OGNL（ObjectGraphNavigationLanguage），是一种表达式语言。使用这种表达式语言，你可以通过某种表达式语法，存取Java对象树中的任意属性、调用Java对象树的方法、同时能够自动实现必要的类型转化。如果我们把表达式看做是一个带有语义的字符串，那么OGNL无疑成为了这个语义字符串与Java对象之间沟通的桥梁。既然OGNL那么强大，那么让我们一起来研究一下他的API，看看如何使用OGNL.
OGNL的API看起来就是两个简单的静态方法：
访问OGNL上下文和Action上下文，#相当于ActionContext.getContext()；下表有几个ActionContext中有用的属性：
名称 | 作用 | 例子 |
parameters | 包含当前HTTP请求参数的Map | #parameters.id[0]作用相当于request.getParameter("id") |
request | 包含当前HttpServletRequest的属性（attribute)的Map | #request.userName相当于request.getAttribute("userName") |
session | 包含当前HttpSession的属性（attribute）的Map | #session.userName相当于session.getAttribute("userName") |
application | 包含当前应用的ServletContext的属性（attribute）的Map | #application.userName相当于application.getAttribute("userName") |
attr | 用于按request >session > application顺序访问其属性（attribute） | #attr.userName相当于按顺序在以上三个范围（scope）内读取userName属性，直到找到为止 |
用于过滤和投影（projecting)集合，如books.{?#this.price<100}；构造Map，如#{'foo1':'bar1', 'foo2':'bar2'}。%符号的用途是在标志的属性为字符串类型时，计算OGNL表达式的值，这个类似js中的eval，很暴力。
"$"符号$符号主要有两个方面的用途。
— 在国际化资源文件中，引用OGNL表达式，例如国际化资源文件中的代码：reg.agerange=国际化资源信息：年龄必须在${min}同${max}之间。
— 在Struts 2框架的配置文件中引用OGNL表达式，例如：
<action name="AddPhoto" class="addPhoto"> <interceptor-ref name="fileUploadStack" /> <result type="redirect">ListPhotos.action?albumId=${albumId}</result> </action>下面代码实例对OGNL有更深的了解：Cat.java
package com.oumyye.struts2.ognl;public class Cat { private Dog friend; public Dog getFriend() { return friend; } public void setFriend(Dog friend) { this.friend = friend; } public String miaomiao() { return "miaomiao"; }}Dog.java
package com.oumyye.struts2.ognl;public class Dog { private String name; public Dog() { } public Dog(String name) { super(); this.name = name; } public String getName() { return name; } public void setName(String name) { this.name = name; } @Override public String toString() { return "dog: " + name; }}User.java
package com.oumyye.struts2.ognl;public class User { private int age = 8; public User() { } public User(int age) { super(); this.age = age; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } @Override public String toString() { return "user" + age; }}OgnlAction.java
package com.oumyye.struts2.ognl;import java.util.ArrayList;import java.util.HashMap;import java.util.HashSet;import java.util.List;import java.util.Map;import java.util.Set;import com.opensymphony.xwork2.ActionSupport;public class OgnlAction extends ActionSupport { private Cat cat; private Map<String, Dog> dogMap = new HashMap<String, Dog>(); private Set<Dog> dogs = new HashSet<Dog>(); private String passWord; private User user; private String username; private List<User> users = new ArrayList<User>(); public OgnlAction() { users.add(new User(1)); users.add(new User(2)); users.add(new User(3)); dogs.add(new Dog("dog1")); dogs.add(new Dog("dog2")); dogs.add(new Dog("dog3")); dogMap.put("dog100", new Dog("dog100")); dogMap.put("dog101", new Dog("dog101")); dogMap.put("dog102", new Dog("dog102")); } public String execute() { return SUCCESS; } public Cat getCat() { return cat; } public Map<String, Dog> getDogMap() { return dogMap; } public Set<Dog> getDogs() { return dogs; } public String getPassword() { return password; } public User getUser() { return user; } public String getUsername() { return username; } public List<User> getUsers() { return users; } public String m() { return "hello"; } public void setCat(Cat cat) { this.cat = cat; } public void setDogMap(Map<String, Dog> dogMap) { this.dogMap = dogMap; } public void setDogs(Set<Dog> dogs) { this.dogs = dogs; } public void setPassword(String password) { this.password = password; } public void setUser(User user) { this.user = user; } public void setUsername(String username) { this.username = username; } public void setUsers(List<User> users) { this.users = users; }}ognl.jsp
