What are the JSP expression languages

①el keyword List

Div-except (/) mod-fetch (%) eq-equals (= =) ne-Not equal to (!=)
Lt-Less than (<) GT-greater Than (>) le-less than or equal to (<=) GE-greater than or equal to (>=)
And-Logical with (&&) or-Logical OR (| |)         Not-logical non (!) True-Logical true
False-logical false null-null value instanceof-empty-NULL test operator

②el built-in objects

----------------------------------------------------
Suppressed objects                 type                        Description
-------------------------------------------- --------
pagecontext         java.util.map     The PageContext object that represents this JSP
pagescope           Java.util.Map      get the value of the property name for the page share range
requestscope        java.util.map     gets the value of the property name for the request shared range
sessionscope         java.util.map     Gets the value of the property name for the session shared range
applicationscope    java.util.map     gets the value of the property name for the application shared range
param                java.util.map     function with Request.getparameter () method
paramvalue          java.util.map     Feature with Request.getparametervalues () method
header               java.util.map     function with Request.getheader () method
heradervalues        java.util.map     function with Request.getheaders () method
cookie               java.util.map     Feature with Request.getcookies () method
initparam           The java.util.map     function is the same as the Application.getinitparameter () method
---------------------------------- ------------------------

③el operator

[] .
()
- ! Empty
* / %
+ -
< > <= >=
== !=
&&
||
${A? B:C}

Custom Functions in ④el

1. Write function definition, function declaration must be static in class
Package elfunction;
public class Functions {
public static string ToUpper (string value) {
return Value.touppercase ();
}
}

2. Write the custom function of the TLD file, placed in the <taglib> tag
<function>
<name>toUpper</name>
<function-class>elfunction.functions</function-class>
<function-signature>
Java.lang.String ToUpper (java.lang.String)
</function-signature>
<description>a to Upper function.</description>
</function>

3. Introduction of label definition files in Web.xml
<taglib>
<taglib-uri>/WEB-INF/tlds/elfunction.tld</taglib-uri>
<taglib-location>/WEB-INF/tlds/elfunction.tld</taglib-location>
</taglib>

4. Introduce custom functions in the form of a custom label
   <%@ taglib uri= "/web-inf/tlds/elfunction.tld" prefix= "Elfun"%>
   ${elfun:toupper ("ABCDEFG")}