Struts2中OGNL表达式的原理是什么，相信很多没有经验的人对此束手无策，为此本文总结了问题出现的原因和解决方法，通过这篇文章希望你能解决这个问题。
一、OGNL表达式基础知识
二、OGNL与Struts2
OGNL表达式
OGNL，全称为Object-Graph Navigation Language，它是一个功能强大的表达式语言，用来获取和设置Java对象的属性，它旨在提供一个更高的更抽象的层次来对Java对象图进行导航。
OGNL表达式的基本单位是"导航链"，一般导航链由如下几个部分组成：
属性名称（property）
方法调用（method invoke）
数组元素
所有的OGNL表达式都基于当前对象的上下文来完成求值运算，链的前面部分的结果将作为后面求值的上下文。例如：names[0].length()。
示例：第一个OGNL程序
public class OGNL1 { public static void main(String[] args) { Person person = new Person(); person.setName("zhangsan"); try { Object value = Ognl.getValue("name", person); System.out.println(value); } catch (OgnlException e) { e.printStackTrace(); } } } class Person { private String name; public String getName() { return name; } public void setName(String name) { this.name = name; } }控制台输出：
zhangsan可以看到我们正确的取得了person对象的name属性值，该getValue声明如下：
public static <T> T getValue(String expression,Object root)throws OgnlException Convenience method that combines calls to parseExpression and getValue. Parameters: expression - the OGNL expression to be parsed root - the root object for the OGNL expression Returns: the result of evaluating the expressionOGNL会根据表达式从根对象（root）中提取值。
示例：上下文环境中使用OGNL
public class OGNL1 { public static void main(String[] args) { Map<String , Object> context = new HashMap<String , Object>(); Person person1 = new Person(); person1.setName("zhangsan"); Person person2 = new Person(); person2.setName("lisi"); Person person3 = new Person(); person3.setName("wangwu"); Person person4 = new Person(); person4.setName("zhaoliu"); context.put("person1", person1); context.put("person2", person2); context.put("person3", person3); try { Object value = Ognl.getValue("name", context, person2); System.out.println("ognl expression \"name\" evaluation is : " + value); Object value2 = Ognl.getValue("#person2.name", context, person2); System.out.println("ognl expression \"#person2.name\" evaluation is : " + value2); Object value3 = Ognl.getValue("#person1.name", context, person2); System.out.println("ognl expression \"#person1.name\" evaluation is : " + value3); Object value4 = Ognl.getValue("name", context, person4); System.out.println("ognl expression \"name\" evaluation is : " + value4); Object value5 = Ognl.getValue("#person4.name", context, person4); System.out.println("ognl expression \"person4.name\" evaluation is : " + value5); // Object value6 = Ognl.getValue("#person4.name", context, person2); // System.out.println("ognl expression \"#person4.name\" evaluation is : " + value6); } catch (OgnlException e) { e.printStackTrace(); } } } class Person { private String name; public String getName() { return name; } public void setName(String name) { this.name = name; } }控制台输出：
ognl expression "name" evaluation is : lisi ognl expression "#person2.name" evaluation is : lisi ognl expression "#person1.name" evaluation is : zhangsan ognl expression "name" evaluation is : zhaoliu ognl.OgnlException: source is null for getProperty(null, "name") at ognl.OgnlRuntime.getProperty(OgnlRuntime.java:2296) at ognl.ASTProperty.getValueBody(ASTProperty.java:114) at ognl.SimpleNode.evaluateGetValueBody(SimpleNode.java:212) at ognl.SimpleNode.getValue(SimpleNode.java:258) at ognl.ASTChain.getValueBody(ASTChain.java:141) at ognl.SimpleNode.evaluateGetValueBody(SimpleNode.java:212) at ognl.SimpleNode.getValue(SimpleNode.java:258) at ognl.Ognl.getValue(Ognl.java:494) at ognl.Ognl.getValue(Ognl.java:596) at ognl.Ognl.getValue(Ognl.java:566) at com.beliefbetrayal.ognl.OGNL1.main(OGNL1.java:53)对于使用上下文的OGNL，若不指定从哪一个对象中查找"name"属性，则OGNL直接从根对象(root)查找，若指定查找对象(使用'#'号指定，如#person1),则从指定的对象中查找，若指定对象不在上下文中则会抛出异常，换句话说就是是#person1.name形式指定查找对象则必须要保证指定对象在上下文环境中。
示例：使用OGNL调用方法
