling JSP.
     * </p>
     * <p>
     * It is only valid to call this method from a <code> Thread </code> executing within a
     * <code> _jspService(...) </code> method of a JSP.
     * </p>
     * <p>
     * Once this method has been called successfully, it is illegal for the calling <code> Thread </code> to attempt to
     * modify the <code>
     * ServletResponse </code> object. Any such attempt to do so, shall result in undefined behavior. Typically, callers
     * immediately return from <code> _jspService(...) </code> after calling this method.
     * </p>
     *
     * @param relativeUrlPath specifies the relative URL path to the target resource as described above
     *
     * @throws IllegalStateException if <code> ServletResponse </code> is not in a state where a forward can be
     *                                   performed
     * @throws ServletException      if the page that was forwarded to throws a ServletException
     * @throws IOException           if an I/O error occurred while forwarding
     */

    public abstract void forward(String relativeUrlPath) throws ServletException, IOException;

    /**
     * <p>
     * Causes the resource specified to be processed as part of the current ServletRequest and ServletResponse being
     * processed by the calling Thread. The output of the target resources processing of the request is written directly
     * to the ServletResponse output stream.
     * </p>
     * <p>
     * The current JspWriter "out" for this JSP is flushed as a side-effect of this call, prior to processing the
     * include.
     * </p>
     * <p>
     * If the <I> relativeUrlPath </I> begins with a "/" then the URL specified is calculated relative to the DOCROOT of
     * the <code>ServletContext</code> for this JSP. If the path does not begin with a "/" then the URL specified is
     * calculated relative to the URL of the request that was mapped to the calling JSP.
     * </p>
     * <p>
     * It is only valid to call this method from a <code> Thread </code> executing within a
     * <code> _jspService(...) </code> method of a JSP.
     * </p>
     *
     * @param relativeUrlPath specifies the relative URL path to the target resource to be included
     *
     * @throws ServletException if the page that was forwarded to throws a ServletException
     * @throws IOException      if an I/O error occurred while forwarding
     */
    public abstract void include(String relativeUrlPath) throws ServletException, IOException;

    /**
     * <p>
     * Causes the resource specified to be processed as part of the current ServletRequest and ServletResponse being
     * processed by the calling Thread. The output of the target resources processing of the request is written directly
     * to the current JspWriter returned by a call to getOut().
     * </p>
     * <p>
     * If flush is true, The current JspWriter "out" for this JSP is flushed as a side-effect of this call, prior to
     * processing the include. Otherwise, the JspWriter "out" is not flushed.
     * </p>
     * <p>
     * If the <i>relativeUrlPath</i> begins with a "/" then the URL specified is calculated relative to the DOCROOT of
     * the <code>ServletContext</code> for this JSP. If the path does not begin with a "/" then the URL specified is
     * calculated relative to the URL of the request that was mapped to the calling JSP.
     * </p>
     * <p>
     * It is only valid to call this method from a <code> Thread </code> executing within a
     * <code> _jspService(...) </code> method of a JSP.
     * </p>
     *
     * @param relativeUrlPath specifies the relative URL path to the target resource to be included
     * @param flush           True if the JspWriter is to be flushed before the include, or false if not.
     *
     * @throws ServletException if the page that was forwarded to throws a ServletException
     * @throws IOException      if an I/O error occurred while forwarding
     *
     * @since JSP 2.0
     */
    public abstract void include(String relativeUrlPath, boolean flush) throws ServletException, IOException;

    /**
     * <p>
     * This method is intended to process an unhandled 'page' level exception by forwarding the exception to the
     * specified error page for this JSP. If forwarding is not possible (for example because the response has already
     * been committed), an implementation dependent mechanism should be used to invoke the error page (e.g. "including"
     * the error page instead).
     * <p>
     * If no error page is defined in the page, the exception should be rethrown so that the standard servlet error
     * handling takes over.
     * <p>
     * A JSP implementation class shall typically clean up any local state prior to invoking this and will return
     * immediately thereafter. It is illegal to generate any output to the client, or to modify any ServletResponse
     * state after invoking this call.
     * <p>
     * This method is kept for backwards compatibility reasons. Newly generated code should use
     * PageContext.handlePageException(Throwable).
     *
     * @param e the exception to be handled
     *
     * @throws ServletException     if an error occurs while invoking the error page
     * @throws IOException          if an I/O error occurred while invoking the error page
     * @throws NullPointerException if the exception is null
     *
     * @see #handlePageException(Throwable)
     */

    public abstract void handlePageException(Exception e) throws ServletException, IOException;

    /**
     * <p>
     * This method is intended to process an unhandled 'page' level exception by forwarding the exception to the
     * specified error page for this JSP. If forwarding is not possible (for example because the response has already
     * been committed), an implementation dependent mechanism should be used to invoke the error page (e.g. "including"
     * the error page instead).
     * <p>
     * If no error page is defined in the page, the exception should be rethrown so that the standard servlet error
     * handling takes over.
     * <p>
     * This method is intended to process an unhandled "page" level exception by redirecting the exception to either the
     * specified error page for this JSP, or if none was specified, to perform some implementation dependent action.
     * <p>
     * A JSP implementation class shall typically clean up any local state prior to invoking this and will return
     * immediately thereafter. It is illegal to generate any output to the client, or to modify any ServletResponse
     * state after invoking this call.
     *
     * @param t the throwable to be handled
     *
     * @throws ServletException     if an error occurs while invoking the error page
     * @throws IOException          if an I/O error occurred while invoking the error page
     * @throws NullPointerException if the exception is null
     *
     * @see #handlePageException(Exception)
     */

    public abstract void handlePageException(Throwable t) throws ServletException, IOException;

    /**
     * Return a new BodyContent object, save the current "out" JspWriter, and update the value of the "out" attribute in
     * the page scope attribute namespace of the PageContext.
     *
     * @return the new BodyContent
     */

    public BodyContent pushBody() {
        return null; // XXX to implement
    }


    /**
     * Provides convenient access to error information.
     *
     * @return an ErrorData instance containing information about the error, as obtained from the request attributes, as
     *             per the Servlet specification. If this is not an error page (that is, if the isErrorPage attribute of
     *             the page directive is not set to "true"), the information is meaningless.
     *
     * @since JSP 2.0
     */
    public ErrorData getErrorData() {
        int status = 0;

        Integer status_code = (Integer) getRequest().getAttribute(RequestDispatcher.ERROR_STATUS_CODE);
        // Avoid NPE if attribute is not set
        if (status_code != null) {
            status = status_code.intValue();
        }

        return new ErrorData((Throwable) getRequest().getAttribute(RequestDispatcher.ERROR_EXCEPTION), status,
                (String) getRequest().getAttribute(RequestDispatcher.ERROR_REQUEST_URI),
                (String) getRequest().getAttribute(RequestDispatcher.ERROR_SERVLET_NAME));
    }

}