package org.apache.jasper.compiler;

import javax.servlet.ServletContext;

import org.apache.jasper.JspCompilationContext;

/**
 * ELInterpreter Factory
 * 
 * Supports how to get a right ELInterpreter from ServletContext.
 * 
 * 1. It supports get ELInterpreter instance from ServletContext, so that Web Application can provide its own ELInterpreter.
 * 2. Customized ELInterpreter can be passed-in by setting "org.apache.jasper.compiler.ELInterpreter=xxx" in ServletContext's initialization parameters.
 * 3. If there is no any ELInterpreter, the DefaultELInterpreter will be used
 * 
 * @author Sheldon Shao (XShao@ebay.com)
 */
public class ELInterpreterFactory {

    private ELInterpreterFactory() {
    }

    public static final String EL_INTERPRETER_CLASS_NAME = ELInterpreter.class.getName();
    
    /**
     * Default Instance
     */
    private static final ELInterpreter DEFAULT_INSTANCE = new DefaultELInterpreter();
    
    public static class DefaultELInterpreter implements ELInterpreter {

        /**
         * Produces a String representing a call to the EL interpreter.
         *
         * @param context JspCompilationContext
         * @param expression
         *            a String containing zero or more "${}" expressions
         * @param expectedType
         *            the expected type of the interpreted result
         * @param fnmapvar
         *            Variable pointing to a function map.
         * @param XmlEscape
         *            True if the result should do XML escaping
         * @return a String representing a call to the EL interpreter.
         */
        @Override
        public String interpreterCall(JspCompilationContext context,
                boolean isTagFile, String expression,
                Class<?> expectedType, String fnmapvar, boolean xmlEscape) {
            return JspUtil.interpreterCall(isTagFile, expression, expectedType, fnmapvar, xmlEscape);
        }
    }

    /**
     * Retrieve ELInterpreter implementation from ServletContext
     * 
     * @param context ServletContext
     */
    public static ELInterpreter getELInterpreter(ServletContext context) throws Exception {
        ELInterpreter interpreter = null;
        //Check ELInterpreter in 
        Object conf = context.getAttribute(EL_INTERPRETER_CLASS_NAME);
        if (conf == null) {
            String className = context.getInitParameter(EL_INTERPRETER_CLASS_NAME);
            if (className == null) { //If there is no any parameter from Context, try to resolve the implementation from System property
            	className = System.getProperty(EL_INTERPRETER_CLASS_NAME);
            }
            if (className != null) {
                interpreter = toInterpreter(context, className);
            }
            else {
                interpreter =  DEFAULT_INSTANCE;
            }
        }
        else if (conf instanceof ELInterpreter) {
            interpreter = (ELInterpreter)conf;
            return interpreter;
        }
        else if (conf instanceof String) {
            interpreter = toInterpreter(context, (String)conf);
        }
        context.setAttribute(EL_INTERPRETER_CLASS_NAME, interpreter);
        return interpreter;
    }
    
    /**
     * Create ELInterpreter by given class name
     * 
     * @param context ServletContext
     * @param className Class Name
     * @return
     */
    private static ELInterpreter toInterpreter(ServletContext context, String className) throws Exception {
        return (ELInterpreter)context.getClassLoader().loadClass(className).newInstance();
    }
}