public class OGNL2 { public static void main(String[] args) { OgnlContext context = new OgnlContext(); People people1 = new People(); people1.setName("zhangsan"); People people2 = new People(); people2.setName("lisi"); People people3 = new People(); people3.setName("wangwu"); context.put("people1", people1); context.put("people2", people2); context.put("people3", people3); context.setRoot(people1); try { Object value = Ognl.getValue("name.length()", context, context.getRoot()); System.out.println("people1 name length is :" + value); Object upperCase = Ognl.getValue("#people2.name.toUpperCase()", context, context.getRoot()); System.out.println("people2 name upperCase is :" + upperCase); Object invokeWithArgs = Ognl.getValue("name.charAt(5)", context, context.getRoot()); System.out.println("people1 name.charAt(5) is :" + invokeWithArgs); Object min = Ognl.getValue("@java.lang.Math@min(4,10)", context, context.getRoot()); System.out.println("min(4,10) is :" + min); Object e = Ognl.getValue("@java.lang.Math@E", context, context.getRoot()); System.out.println("E is :" + e); } catch (OgnlException e) { e.printStackTrace(); } } } class People { private String name; public String getName() { return name; } public void setName(String name) { this.name = name; } }控制台输出：
people1 name length is :8 people2 name upperCase is :LISI people1 name.charAt(5) is :s min(4,10) is :4 E is :2.718281828459045使用OGNL调用方法也十分简单，对于成员方法调用，只需要给出方法的名称+()，若有参数，直接写在括号内，与一般调用Java方法一致。对于静态方法的调用，需要使用如下格式：@ClassName@method，对于静态变量需要使用如下格式：@ClassName@field。
示例：使用OGNL操作集合
public class OGNL3 { public static void main(String[] args) throws Exception { OgnlContext context = new OgnlContext(); Classroom classroom = new Classroom(); classroom.getStudents().add("zhangsan"); classroom.getStudents().add("lisi"); classroom.getStudents().add("wangwu"); classroom.getStudents().add("zhaoliu"); classroom.getStudents().add("qianqi"); Student student = new Student(); student.getContactWays().put("homeNumber", "110"); student.getContactWays().put("companyNumber", "119"); student.getContactWays().put("mobilePhone", "112"); context.put("classroom", classroom); context.put("student", student); context.setRoot(classroom); Object collection = Ognl.getValue("students", context, context.getRoot()); System.out.println("students collection is ：" + collection); Object firstStudent = Ognl.getValue("students[0]", context, context.getRoot()); System.out.println("first student is : " + firstStudent); Object size = Ognl.getValue("students.size()", context, context.getRoot()); System.out.println("students collection size is :" + size); System.out.println("--------------------------飘逸的分割线--------------------------"); Object mapCollection = Ognl.getValue("#student.contactWays", context, context.getRoot()); System.out.println("mapCollection is :" + mapCollection); Object firstElement = Ognl.getValue("#student.contactWays['homeNumber']", context, context.getRoot()); System.out.println("the first element of contactWays is :" + firstElement); System.out.println("--------------------------飘逸的分割线--------------------------"); Object createCollection = Ognl.getValue("{'aa','bb','cc','dd'}", context, context.getRoot()); System.out.println(createCollection); Object createMapCollection = Ognl.getValue("#{'key1':'value1','key2':'value2'}", context, context.getRoot()); System.out.println(createMapCollection); } } class Classroom { private List<String> students = new ArrayList<String>(); public List<String> getStudents() { return students; } public void setStudents(List<String> students) { this.students = students; } } class Student { private Map<String , Object> contactWays = new HashMap<String , Object>(); public Map<String , Object> getContactWays() { return contactWays; } public void setContactWays(Map<String , Object> contactWays) { this.contactWays = contactWays; } }控制台的输出：
students collection is ：[zhangsan, lisi, wangwu, zhaoliu, qianqi] first student is : zhangsan students collection size is :5 --------------------------飘逸的分割线-------------------------- mapCollection is :{homeNumber=110, mobilePhone=112, companyNumber=119} the first element of contactWays is :110 --------------------------飘逸的分割线-------------------------- [aa, bb, cc, dd] {key1=value1, key2=value2}OGNL不仅可以操作集合对象，还可以创建集合对象，对集合操作与对属性的操作没什么不同，需要注意的是OGNL认为List与Array是一样的。使用OGNL创建List集合时使用{}，创建Map对象时使用#{}。
