JSTL and EL expressions

2010-03-27  来源:本站原创  分类:Java  人气:370 

Session of the values obtained

<c:out value="${sessionScope.user.userId}"> </ c: out> <br>

<c:out value="${user.userLoginName}"> </ c: out> <br>

<s:property value="#session.user.userId"/> <br>

$ (Session.user.userId) <br>

$ (SessionScope.user.userId) <br>

Basic syntax

1, EL Introduction
1. Grammatical structure
$ (Expression)
2. [] And. Operator
EL provide. And [] of two operators to access the data.
When you want to access the property name contains special characters, such as. Or? Such as letters or numbers are not symbols, they have to use []. For example:
$ (User.My-Name) should be replaced by $ (user ["My-Name"])
If you want to when the dynamic value, you can use [] to do, but. Can not be dynamic values. For example:
$ (SessionScope.user [data]) is a variable in the data
3. Variable
EL access the variable data is simple, for example: $ (username). It means that out of a range variable named username.
Because we did not specify which of a range of username, so it will sequence from the Page, Request, Session, Application range search.
If the way to find the username, the direct return, not to look down, but if all of the range are found, would return null.
Properties range in the name of the EL
Page PageScope
Request RequestScope
Session SessionScope
Application ApplicationScope

2, EL implicit objects
1. And scope of the hidden object and scope of the EL implicit object contains the following four: pageScope, requestScope, sessionScope and applicationScope;
They are basically on and JSP's pageContext, request, session and application the same;
In the EL in the four implicit object attribute values can only be used to get range, that getAttribute (String name), they can not get other related information.

For example: we are to make session username stored in the value of a property, you can use the following methods:
session.getAttribute ("username") to obtain the value of username,
Used in the EL in the following way
$ (SessionScope.username)

2. And enter the hidden object and enter the hidden object has two: param and paramValues, they are the more special EL implicit object.

For example, we have to get the user's request parameters, you can use the following methods:
request.getParameter (String name)
request.getParameterValues (String name)
You can use in the EL in both param and paramValues to obtain the data.
$ (Param.name)
$ (ParamValues.name)

3. Other implicit objects

cookie
JSTL and set the cookie does not provide the action,
Example: To obtain the cookie has a name for userCountry value set, you can use $ (cookie.userCountry) to get it.

header and headerValues
header store the user's browser and server to communicate with the data Example: To get the user's browser version, you can use $ (header ["User-Agent"]).
Also in very few opportunities, there may be the same header name has a different value, this time must be used instead headerValues to get these values.

initParam
initParam web site to obtain environmental parameters set (Context)
Example: a general method of String userid = (String) application.getInitParameter ("userid");
You can use $ (initParam.userid) to get the name for the userid

pageContext
pageContext to obtain user requirements or other relevant page for more information.
$ (PageContext.request.queryString) parameter string to obtain the requested
$ (PageContext.request.requestURL) to obtain the requested URL, but does not include the request parameters string
$ (PageContext.request.contextPath) The name of the web application services
$ (PageContext.request.method) HTTP access method (GET, POST)
$ (PageContext.request.protocol) made use of the protocol (HTTP/1.1, HTTP/1.0)
$ (PageContext.request.remoteUser) to obtain the user name
$ (PageContext.request.remoteAddr) to obtain the user's IP address
$ (PageContext.session.new) determine whether the new session
$ (PageContext.session.id) to obtain the ID session
$ (PageContext.servletContext.serverInfo) to obtain information on the host side of the service

3, EL operator
1. Arithmetic operators have five :+,-,* or $, / or div,% or mod
2. There are six relational operators: == or eq,! = Or ne, <or lt,> or gt, <= or le,> = or ge
3. There are three logical operators: & & or and, | | or or,! Or not
4. Other operators have three: Empty operator, the conditional operator, () operator cases: $ (empty param.name), $ (A? B: C), $ (A * (B + C))

4, EL functions (functions).
Syntax: ns: function (arg1, arg2, arg3 .... ArgN)
Ns for the front of which the name (prefix), it must be the name of a taglib directive home front

---------------------------------------------

Added:

