intro

为什么要记录一下OGNL，因为打算调试Struts2漏洞了，作为基础储备知识，简单搜集和记录一下，言归正传。

OGNL是Object Graphic Navigation Language（对象图导航语言）的缩写。

* 所谓对象图，即以任意一个对象为根，通过OGNL可以访问与这个对象关联的其它对象
* 通过它简单一致的表达式语法，可以存取对象的任意属性，调用对象的方法，遍历整个对象的结构图，实现字段类型转化等功能。它使用相同的表达式去存取对象的属性

features

OGNL提供五大类功能

* 支持对象方法调用
* 支持类静态的方法调用和值访问
* 访问OGNL上下文（OGNL context）和ActionContext
* 支持赋值操作和表达式串联
* 操作集合对象

elements

表达式（Expression）

　　表达式是整个OGNL的核心，所有的OGNL操作都是针对表达式的解析后进行的。表达式会规定此次OGNL操作到底要干什么，OGNL支持很多类型的表达式，之后我们会看到更多。
　　

根对象（Root Object）

根对象可以理解为OGNL的操作对象。在表达式规定了“干什么”以后，你还需要指定到底“对谁干”。

上下文环境（Context）

有了表达式和根对象，我们实际上已经可以使用OGNL的基本功能。例如，根据表达式对根对象进行取值或者设值工作。不过实际上，在OGNL的内部，所有的操作都会在一个特定的环境中运行，这个环境就是OGNL的上下文环境（Context）。说得再明白一些，就是这个上下文环境（Context），将规定OGNL的操作“在哪里干”。

　OGNL的上下文环境是一个Map结构，称之为OgnlContext。上面我们提到的根对象（Root Object），事实上也会被加入到上下文环境中去，并且这将作为一个特殊的变量进行处理，具体就表现为针对根对象（Root Object）的存取操作的表达式是不需要增加#符号进行区分的。

OgnlContext不仅提供了OGNL的运行环境。在这其中，我们还能设置一些自定义的parameter到Context中，以便我们在进行OGNL操作的时候能够方便的使用这些parameter。不过正如我们上面反复强调的，我们在访问这些parameter时，需要使用#作为前缀才能进行。

refernce

什么是OGNL表达式

ognl概念和原理详解

OGNL表达式语言详解

607 对 “Less About OGNL”的想法；

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