/*
* 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 javax.servlet.jsp;

import java.util.Enumeration;

import javax.servlet.jsp.el.ExpressionEvaluator;
import javax.servlet.jsp.el.VariableResolver;

/**
 * <p>
 * <code>JspContext</code> serves as the base class for the 
 * PageContext class and abstracts all information that is not specific
 * to servlets.  This allows for Simple Tag Extensions to be used
 * outside of the context of a request/response Servlet.
 * <p>
 * The JspContext 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 mechanism to obtain the JspWriter for output
 * <li>a mechanism to expose page directive attributes to the 
 *     scripting environment
 * </ul>
 *
 * <p><B>Methods Intended for Container Generated Code</B>
 * <p>
 * The following methods enable the <B>management of nested</B> JspWriter 
 * streams to implement Tag Extensions: <code>pushBody()</code> and
 * <code>popBody()</code>
 *
 * <p><B>Methods Intended for JSP authors</B>
 * <p>
 * Some methods provide <B>uniform access</B> to the diverse objects
 * representing scopes.
 * The implementation must use the underlying machinery
 * corresponding to that scope, so information can be passed back and
 * forth between the underlying environment (e.g. Servlets) and JSP pages.
 * The methods are:
 * <code>setAttribute()</code>,  <code>getAttribute()</code>,
 * <code>findAttribute()</code>,  <code>removeAttribute()</code>,
 * <code>getAttributesScope()</code> and 
 * <code>getAttributeNamesInScope()</code>.
 * 
 * <p>
 * The following methods provide <B>convenient access</B> to implicit objects:
 * <code>getOut()</code>
 *
 * <p>
 * The following methods provide <B>programmatic access</b> to the 
 * Expression Language evaluator:
 * <code>getExpressionEvaluator()</code>, <code>getVariableResolver()</code>
 *
 * @since 2.0
 */

public abstract class JspContext {

    /**
     * Sole constructor. (For invocation by subclass constructors, 
     * typically implicit.)
     */
    public JspContext() {
    }
    
    /** 
     * Register the name and value specified with page scope semantics.
     * If the value passed in is <code>null</code>, this has the same 
     * effect as calling 
     * <code>removeAttribute( name, PageContext.PAGE_SCOPE )</code>.
     *
     * @param name the name of the attribute to set
     * @param value the value to associate with the name, or null if the
     *     attribute is to be removed from the page scope.
     * @throws NullPointerException if the name is null
     */

    abstract public void setAttribute(String name, Object value);

    /**
     * Register the name and value specified with appropriate 
     * scope semantics.  If the value passed in is <code>null</code>, 
     * this has the same effect as calling
     * <code>removeAttribute( name, scope )</code>.
     * 
     * @param name the name of the attribute to set
     * @param value the object to associate with the name, or null if
     *     the attribute is to be removed from the specified scope.
     * @param scope the scope with which to associate the name/object
     * 
     * @throws NullPointerException if the name is null
     * @throws IllegalArgumentException if the scope is invalid
     * @throws IllegalStateException if the scope is 
     *     PageContext.SESSION_SCOPE but the page that was requested
     *     does not participate in a session or the session has been
     *     invalidated.
     */

    abstract public void setAttribute(String name, Object value, int scope);

    /**
     * Returns the object associated with the name in the page scope or null
     * if not found.
     *
     * @param name the name of the attribute to get
     * @return the object associated with the name in the page scope 
     *     or null if not found.
     * 
     * @throws NullPointerException if the name is null
     */

    abstract public Object getAttribute(String name);

    /**
     * Return the object associated with the name in the specified
     * scope or null if not found.
     *
     * @param name the name of the attribute to set
     * @param scope the scope with which to associate the name/object
     * @return the object associated with the name in the specified
     *     scope or null if not found.
     * 
     * @throws NullPointerException if the name is null
     * @throws IllegalArgumentException if the scope is invalid 
     * @throws IllegalStateException if the scope is 
     *     PageContext.SESSION_SCOPE but the page that was requested
     *     does not participate in a session or the session has been
     *     invalidated.
     */

    abstract public Object getAttribute(String name, int scope);

