浅析=======Struts2之==========valueStack

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今天刚学习了struts2的valueStack，在这里把自己学到的做个分享，说道valueStack就不得不提OGNL表达式===

struts2工作流程

1.OGNL（Object Graph Navigation Language）对象导航图语言

　　Struts2框架使用OGNL作为默认的表达式语言，OGNL（Object Graph Navigation Language），是一种表达式语言，目的是为了在不能写Java代码的地方执行java代码；主要作用是用来存数据和取数据的。

2 valueStack

valueStack是struts2的值栈空间，是struts2存储数据的空间

　　Strut2的Action类通过属性可以获得所有请求相关的值，要在Action类中声明与参数同名的属性或者以类名小写加属性名如：user.name,就可以获得这些参数值，在Struts2调用Action类的Action方法（默认是execute方法）之前，就会为相应的Action属性赋值。这项功能Struts2要依赖于ValueStack对象来实现，每个Action类的对象实例会拥有一个ValueStack对象，这个对象贯穿整个Action的生命周期，

　　当Struts2接收到一个请求后，会先建立Action类的对象实例，但并不会立即调用Action方法，而是先将Action类的相应属性放到ValueStack实现类ONGLValueStack对象root对象的顶层节点。只是所有的属性值都是默认的值，如String类型的属性值为null，int类型的属性值为0等。在处理完上述工作后，Struts 2就会调用拦截器链中的拦截器，这些拦截器会根据用户请求参数值去更新ValueStack对象顶层节点的相应属性的值，最后会传到Action对象，并将ValueStack对象中的属性值，赋给Action类的相应属性。当调用完所有的拦截器后，才会调用Action类的Action方法。ValueStack会在请求开始时被创建，请求结束时消亡。

3.valueStack底层相关的数据结构

CompoundRoot是继承了Arraylist的集合，里面存放Action的实例、代理对象、自定义对象（Student）等数据

5 代码展示valueStack

需求：通过页面跳转到指定的Action的方法中，在方法中通过

创建student实体类

package com.xsh.pojo;public class Student {    private String name;    public Student(String name) {        super();        this.name = name;    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }}

struts.xml配置文件

<?xml version="1.0" encoding="UTF-8" ?><!DOCTYPE struts PUBLIC    "-//Apache Software Foundation//DTD Struts Configuration 2.3//EN"    "http://struts.apache.org/dtds/struts-2.3.dtd"><struts>    <constant name="struts.devMode" value="true"/>    <package name="default" namespace="/user" extends="struts-default">        <action name="login" class="com.xsh.test.LoginAction" method="execute">            <result>/success.jsp</result>        </action>    </package></struts>

Action类

package com.xsh.test;import java.util.Map;import com.opensymphony.xwork2.ActionContext;import com.opensymphony.xwork2.ActionSupport;import com.opensymphony.xwork2.util.ValueStack;import com.xsh.pojo.Student;public class LoginAction extends ActionSupport {    public String execute(){        System.out.println("进入方法=====");        Map<String, Object> session = ActionContext.getContext().getSession();        session.put("sessionName", "sessionName");        Map<String, Object> request = (Map<String, Object>) ActionContext.getContext().get("request");        request.put("requestName", "requestName");        Map<String, Object> application = ActionContext.getContext().getApplication();        application.put("applicationName", "applicationName");        ValueStack valueStack = (ValueStack) request.get("struts.valueStack");//通过request对象获得valueStack对象

System.out.println("valueStack的值======"); valueStack.getRoot().add(new Student("小白白"));//向valueStack的root属性中添加自定义值 

 System.out.println(valueStack); return SUCCESS; } }

strut2的底层getroot()是接口ValueStack 定义的方法，返回CompoundRoot继承了Arraylist的集合

 /**     * Get the CompoundRoot which holds the objects pushed onto the stack     *     * @return the root     */    public abstract CompoundRoot getRoot();

/** * A Stack that is implemented using a List. *  * @author plightbo * @version $Revision$ */public class CompoundRoot extends ArrayList {    public CompoundRoot() {    }    public CompoundRoot(List list) {        super(list);    }    public CompoundRoot cutStack(int index) {        return new CompoundRoot(subList(index, size()));    }    public Object peek() {        return get(0);    }    public Object pop() {        return remove(0);    }    public void push(Object o) {        add(0, o);    }}

登录实现跳转页面login.jsp

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%><%String path = request.getContextPath();String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html>  <head>    <base href="<%=basePath%>">        <title>My JSP 'login.jsp' starting page</title>        <meta http-equiv="pragma" content="no-cache">    <meta http-equiv="cache-control" content="no-cache">    <meta http-equiv="expires" content="0">        <meta http-equiv="keywords" content="keyword1,keyword2,keyword3">    <meta http-equiv="description" content="This is my page">    <!--    <link rel="stylesheet" type="text/css" href="styles.css">    -->  </head>  <body>      <h1><a href="user/login">Check ValueStack</a></h1>  </body></html>

跳转成功页面success.jsp

String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html>  <head>    <base href="<%=basePath%>">        <title>My JSP 'success.jsp' starting page</title>        <meta http-equiv="pragma" content="no-cache">    <meta http-equiv="cache-control" content="no-cache">    <meta http-equiv="expires" content="0">        <meta http-equiv="keywords" content="keyword1,keyword2,keyword3">    <meta http-equiv="description" content="This is my page">    <!--    <link rel="stylesheet" type="text/css" href="styles.css">    --><%@ taglib prefix="s" uri="/struts-tags" %>  </head>    <body>    <h1><s:debug/></h1>  </body></html>

点击查看ValueStack，标题进入旋转状态（前提在代码内打好断点），查看断电是的valueStack的状态，如图

在debug模式中，f6，继续查看变化，通过getRoot()方法获得的集合对象的add方法添加的Student方法已经添加到

5.运输处结果

打印ValueStack对象的hash码是OnglValueStack，查看ObglValueStack类

可知：

a.OgnlValueStack实现了ValueStack的接口

b.属性root是 CompoundRoot类型，CompoundRoot继承了Arraylist的集合

c.context是一个Map<String, Object>类型的集合

d.通过Context的get(key)方法获得的Session、request、Application对象还是还是一个Map<String, Object>类型的集合

/** * Ognl implementation of a value stack that allows for dynamic Ognl expressions to be evaluated against it. When evaluating an expression, * the stack will be searched down the stack, from the latest objects pushed in to the earliest, looking for a bean with a getter or setter * for the given property or a method of the given name (depending on the expression being evaluated). * * @author Patrick Lightbody * @author tm_jee * @version $Date$ $Id$ */public class OgnlValueStack implements Serializable, ValueStack, ClearableValueStack, MemberAccessValueStack {    public static final String THROW_EXCEPTION_ON_FAILURE = OgnlValueStack.class.getName() + ".throwExceptionOnFailure";    private static final long serialVersionUID = 370737852934925530L;    private static final String MAP_IDENTIFIER_KEY = "com.opensymphony.xwork2.util.OgnlValueStack.MAP_IDENTIFIER_KEY";    private static final Logger LOG = LoggerFactory.getLogger(OgnlValueStack.class);    CompoundRoot root;    transient Map<String, Object> context;    Class defaultType;    Map<Object, Object> overrides;    transient OgnlUtil ognlUtil;    transient SecurityMemberAccess securityMemberAccess;    private boolean devMode;

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