<% @ Taglib prefix = "c" uri = "http://java.sun.com/jstl/core_rt"%>

FOREACH:

<C: forEach items = "$ (messages)"
var = "item"
begin = "0"
end = "9"
step = "1"
varStatus = "var">
... ...
</ C: forEach>

OUT:

<c:out value="${logininfo.username}"/>
c: out> the value of the contents of the output to the current location, where the object is to logininfo
username property value output to the page here.
$ (... ...) Is JSP2.0 the Expression Language (EL) syntax. It defines an expression,
Which can be a constant expression (as above), it can be a concrete expression of statements (such as forEach loop in the case). Typical cases are as follows:
? $ (Logininfo.username)
This indicates that the object reference logininfo username property. We can "." Operator property of the object reference, you can also use the "[]" reference object properties, such as $ (logininfo [username])
And $ (logininfo.username) achieved the same effect.
"[]" References the way, is that if the property name appears in the special characters, such as "." Or "-"
At this point we must use the "[]" syntax for property values in order to avoid a conflict (the system should try to avoid the development of this phenomenon).
With equivalent JSP Script as follows:
LoginInfo logininfo =
(LoginInfo) session.getAttribute ("logininfo");
String username = logininfo.getUsername ();
Can be seen, EL significant savings in coding capacity.
Leads to another question here is, EL logininfo from where to find the object, for
$ (Logininfo.username) this expression, the first page of search from the current definition of a variable before it logininfo, if not found then turn to the Request, Session,
Application range of search, but until then. If still not found until the last match of the variables, it returns null.
If we need to specify the variable radius can be specified in the EL expression search:
$ (PageScope.logininfo.username)
$ (RequestScope.logininfo.username)
$ (SessionScope.logininfo.username)
$ (ApplicationScope.logininfo.username)
In the Spring, all the logic processing unit returned results data will be placed as an Attribute to the HttpServletRequest object returns (realization can be found in the Spring source
org.springframework.web.servlet.view.InternalResourceView.
exposeModelAsRequestAttributes method implementation code), that is Spring
MVC, the results of data objects by default are requestScope. Therefore, in the Spring MVC in
Addressing the following method should be used with caution:
$ (SessionScope.logininfo.username)
$ (ApplicationScope.logininfo.username)
? $ (1 +2)
The results for the expression results, the integer value 3.
? $ (I> 1)
If the variable value i> 1, then will return bool type true. Compared with the previous cases, we find that EL will be automatically calculated according to the expression to return different data types.
Java code written expression and written expression in much the same way.

IF / CHOOSE:

<c:if test="${var.index% 2 == 0}">
*
</ C: if>
Determine the general conditions for a EL expression.
<c:if> else clause does not provide, for use when there may be some inconvenience, then we can <c:choose>
tag to achieve a similar purpose:
<c:choose>
<c:when test="${var.index% 2 == 0}">
*
</ C: when>
<c:otherwise>
!
</ C: otherwise>
</ C: choose>
Similar to Java's switch statement, <c:choose> determined to provide the complex under the conditions of the simplified approach. Clause similar to the case in which <c:when> clause, can occur many times. The above code, the output in the odd rows with "*",
The even rows output "!."
---------------------------------------------

Add:

1 EL expression with $ () that can be used in all the HTML and JSP tags in JSP pages to replace the role of the complex JAVA code.

2 EL expressions operational variables and constants implicit object. The most commonly used implicit object has $ (param) and $ (paramValues). $ (Param) request parameters that returns a single string value. $ (ParamValues) said Return the request parameters of a set of values. pageScope indicate that the page scope variable. requestScope said the request object's variables. sessionScope that session variables within. applicationScope that the variable range of applications.

3 <% @ page isELIgnored = "true"%> that are disabled for EL language, TRUE is prohibited. FALSE said they were not prohibited. JSP2.0 EL is enabled in the default language.

4 EL language to display the logical expression such as $ (true and false) The result is false relational expression such as $ (5> 6) The result is false arithmetic expressions such as $ (5) The result is 10 +5

5 EL variables search is: page request session application point operator (.) And "[]" are said to obtain the variable value. Difference is that [] to display non-variable parts of speech

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