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：
1 /*
2 * Copyright 2002-2007,2009 The Apache Software Foundation.
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16 package com.opensymphony.xwork2.util;
17
18 import java.util.Map;
19
20 /**
21 * ValueStack allows multiple beans to be pushed in and dynamic EL expressions to be evaluated against it. When
22 * evaluating an expression, the stack will be searched down the stack, from the latest objects pushed in to the
23 * earliest, looking for a bean with a getter or setter for the given property or a method of the given name (depending
24 * on the expression being evaluated).
25 */
26 public interface ValueStack {
27
28 public static final String VALUE_STACK = "com.opensymphony.xwork2.util.ValueStack.ValueStack";
29
30 public static final String REPORT_ERRORS_ON_NO_PROP = "com.opensymphony.xwork2.util.ValueStack.ReportErrorsOnNoProp";
31
32 /**
33 * Gets the context for this value stack. The context holds all the information in the value stack and it‘s surroundings.
34 *
35 * @return the context.
36 */
37 public abstract Map<String, Object> getContext();
38
39 /**
40 * Sets the default type to convert to if no type is provided when getting a value.
41 *
42 * @param defaultType the new default type
43 */
44 public abstract void setDefaultType(Class defaultType);
45
46 /**
47 * Set a override map containing <code>key -> values</code> that takes precedent when doing find operations on the ValueStack.
48 * <p/>
49 * See the unit test for ValueStackTest for examples.
50 *
51 * @param overrides overrides map.
52 */
53 public abstract void setExprOverrides(Map<Object, Object> overrides);
54
55 /**
56 * Gets the override map if anyone exists.
57 *
58 * @return the override map, <tt>null</tt> if not set.
59 */
60 public abstract Map<Object, Object> getExprOverrides();
61
62 /**
63 * Get the CompoundRoot which holds the objects pushed onto the stack
64 *
65 * @return the root
66 */
67 public abstract CompoundRoot getRoot();
68
69 /**
70 * Attempts to set a property on a bean in the stack with the given expression using the default search order.
71 *
72 * @param expr the expression defining the path to the property to be set.
73 * @param value the value to be set into the named property
74 */
75 public abstract void setValue(String expr, Object value);
76
77 /**
78 * Attempts to set a property on a bean in the stack with the given expression using the default search order.
79 * N.B.: unlike #setValue(String,Object) it doesn‘t allow eval expression.
80 * @param expr the expression defining the path to the property to be set.
81 * @param value the value to be set into the named property
82 */
83 void setParameter(String expr, Object value);
84
85 /**
86 * Attempts to set a property on a bean in the stack with the given expression using the default search order.
87 *
88 * @param expr the expression defining the path to the property to be set.
89 * @param value the value to be set into the named property
90 * @param throwExceptionOnFailure a flag to tell whether an exception should be thrown if there is no property with
91 * the given name.
92 */
93 public abstract void setValue(String expr, Object value, boolean throwExceptionOnFailure);
94
95 public abstract String findString(String expr);
96 public abstract String findString(String expr, boolean throwExceptionOnFailure);
97
98 /**
99 * Find a value by evaluating the given expression against the stack in the default search order.
100 *
101 * @param expr the expression giving the path of properties to navigate to find the property value to return
102 * @return the result of evaluating the expression
103 */
104 public abstract Object findValue(String expr);
105
106 public abstract Object findValue(String expr, boolean throwExceptionOnFailure);
107
108 /**
109 * Find a value by evaluating the given expression against the stack in the default search order.
110 *
111 * @param expr the expression giving the path of properties to navigate to find the property value to return
112 * @param asType the type to convert the return value to
113 * @return the result of evaluating the expression
114 */
115 public abstract Object findValue(String expr, Class asType);
116 public abstract Object findValue(String expr, Class asType, boolean throwExceptionOnFailure);
117
118 /**
119 * Get the object on the top of the stack <b>without</b> changing the stack.
120 *
121 * @return the object on the top.
122 * @see CompoundRoot#peek()
123 */
124 public abstract Object peek();
125
126 /**
127 * Get the object on the top of the stack and <b>remove</b> it from the stack.
128 *
129 * @return the object on the top of the stack
130 * @see CompoundRoot#pop()
131 */
132 public abstract Object pop();
133
134 /**
135 * Put this object onto the top of the stack
136 *
137 * @param o the object to be pushed onto the stack
138 * @see CompoundRoot#push(Object)
139 */
140 public abstract void push(Object o);
141
142 /**
143 * Sets an object on the stack with the given key
144 * so it is retrievable by {@link #findValue(String)}, {@link #findValue(String, Class)}
145 *
146 * @param key the key
147 * @param o the object
148 */
149 public abstract void set(String key, Object o);
150
151 /**
152 * Get the number of objects in the stack
153 *
154 * @return the number of objects in the stack
155 */
156 public abstract int size();
157
158 }
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，对应一个action,这个时候你要用相应的Action类实例化相应的action去处理，这个时候，如果你在这个Action中，加上一个ActionContext(或者其他的几个接口)，如下：
ActionContext.getContext().put("list", leaveWordsList);
这个时候，这个list就会被放入值栈（为什么list会放入值栈将在后面写出）。
然后，你就能在另一个Action，或者是jsp页面（通过el表达式等）得到值栈中的属性。
private LeaveWordDao leaveWordDao;
private LeaveWord leaveWord;
private LeaveWord resultLeaveWord;
public void setResultLeaveWord(LeaveWord resultLeaveWord) {
this.resultLeaveWord = resultLeaveWord;
}
@Override
public void setSession(Map<String, Object> arg0) {
sessionMap = arg0;
}
@Override
public void setApplicationContext(ApplicationContext arg0) throws BeansException {
this.applicationContext = arg0;
}
通过你在另一个Action中的属性以及setXX方法去得到，当然，这只是会得到栈顶开始出栈的第一个匹配的同名属性，如果你要得到第二个，第三个，还有别的方法。
当然，你也可以在jsp页面中得到，也就是ognl，如下：
<%@ 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>
接下来，说说为什么ActionContext起到这样的作用：
1 /**
2 * Sets the OGNL value stack.
3 *
4 * @param stack the OGNL value stack.
5 */
6 public void setValueStack(ValueStack stack) {
7 put(VALUE_STACK, stack);
8 }
这是ActionContext中的一部分，可以看到，它持有了ValueStack实例，既然持有实例，那么将属性存储进去就很容易了吧。
另外，值栈的生命周期 = Request的生命周期
博文为原创，如需转载，请标注原处，谢谢。
时间： 2024-10-06 20:26:00