Struts2中关于Model-driven的问题

在使用Struts2的时候，使用了model-driven的形式(implements ModelDriven),但是在使用的过程中，缺发现没请求一次action则getModel方法都会被调用两次,这个真是让人郁闷的事情，当然也可能是我的是偶那个方式本身就是错误的，因为在getModel中的逻辑是:
    查询数据库得到数据信息，
    包装为一个model对象
    返回包装后的对象

从上面的逻辑可知，如果getModel被调用两次的话，则也就意味着要做两次数据库请求哈!!

    出现问题的资源文件的版本:
struts2-core-2.0.11.2.jar,  xwork-2.0.4.jar

    配置文件:

<action name="modelDrivenRequest" class="modelDrivenRequest">
	<interceptor-ref name="myStack"/>
	    <interceptor-ref name="modelDriven"/>
	    <interceptor-ref name="params"/>
        <result>/question/modeldriven.jsp</result>
</action>

 
     后来经过跟踪ModelDrivenInterceptor的源代码发现:

if (modelDriven.getModel() !=  null) {
    stack.push(modelDriven.getModel());
}

     这种实现方式，大概是为了确保，stack.push操作的model对象是最新的吧，但这也正是getModel方法被调用了两次的原因, 
     后来考虑干脆不再使用modelDriven的实现形式，感觉这个东西的意义不大，尤其在如下形式的时候，更是不知所措:
     implements ModelDriven<List<MyData>> ,也就是model是一个集合的情况; 舍弃ModelDriven是我的选择，当然也可能是对它的理解有偏差的缘故吧，

     最后为了解决调用getModel方法两次的问题，可以参看xwork-2.1.3的实现版本，这里给出它的源代码:

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

import com.opensymphony.xwork2.ActionInvocation;
import com.opensymphony.xwork2.ModelDriven;
import com.opensymphony.xwork2.util.CompoundRoot;
import com.opensymphony.xwork2.util.ValueStack;


/**
 * <!-- START SNIPPET: description -->
 *
 * Watches for {@link ModelDriven} actions and adds the action's model on to the value stack.
 *
 * <p/> <b>Note:</b>  The ModelDrivenInterceptor must come before the both {@link StaticParametersInterceptor} and
 * {@link ParametersInterceptor} if you want the parameters to be applied to the model.
 * 
 * <p/> <b>Note:</b>  The ModelDrivenInterceptor will only push the model into the stack when the
 * model is not null, else it will be ignored.
 *
 * <!-- END SNIPPET: description -->
 *
 * <p/> <u>Interceptor parameters:</u>
 *
 * <!-- START SNIPPET: parameters -->
 *
 * <ul>
 *
 * <li>refreshModelBeforeResult - set to true if you want the model to be refreshed on the value stack after action
 * execution and before result execution.  The setting is useful if you want to change the model instance during the
 * action execution phase, like when loading it from the data layer.  This will result in getModel() being called at
 * least twice.</li>
 *
 * </ul>
 *
 * <!-- END SNIPPET: parameters -->
 *
 * <p/> <u>Extending the interceptor:</u>
 *
 * <p/>
 *
 * <!-- START SNIPPET: extending -->
 *
 * There are no known extension points to this interceptor.
 *
 * <!-- END SNIPPET: extending -->
 *
 * <p/> <u>Example code:</u>
 *
 * <pre>
 * <!-- START SNIPPET: example -->
 * &lt;action name="someAction" class="com.examples.SomeAction"&gt;
 *     &lt;interceptor-ref name="modelDriven"/&gt;
 *     &lt;interceptor-ref name="basicStack"/&gt;
 *     &lt;result name="success"&gt;good_result.ftl&lt;/result&gt;
 * &lt;/action&gt;
 * <!-- END SNIPPET: example -->
 * </pre>
 * 
 * @author tm_jee
 * @version $Date: 2008-06-21 17:29:39 +0800 (星期六, 21 六月 2008) $ $Id: ModelDrivenInterceptor.java 1833 2008-06-21 09:29:39Z rainerh $
 */
public class ModelDrivenInterceptor extends AbstractInterceptor {

    protected boolean refreshModelBeforeResult = false;

    public void setRefreshModelBeforeResult(boolean val) {
        this.refreshModelBeforeResult = val;
    }

    @Override
    public String intercept(ActionInvocation invocation) throws Exception {
        Object action = invocation.getAction();

        if (action instanceof ModelDriven) {
            ModelDriven modelDriven = (ModelDriven) action;
            ValueStack stack = invocation.getStack();
            Object model = modelDriven.getModel();
            if (model !=  null) {
            	stack.push(model);
            }
            if (refreshModelBeforeResult) {
                invocation.addPreResultListener(new RefreshModelBeforeResult(modelDriven, model));
            }
        }
        return invocation.invoke();
    }

    /**
     * Refreshes the model instance on the value stack, if it has changed
     */
    protected static class RefreshModelBeforeResult implements PreResultListener {
        private Object originalModel = null;
        protected ModelDriven action;


        public RefreshModelBeforeResult(ModelDriven action, Object model) {
            this.originalModel = model;
            this.action = action;
        }

        public void beforeResult(ActionInvocation invocation, String resultCode) {
            ValueStack stack = invocation.getStack();
            CompoundRoot root = stack.getRoot();

            boolean needsRefresh = true;
            Object newModel = action.getModel();

            // Check to see if the new model instance is already on the stack
            for (Object item : root) {
                if (item == newModel) {
                    needsRefresh = false;
                }
            }

            // Add the new model on the stack
            if (needsRefresh) {

                // Clear off the old model instance
                if (originalModel != null) {
                    root.remove(originalModel);
                }
                if (newModel != null) {
                    stack.push(newModel);
                }
            }
        }
    }
}

    通过这个实现我们可以通过配置来决定是否调用getModel方法两次:

     <action name="modelDrivenRequest" class="modelDrivenRequest">
		    <interceptor-ref name="myStack"/>
			<interceptor-ref name="modelDriven213">
			    <param name="refreshModelBeforeResult">false</param>
			</interceptor-ref>
			<interceptor-ref name="params"/>
            <result>/question/modeldriven.jsp</result>
        </action>