<?xml version="1.0" encoding="UTF-8" ?><%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%><%@ taglib uri="/struts-tags" prefix="s" %><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>OGNL表达式语言学习</title></head><body> <ol> <li>访问值栈中的action的普通属性: username = <s:property value="username"/> </li> <li>访问值栈中对象的普通属性(get set方法)：<s:property value="user.age"/> | <s:property value="user['age']"/> | <s:property value="user[/"age/"]"/> | wrong: <%--<s:property value="user[age]"/>--%></li> <li>访问值栈中对象的普通属性(get set方法): <s:property value="cat.friend.name"/></li> <li>访问值栈中对象的普通方法：<s:property value="password.length()"/></li> <li>访问值栈中对象的普通方法：<s:property value="cat.miaomiao()" /></li> <li>访问值栈中action的普通方法：<s:property value="m()" /></li> <hr /> <li>访问静态方法：<s:property value="@com.oumyye.struts2.ognl.S@s()"/></li> <li>访问静态属性：<s:property value="@com.oumyye.struts2.ognl.S@STR"/></li> <li>访问Math类的静态方法：<s:property value="@@max(2,3)" /></li> <hr /> <li>访问普通类的构造方法：<s:property value="new com.oumyye.struts2.ognl.User(8)"/></li> <hr /> <li>访问List:<s:property value="users"/></li> <li>访问List中某个元素:<s:property value="users[1]"/></li> <li>访问List中元素某个属性的集合:<s:property value="users.{age}"/></li> <li>访问List中元素某个属性的集合中的特定值:<s:property value="users.{age}[0]"/> | <s:property value="users[0].age"/></li> <li>访问Set:<s:property value="dogs"/></li> <li>访问Set中某个元素:<s:property value="dogs[1]"/></li> <li>访问Map:<s:property value="dogMap"/></li> <li>访问Map中某个元素:<s:property value="dogMap.dog101"/> | <s:property value="dogMap['dog101']"/> | <s:property value="dogMap[/"dog101/"]"/></li> <li>访问Map中所有的key:<s:property value="dogMap.keys"/></li> <li>访问Map中所有的value:<s:property value="dogMap.values"/></li> <li>访问容器的大小：<s:property value="dogMap.size()"/> | <s:property value="users.size"/> </li> <hr /> <li>投影(过滤)：<s:property value="users.{?#this.age==1}[0]"/></li> <li>投影：<s:property value="users.{^#this.age>1}.{age}"/></li> <li>投影：<s:property value="users.{$#this.age>1}.{age}"/></li> <li>投影：<s:property value="users.{$#this.age>1}.{age} == null"/></li> <hr /> <li>[]:<s:property value="[0].username"/></li> </ol> <s:debug></s:debug></body></html>
index.jsp
<?xml version="1.0" encoding="GB18030" ?><%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%><%String contextPath = request.getContextPath(); %><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Insert title here</title></head><body> 访问属性 <a href="<%=contextPath %>/ognl.action?username=u&password=p">ognl</a></body></html>
Struts.xml
<?xml version="1.0" encoding="UTF-8" ?><!DOCTYPE struts PUBLIC "-//Apache Software Foundation//DTD Struts Configuration 2.0//EN" "http://struts.apache.org/dtds/struts-2.0.dtd"><struts> <constant name="struts.enable.DynamicMethodInvocation" value="false" /> <constant name="struts.ognl.allowStaticMethodaccess" value="true"></constant> <package name="ognl" extends="struts-default"> <action name="ognl" class="com.oumyye.struts2.ognl.OgnlAction"> <result>/ognl.jsp</result> </action> <action name="test" class="com.oumyye.struts2.ognl.TestAction"> <result type="chain">ognl</result> </action> </package></struts>
结果如下
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