Struts example source code file (InstantiatingNullHandler.java)

This example Struts source code file (InstantiatingNullHandler.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Java - Struts tags/keywords

arraylist, bean, class, entering, entering, exception, hashmap, inject, inject, javabean, map, object, object, objectfactory, string, string, util

The Struts InstantiatingNullHandler.java source code

/*
 * Copyright 2002-2006,2009 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 com.opensymphony.xwork2.conversion.impl;

import com.opensymphony.xwork2.ObjectFactory;
import com.opensymphony.xwork2.conversion.NullHandler;
import com.opensymphony.xwork2.conversion.ObjectTypeDeterminer;
import com.opensymphony.xwork2.inject.Inject;
import com.opensymphony.xwork2.util.logging.Logger;
import com.opensymphony.xwork2.util.logging.LoggerFactory;
import com.opensymphony.xwork2.util.reflection.ReflectionContextState;
import com.opensymphony.xwork2.util.reflection.ReflectionProvider;

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


/**
 * <!-- START SNIPPET: javadoc -->
 *
 * Provided that the key {@link ReflectionContextState#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.
 *
 * <p/> The following rules are used when handling null references:
 *
 * <ul>
 *
 * <li>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.</li>
 *
 * <li>If the property is declared as a {@link Map}, then a HashMap will be returned and assigned to the null
 * references.</li>
 *
 * <li>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.</li>
 *
 * </ul>
 *
 * <!-- END SNIPPET: javadoc -->
 *
 * <!-- START SNIPPET: example -->
 *
 * For example, if a form element has a text field named <b>person.name and the expression person evaluates
 * to null, then this class will be invoked. Because the <i>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.
 *
 * <!-- END SNIPPET: example>
 *
 * @author Matt Ho
 * @author Patrick Lightbody
 */
public class InstantiatingNullHandler implements NullHandler {

    /**
     * @deprecated Use {@link ReflectionContextState#CREATE_NULL_OBJECTS} instead
     */
    @Deprecated public static final String CREATE_NULL_OBJECTS = ReflectionContextState.CREATE_NULL_OBJECTS;
    private static final Logger LOG = LoggerFactory.getLogger(InstantiatingNullHandler.class);
    private ReflectionProvider reflectionProvider;
    private ObjectFactory objectFactory;
    private ObjectTypeDeterminer objectTypeDeterminer;

    @Inject
    public void setObjectTypeDeterminer(ObjectTypeDeterminer det) {
        this.objectTypeDeterminer = det;
    }

    @Inject
    public void setReflectionProvider(ReflectionProvider prov) {
        this.reflectionProvider = prov;
    }

    @Inject
    public void setObjectFactory(ObjectFactory fac) {
        this.objectFactory = fac;
    }

    public Object nullMethodResult(Map<String, Object> context, Object target, String methodName, Object[] args) {
        if (LOG.isDebugEnabled()) {
            LOG.debug("Entering nullMethodResult ");
        }

        return null;
    }

    public Object nullPropertyValue(Map<String, Object> context, Object target, Object property) {
        if (LOG.isDebugEnabled()) {
            LOG.debug("Entering nullPropertyValue [target="+target+", property="+property+"]");
        }

        boolean c = ReflectionContextState.isCreatingNullObjects(context);

        if (!c) {
            return null;
        }

        if ((target == null) || (property == null)) {
            return null;
        }

        try {
            String propName = property.toString();
            Object realTarget = reflectionProvider.getRealTarget(propName, context, target);
            Class clazz = null;

            if (realTarget != null) {
                PropertyDescriptor pd = reflectionProvider.getPropertyDescriptor(realTarget.getClass(), propName);
                if (pd == null) {
                    return null;
                }

                clazz = pd.getPropertyType();
            }

            if (clazz == null) {
                // can't do much here!
                return null;
            }

            Object param = createObject(clazz, realTarget, propName, context);

            reflectionProvider.setValue(propName, context, realTarget, param);

            return param;
        } catch (Exception e) {
            LOG.error("Could not create and/or set value back on to object", e);
        }

        return null;
    }

    private Object createObject(Class clazz, Object target, String property, Map<String, Object> context) throws Exception {
        if (Collection.class.isAssignableFrom(clazz)) {
            return new ArrayList();
        } else if (clazz == Map.class) {
            return new HashMap();
        } else if (clazz == EnumMap.class) {
            Class keyClass = objectTypeDeterminer.getKeyClass(target.getClass(), property);
            return new EnumMap(keyClass);
        }

        return objectFactory.buildBean(clazz, context);
    }
}