JSP Expression Language

JSP Expression Language (2)

Point operators and index operators can be used to access map. For example, the following two El expressions return the value corresponding to the key named mykey:

However, there is a small difference between the two: if the key name contains characters that can confuse El, you cannot use the dot operator. For example, if $ {header ["User-Agent"]} is used, there is no problem, but $ {header. the User-Agent} error occurs because the break number between the user and the agent in the second expression is parsed as a minus sign. Unless there is a variable named agent, and header. User and agent are both null, according to the El specification documentation, the $ {null-null} result is zero. The first expression may return zero. If the map key name contains a vertex, you may encounter different but more serious problems. For example, you can use $ {Param ["My. Par"]} normally, and $ {Param. My. Par} may get null. This will be a serious problem, because null is a possible valid result. We recommend that you use square brackets in all scenarios and forget these questions.

Like JSP, El also contains implicit objects, as shown in Table 2-7.

Table 2-7 El implicit objects

Object	Description
Pagecontext	JSP page context. In particular, pagecontext. Reference obtained by servletcontext and implicit in JSP Type Variable Application references to the same object. Similarly, pagecontext. Session, etc. Price for sessions in JSP, pagecontext. Request is equivalent to the request in JSP, Pagecontext. response is equivalent Response in JSP
Param	Map the request parameter name to its first value.
Paramvalues	Map the request parameter name to an array containing its values.
Header	Map the request header name to its first value.
Headervalues	Map the request header name to an array containing its values
Cookie	Map the cookie name to a unique cookie.
Initparam	Map the context initialization parameter to its value
Pagination	Map the page-scoped variable names to their values
Requestscope	Map the names of requset-scoped variables to their values.
Sessionscope	Map the seesion-scoped variable names to their values.
Applicationscope	Map application-scoped variable names to their values

Note that JSP script variables are invalid in El expressions.

You may have noticed that El does not contain any methods to declare variables. In El expressions, you can use the jstl Core Action C: Set (as described in the next section) or the variables set by the scope attribute (scoped attributes. For example, the following definitions allow you to use the El expression $ {xyz }.

However, you must pay more attention to the scope. C: The set variable and the attribute in pagecontext are the same variable. That is to say, C: Set defines an attribute in the page context. The attributes in sessioncontext are different from other variables, and you cannot access them in $ {xyz} because they "hide" the attributes with the same name in the page context. To access the session attribute, you must prefix sessionscope before the attribute name, such as $ {sessionscope. xyz. If you do not specify a scope, El will first search for attributes in the page, then request, session, and finally apply scope.

Note that El expressions cannot be nested. An expression like ${expr1 [$ {expr2}]} is invalid.

You can combine several El expressions and additional texts into a composite expression, as shown in the following example:

However, you cannot mix $ {}and # {} formats #{}.

From: http://book.51cto.com/art/200909/150189.htm