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XWork is an command-pattern framework that is used to power WebWork as well as other applications. XWork provides an Inversion of Control container, a powerful expression language, data type conversion, validation, and pluggable configuration.

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/*
 * Copyright (c) 2002-2006 by OpenSymphony
 * All rights reserved.
 */
package com.opensymphony.xwork2.util;

import com.opensymphony.xwork2.ObjectFactory;

import ognl.NullHandler;
import ognl.Ognl;
import ognl.OgnlRuntime;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import java.beans.PropertyDescriptor;
import java.util.*;


/**
 * 
 *
 * Provided that the key {@link #CREATE_NULL_OBJECTS} is in the action context with a value of true (this key is set
 * only during the execution of the {@link com.opensymphony.xwork2.interceptor.ParametersInterceptor}), OGNL expressions
 * that have caused a NullPointerException will be temporarily stopped for evaluation while the system automatically
 * tries to solve the null references by automatically creating the object.
 *
 * 
 The following rules are used when handling null references:
 *
 * 
    
 *
 * 
  • If the property is declared exactly as a {@link Collection} or {@link List}, then an ArrayList shall be
 * returned and assigned to the null references.
    • 
 *
 * 
  • If the property is declared as a {@link Map}, then a HashMap will be returned and assigned to the null
 * references.
    • 
 *
 * 
  • If the null property is a simple bean with a no-arg constructor, it will simply be created using the {@link
 * ObjectFactory#buildBean(java.lang.Class, java.util.Map)} method.
    • 
 *
 * 

 *
 * 
 *
 * 
 *
 * For example, if a form element has a text field named person.name and the expression person evaluates
 * to null, then this class will be invoked. Because the person expression evaluates to a Person class, a
 * new Person is created and assigned to the null reference. Finally, the name is set on that object and the overall
 * effect is that the system automatically created a Person object for you, set it by calling setUsers() and then
 * finally called getUsers().setName() as you would typically expect.
 *
 *