示例：使用OGNL过滤集合与投影集合
public class OGNL4 { public static void main(String[] args) throws Exception { OgnlContext context = new OgnlContext(); Humen humen = new Humen(); humen.setName("qiuyi"); humen.setSex("n"); humen.setAge(22); humen.getFriends().add(new Humen("zhangsan" , "n" , 22)); humen.getFriends().add(new Humen("lisi" , "f" , 21)); humen.getFriends().add(new Humen("wangwu" , "n" , 23)); humen.getFriends().add(new Humen("zhaoliu" , "n" , 22)); humen.getFriends().add(new Humen("qianqi" , "n" , 22)); humen.getFriends().add(new Humen("sunba" , "f" , 20)); humen.getFriends().add(new Humen("yangqiu" , "f" , 25)); context.put("humen", humen); context.setRoot(humen); Object filterCollection = Ognl.getValue("friends.{? #this.name.length() > 7}", context, context.getRoot()); System.out.println("filterCollection is :" + filterCollection); System.out.println("--------------------------飘逸的分割线--------------------------"); Object projectionCollection = Ognl.getValue("friends.{name}", context, context.getRoot()); System.out.println("projectionCollection is :" + projectionCollection); } } class Humen { private String name; private String sex; private int age; private List<Humen> friends = new ArrayList<Humen>(); public Humen() { } public Humen(String name , String sex , int age) { this.name = name; this.sex = sex; this.age = age; } public String getName() { return name; } public void setName(String name) { this.name = name; } public String getSex() { return sex; } public void setSex(String sex) { this.sex = sex; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public List<Humen> getFriends() { return friends; } public void setFriends(List<Humen> friends) { this.friends = friends; } @Override public String toString() { return "Humen [name=" + name + ", sex=" + sex + ", age=" + age + "]"; } }控制台输出：
filterCollection is :[Humen [name=zhangsan, sex=n, age=22]] --------------------------飘逸的分割线-------------------------- projectionCollection is :[zhangsan, lisi, wangwu, zhaoliu, qianqi, sunba, yangqiu]OGNL可以对集合进行过滤与投影操作，过滤的语法为collection.{? expression}，其中使用"#this"表示集合当前对象（可以与for-each循环比较）。投影的语法为collection.{expression}。投影和过滤可以看做是数据库中对表取列和取行的操作。
Struts2与OGNL
Struts 2支持以下几种表达式语言： |
平时使用Struts2标签时会出现一些很奇特的问题，对于OGNL不了解的人可能对问题的出现无能为力或者就算解决了问题也不知道是如何解决的。下面总结一些使用Struts2标签容易出现的困惑：
问题一：#,%{},$符号
在Struts2标签属性中经常会出现"#"或者"%{}"的符号出现，通过上面OGNL表达式基础的介绍，知道了OGNL上下文中有且仅有一个根对象。Struts2为我们定义了许多明明对象，他们分别是"ValueStack"，"Parameters"，"Session"，"Request"， "Appliction"，"Attr"，其中"ValueStack"被设置为上下文的根对象。访问非根对象必须加上"#"号，这就是出现"#"的原因。Struts2中的标的处理类，并不是所有都将标签的属性作为OGNL表达式来看待，有时候我们需要设置动态地值，则必须告诉标签的处理类该字符串按照OGNL表达式来处理，%{}符号的作用就是告诉标签的处理类将它包含的字符串按照OGNL表达式处理。 "$"符号用于XML文件中用于获取动态值，与%{}作用类似。
问题二：%{}符号的影响
Struts2的标签几十几百个，要记住哪一个标签的处理类将标签的属性作为OGNL表达式是一件很困难的事情，在不清楚处理类的处理方式时怎么办，%{}对于标签处理类来说，若处理类将属性值作为普通字符串则%{}符号包含的字符串当做OGNL表达式，若处理类将属性值作为OGNL表达式来处理，则直接忽略%{}符号。换句话说，不清楚处理方式的话，可以都使用%{}符号。
问题三：标签是如何获得数据
下面是ValueStack的官方描述：
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).
大致意思：ValueStack允许保存多个bean（也就是Action），并且可以使用表达式语言获得他们。当评估一个表达式，ValueStack将会从栈顶到栈底的方向被搜索一遍，对于给定的属性名称寻找bean的getter或setter方法或寻找给定的方法。
每当一个请求到达Action时，Struts2会将Action对象推入ValueStack中。
<body> username:<s:property value="username"/><br /> -------------------诡异的分割线-------------------<br /> username:<%= ((HelloWorldAction)ActionContext.getContext().getValueStack().peek()).getUsername() %><br /> </body>页面显示结果：
username:zhangsan -------------------诡异的分割线------------------- username:zhangsan可以看到标签取值与用Java代码取值的结果相同，明显标签的取值方式更简练简洁。OGNL表达式"username"表示了从根对象ValueStack中取出属性username的值。它会从栈顶到栈底遍历ValueStack，直到找某一个Action中的"username"属性。
看完上述内容，你们掌握Struts2中OGNL表达式的原理是什么的方法了吗？如果还想学到更多技能或想了解更多相关内容，欢迎关注编程网行业资讯频道，感谢各位的阅读！
