struts2 ValueStack（值栈）解析

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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