    /**
     * Searches for the named attribute in page, request, session (if valid),
     * and application scope(s) in order and returns the value associated or
     * null.
     *
     * @param name the name of the attribute to search for
     * @return the value associated or null
     * @throws NullPointerException if the name is null
     */

    abstract public Object findAttribute(String name);

    /**
     * Remove the object reference associated with the given name
     * from all scopes.  Does nothing if there is no such object.
     *
     * @param name The name of the object to remove.
     * @throws NullPointerException if the name is null
     */

    abstract public void removeAttribute(String name);

    /**
     * Remove the object reference associated with the specified name
     * in the given scope.  Does nothing if there is no such object.
     *
     * @param name The name of the object to remove.
     * @param scope The scope where to look.
     * @throws IllegalArgumentException if the scope is invalid
     * @throws IllegalStateException if the scope is 
     *     PageContext.SESSION_SCOPE but the page that was requested
     *     does not participate in a session or the session has been
     *     invalidated.
     * @throws NullPointerException if the name is null
     */

    abstract public void removeAttribute(String name, int scope);

    /**
     * Get the scope where a given attribute is defined.
     *
     * @param name the name of the attribute to return the scope for
     * @return the scope of the object associated with the name specified or 0
     * @throws NullPointerException if the name is null
     */

    abstract public int getAttributesScope(String name);

    /**
     * Enumerate all the attributes in a given scope.
     *
     * @param scope the scope to enumerate all the attributes for
     * @return an enumeration of names (java.lang.String) of all the 
     *     attributes the specified scope
     * @throws IllegalArgumentException if the scope is invalid
     * @throws IllegalStateException if the scope is 
     *     PageContext.SESSION_SCOPE but the page that was requested
     *     does not participate in a session or the session has been
     *     invalidated.
     */

    abstract public Enumeration getAttributeNamesInScope(int scope);

    /**
     * The current value of the out object (a JspWriter).
     *
     * @return the current JspWriter stream being used for client response
     */
    abstract public JspWriter getOut();
    
    /**
     * Provides programmatic access to the ExpressionEvaluator.
     * The JSP Container must return a valid instance of an 
     * ExpressionEvaluator that can parse EL expressions.
     *
     * @return A valid instance of an ExpressionEvaluator.
     * @since 2.0
     */
    public abstract ExpressionEvaluator getExpressionEvaluator();
    
    /**
     * Returns an instance of a VariableResolver that provides access to the
     * implicit objects specified in the JSP specification using this JspContext
     * as the context object.
     *
     * @return A valid instance of a VariableResolver.
     * @since 2.0
     */
    public abstract VariableResolver getVariableResolver();
    
    /**
     * Return a new JspWriter object that sends output to the
     * provided Writer.  Saves the current "out" JspWriter,
     * and updates the value of the "out" attribute in the
     * page scope attribute namespace of the JspContext.
     * <p>The returned JspWriter must implement all methods and
     * behave as though it were unbuffered.  More specifically:
     * <ul>
     *   <li>clear() must throw an IOException</li>
     *   <li>clearBuffer() does nothing</li>
     *   <li>getBufferSize() always returns 0</li>
     *   <li>getRemaining() always returns 0</li>
     * </ul>
     * </p>
     *
     * @param writer The Writer for the returned JspWriter to send
     *     output to.
     * @return a new JspWriter that writes to the given Writer.
     * @since 2.0
     */
    public JspWriter pushBody( java.io.Writer writer ) {
        return null; // XXX to implement
    }
    
    /**
     * Return the previous JspWriter "out" saved by the matching
     * pushBody(), and update the value of the "out" attribute in
     * the page scope attribute namespace of the JspContext.
     *
     * @return the saved JspWriter.
     */
    public JspWriter popBody() {
        return null; // XXX to implement
    }
}