Struts2一个重要点就是值栈。
ValueStack，是用来存储一些在各个action，或者说是通过s标签、el表达式等给前台Jsp等页面展示的东西。
ValueStack是一个接口，其内部接口非常简单：
/*
* Copyright 2002-2007,2009 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.util; import java.util.Map; /**
* ValueStack allows multiple beans to be pushed in and dynamic EL 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).
*/
public interface ValueStack { public static final String VALUE_STACK = "com.opensymphony.xwork2.util.ValueStack.ValueStack"; public static final String REPORT_ERRORS_ON_NO_PROP = "com.opensymphony.xwork2.util.ValueStack.ReportErrorsOnNoProp"; /**
* Gets the context for this value stack. The context holds all the information in the value stack and it's surroundings.
*
* @return the context.
*/
public abstract Map<String, Object> getContext(); /**
* Sets the default type to convert to if no type is provided when getting a value.
*
* @param defaultType the new default type
*/
public abstract void setDefaultType(Class defaultType); /**
* Set a override map containing <code>key -> values</code> that takes precedent when doing find operations on the ValueStack.
* <p/>
* See the unit test for ValueStackTest for examples.
*
* @param overrides overrides map.
*/
public abstract void setExprOverrides(Map<Object, Object> overrides); /**
* Gets the override map if anyone exists.
*
* @return the override map, <tt>null</tt> if not set.
*/
public abstract Map<Object, Object> getExprOverrides(); /**
* Get the CompoundRoot which holds the objects pushed onto the stack
*
* @return the root
*/
public abstract CompoundRoot getRoot(); /**
* Attempts to set a property on a bean in the stack with the given expression using the default search order.
*
* @param expr the expression defining the path to the property to be set.
* @param value the value to be set into the named property
*/
public abstract void setValue(String expr, Object value); /**
* Attempts to set a property on a bean in the stack with the given expression using the default search order.
* N.B.: unlike #setValue(String,Object) it doesn't allow eval expression.
* @param expr the expression defining the path to the property to be set.
* @param value the value to be set into the named property
*/
void setParameter(String expr, Object value); /**
* Attempts to set a property on a bean in the stack with the given expression using the default search order.
*
* @param expr the expression defining the path to the property to be set.
* @param value the value to be set into the named property
* @param throwExceptionOnFailure a flag to tell whether an exception should be thrown if there is no property with
* the given name.
*/
public abstract void setValue(String expr, Object value, boolean throwExceptionOnFailure); public abstract String findString(String expr);
public abstract String findString(String expr, boolean throwExceptionOnFailure); /**
* Find a value by evaluating the given expression against the stack in the default search order.
*
* @param expr the expression giving the path of properties to navigate to find the property value to return
* @return the result of evaluating the expression
*/
public abstract Object findValue(String expr); public abstract Object findValue(String expr, boolean throwExceptionOnFailure); /**
* Find a value by evaluating the given expression against the stack in the default search order.
*
* @param expr the expression giving the path of properties to navigate to find the property value to return
* @param asType the type to convert the return value to
* @return the result of evaluating the expression
*/
public abstract Object findValue(String expr, Class asType);
public abstract Object findValue(String expr, Class asType, boolean throwExceptionOnFailure); /**
* Get the object on the top of the stack <b>without</b> changing the stack.
*
* @return the object on the top.
* @see CompoundRoot#peek()
*/
public abstract Object peek(); /**
* Get the object on the top of the stack and <b>remove</b> it from the stack.
*
* @return the object on the top of the stack
* @see CompoundRoot#pop()
*/
public abstract Object pop(); /**
* Put this object onto the top of the stack
*
* @param o the object to be pushed onto the stack
* @see CompoundRoot#push(Object)
*/
public abstract void push(Object o); /**
* Sets an object on the stack with the given key
* so it is retrievable by {@link #findValue(String)}, {@link #findValue(String, Class)}
*
* @param key the key
* @param o the object
*/
public abstract void set(String key, Object o); /**
* Get the number of objects in the stack
*
* @return the number of objects in the stack
*/
public abstract int size(); }
和一个普通的栈没多大区别。
他的实现类就比较复杂了（其实也不复杂...）
public class OgnlValueStack implements Serializable, ValueStack, ClearableValueStack, MemberAccessValueStack {
这里贴一部分。
现在来说说值栈的具体作用：
当用户发出一个请求，产生一个request，随即产生一个valueStack，然后Action中的setXX()方法，添加到值栈中，然后struts2根据响应的内容跳转到下一个jsp页面中，这个页面可以通过s标签或者el表达式去获取属性值。
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<%@ taglib prefix="s" uri="/struts-tags"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
</head>
<body>
<s:iterator value="list" status="li">
<s:property value="name"/>
<s:property value="title"/>
<s:property value="content"/>
<s:property value="lastDate"/>
<form action="reply.action">
<input type="hidden" name="leaveWord_id" value="${id}">
<input type="submit" value="回复">
</form>
</br>
</s:iterator>
</body>
</html>
最后，详细的说一下s标签或者el表达式是如何拿到属性值的（值栈的工作原理）：
/**
* Gets the override map if anyone exists.
*
* @return the override map, <tt>null</tt> if not set.
*/
public abstract Map<Object, Object> getExprOverrides(); /**
* Get the CompoundRoot which holds the objects pushed onto the stack
*
* @return the root
*/
public abstract CompoundRoot getRoot();
上面的root是一个真正的栈结构，而Map就是ActionContext.
当s标签获取属性值时，先从root中找，看有没有，然后再从map中找。
另外，值栈的生命周期 = Request的生命周期，ActionContext的生命周期 = session的生命周期
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