Ognl expression struts2 label "%, #, $"

1. 1. What is ognl and what are its characteristics? 
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3. Ognl (Object-graphNavigationLanguage. Is an open-source expression language that allows you to conveniently operate object attributes. Ognl has the following features: 
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5. 1. Supports object method calls in the form of objname. methodname (); 
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7. 2. Supports static class method calls and value access. The expression format is @ [full class name (including package path)] @ [method name| Value name], for example: 
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9. @ Java. Lang. String @ format ('foo% S','Bar') or @ tutorial. myconstant @ app_name;
    
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11. 3. The assignment operation and expression concatenation are supported, for example: 
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13. Price = 100,Discount =0.8,Calculateprice (), which returns80;
     
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15. 4. Access ognl context (ognlContext) and actioncontext; 
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17. 5. Operation set object. 
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21. Ii. Use ognl expressions 
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23. Ognl should be used with Struts labels. Because of its flexibility, it is easy to confuse people, especially the use of the three symbols "%", "#", and "$. Because $ is widely used in El, it focuses on the usage of the % and # symbols. 
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25. 1. The "#" symbol has three purposes: 
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27. (1) access non-root objects (the struts value stack is the root object) such as ognl context and action context. # It is equivalent to actioncontext. getcontext (). The following table contains several useful attributes in actioncontext: 
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31. Name 
32.  Function 
33.  Example 
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35. Parameters 
36.  Map containing the current HTTP Request Parameters 
37.  # Parameters. ID [0] is equivalent to request. getparameter ("ID ") 
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39. Request 
40.  Map that contains the attribute of the current httpservletrequest 
41.  # Request. username is equivalent to request. getattribute ("Username ") 
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43. Session 
44.  Map that contains the attribute of the current httpsession 
45.  # Session. username is equivalent to session. getattribute ("Username ") 
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47. Application 
48.  Map that contains the attribute of the current application's servletcontext 
49.  # Application. username is equivalent to application. getattribute ("Username ") 
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52. Note: ATTRUsed to press request>Session>Application accesses its attributes in sequence, # ATTR. username is equivalent to reading the username attribute in the preceding three scopes in order until it is found. Used to filter and project sets, such as books .{? #This. Price <100}; construct a map, for example, # {'foo1': 'bar1 ','Foo2': 'bar2 '}.
     
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56. (2) used for filtering and projecting sets, such:Books .{? #This. Price> 35} 
     
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58. Books .{? #This. Price> 35} 
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60. (3) construct a map, such:#{'Foo1': 'bar1 ','Foo2': 'bar2 '} 
     
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62. # {'Foo1': 'bar1 ','Foo2': 'bar2'} is usually used to assign values to tags such as radio, select, and checkbox. If you want to obtain a MAP value in the page, you can write it as follows:
     
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64. <S: PropertyValue ="# Mymap ['foo1']"/>  
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66. <S: PropertyValue ="# Mymap ['foo1']"/> 
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68. 2. The "%" symbol is used to tell the execution environment % {} That the ognl expression is used when the tag property value is understood as a string type. 
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70. This is the most incomprehensible symbol at the beginning, because some resources are not prepared for presentation. After a painful exploration, I finally understood its purpose. Actually, it is to let the expression that is understood as a string and be actually executed as ognl. Similar to the eval_r () function in Javascript, for example: 
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72. VaROdiv=Eval_r ("Document. All. Div" + index)  
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74. VaROdiv=Eval_r ("Document. All. Div" + index) 
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76. When the index variable is 1, the statement is treated as VAROdiv=Document. All. div1 VaROdiv=Document. All. div1. % {} Serves this purpose. Example: 
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78. <S: SetName ="Mymap"Value ="# {'Key1': 'value1', 'key2': 'value2'}"/>   
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80. <S: PropertyValue ="# Mymap ['key1']"/>   
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82. <S: URLValue ="# Mymap ['key1']"/> 
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84. <S: SetName ="Mymap"Value ="# {'Key1': 'value1', 'key2': 'value2'}"/> 
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86. <S: PropertyValue ="# Mymap ['key1']"/> 
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88. <S: URLValue ="# Mymap ['key1']"/> 
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90. The code above 2nd will output "value1" on the page, while line 3rd will output a string such as "# mymap ['key1.IfThe three lines are rewritten as follows:
     
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92. <S: URLValue ="% {# Mymap ['key1']}"/>  
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94. <S: URLValue ="% {# Mymap ['key1']}"/> 
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96. The output is "value1 ". 
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102. This shows that different tags in struts2 have different understandings of ognl expressions. If some labels "do not understand" statements similar to "# mymap ['key1']", use % {} to include the statements and translate them. 
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104. 3. "$" has two purposes: 
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106. (1) Reference ognl expressions in international resource files. 
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108. (2) In struts2. Reference The ognl expression in the configuration file: 
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110. <ActionName ="Saveuser"Class= "Useraction"Method ="Save">
      
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112. <ResultType ="Redirect"> listuser. Action? MSG =$ {MSG} </result> 
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114. </Action> 
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116. <ActionName ="Saveuser"Class= "Useraction"Method ="Save">   
      
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118. <ResultType ="Redirect"> listuser. Action? MSG =$ {MSG} </result> 
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120. </Action>