Using an El expression in a JSP

1. El expression: El full name is expression Language, which is to replace the <%=%> script expression.
2. El main functions:
   • Get Data :

   El expressions are primarily used to replace script expressions in JSP pages to retrieve Java objects from various types of Web domains and get data. (objects in a web domain, accessing JavaBean properties, accessing the list collection, accessing the map collection, accessing an array)

   • Perform the operation:

   The EL expression allows you to perform some basic relational, logical, and arithmetic operations on a JSP page to accomplish some simple logic operations in a JSP page. ${user==null}

   • Get Web Development Common objects

   EL expressions define implicit objects that make it easy for web developers to get a reference to a common Web object to get the data in those objects.

   • Calling the Java method

   The EL expression allows the user to develop a custom El function that invokes a method of the Java class through an El expression in a JSP page.

3. Using El expressions to get data

(1) Use El expression to get data syntax: "${identifier}"    el expression statement when executed, will invoke the Pagecontext.findattribute method, using the identifier as the keyword, from page, request, session, Application

   ( Note that it is not NULL, but an empty string ).     Example: ${user}    <%= ((httpservletrequest) pagecontext.getrequest ()). Getcontextpath ()%>   Parentheses are required after the object is fetched to continue calling the method on the object     ${pagecontext.request.contextpath}  This still needs to be remembered for useful
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(2) El expressions can also easily get JavaBean properties or get data for arrays, Collection, and map type collections, for example:

1. ${STRS[1]}//Gets the elements in the array

${user.address.city}

${USER.LIST[0]}: Access an ordered collection of elements at a location

${map.key}: Gets the value of the specified key in the Map collection

Iterate through the C tag's foreach in the Jstl tag, which we've already done!

4.el Expression execution operations [focus in focus]

(1) Arithmetic operation: All elements involved in the meta-calculation will be converted to a number, if the error cannot be transferred, empty elements participate in the operation as not involved.
(2) Relational operations: must be remembered

  == --->    eq     for example: 5 = = 5 & #x6216; > 5 ==5 5==5 or {5 eq 5}

! =---> NE

<---> lt

>---> GT

<=---> Le

>=---> GE

(3) Logical operation:

&&---> and for example:{a&&b} or {a&&b} or {A and B}

|| ---> or

! ---> Not

(4) Empty/not empty to determine whether an object or collection or array is null or a length of 0

(5) conditional expression ${user!= null? User.Name: "";

(6) [] and. Number operators

5. use El expressions to get web development common objects

The El expression language defines 11 hidden objects , which make it easy to get some common objects in web development and read the data of those objects.

syntax : ${implicit object name}: Getting a reference to an object

PageContext: Represents PageContext object, note and Pagescope are differentiated

Pagescope: Represents the page field, which can be used to get properties in the page field

Reqeustscope: Represents the Reqeust domain, which can be used to obtain properties in the Reqeust domain

Sessionscope: Represents the Session field, which can be used to get the properties in the Session field

Applicationscope: Represents the application domain, which can be used to obtain properties in the application domain

PARAM: The Map collection representing the request parameters ${param.username} is equivalent to Request.getparameter ("UserName")

Paramvalues: Represents a map collection of requests for a host, but the value of this collection is string[], which is used to obtain a multivalued param equivalent to request.getparametervalues ("UserName")

Header: Gets the Map object for all HTTP request fields

Headervalues: Ibid., gets a map consisting of the request header, but value is a string[]

Note: If there is "-" in the head, example accept-encoding, headervalues["accept-encoding"]

Cookie: Gets the Map object composed of a cookie, the value of this map is a cookie object ${cookie.cookiename.cookievalue}

Initparam: Initialization parameters for the entire Web application configured in the map package, XML

<%@ page language= "java" import= "java.util.*" pageencoding= "Utf-8"%>
Using an El expression in a JSP