Java Web Review Article 9 EL expressions and Java Web Article 9 el
EL expression is widely used in development. Now let's review how to operate EL expressions.
1: EL expression syntaxAll EL expressions end with {start. and the [] operator to access the data metaphor $ {user. userName} and $ {user ["userName"]} are equivalent.
2: several important operatorsArithmetic, logic, and Relational operators are the same as our java syntax operations.
2.1: Empty OperatorThe Empty operation is used to determine whether a value is null or "", as shown in $ {! Empty user}
2.2: Three-object OperatorThe EL expression also supports the three-object operator $ {? Result 1: result 2}, for example, ${1 + 1> 2? } The output result is 2
3: Implicit objectIn EL expressions, there are 11 hidden objects. I will list them one by one.
3.1: pageContextPageContext is an instance of javax. servlet. jsp. PageContext. objects such as ServletContext, Request, Response, and Session can be accessed through pageContext.
3.2: paginationWe have seen the scope of Jsp before. This is the same as the scope of Jsp. From the meaning, we can see that this is mainly used to obtain the attributes within the scope of the page. Let's look at an example. We will first create a jsp file.
<% UserBean user1 = new UserBean (); user1.setAge (12); user1.setEmail ("123@outlook.com"); user1.setUserName (""); pageContext. setAttribute ("user1", user1); %>
Then we started to access $ {pagination. user1.userName}. I also added $ {pageContext. request. requestURI} to the first pageContext application}
<body>${pageContext.request.requestURI}${pageScope.user1.userName}</body>
3.3: requestScope
Obtains the attribute values within the request range. The code above is changed to the following code:
<% UserBean user1 = new UserBean (); user1.setAge (12); user1.setEmail ("123@outlook.com"); user1.setUserName (""); request. setAttribute ("user", user1); %>
El in jsp is $ {requestScope. user1.userName}
3.4: sessionScopeThis mainly obtains the attribute values in the session range. Create a Servlet class and assign a value to the Session.
Protected void doGet (HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {UserBean user = new UserBean (); user. setAge (19); user. setEmail ("16455@qq.com"); user. setUserName ("Zhang San"); HttpSession session = request. getSession (); session. setAttribute ("user", user );}
Then get the Session attribute value $ {sessionScope. user. userName} in el. jsp. Remember to run the Servlet class and then run el. jsp as follows:
3.5: applicationScopeThis is the value of the object in the global range. This is much the same as above.
3.6: paramIf we want to obtain the URL (http: // localhost: 8080/taglib-test/el. jsp? UserName = Zhao Si) The request parameter value can be obtained using $ {param. userName }.
3.7: paramValuesThis is mainly used to obtain the information related to the parameter request parameters.
3.8: headerUsed to obtain the request header information, such as $ {header ["User-Agent"]} to obtain the parameter values in the header information.
3.9: headerValuesIt is mainly used to obtain the value of the request header information.
3.10: cookieNeedless to say, too many
3.11: initParamObtains the initialization parameters of web applications.
4: naming VariablesIn EL expressions, the named variable value is pageContext. the findAttribute () method is used to find the completed data. If we do not write their range, we will search for the data by page, Request, Session, and application, so we try to write the data as much as possible, in this way, the program can be optimized.
5: EL expressions use functionsThis is similar to a custom tag. We can also use functions.
First, define a class (I created a very simple one)
public class ElFun { public static String getHelloWorld(String str) { return str; }}
Then we create a tld file.
<? Xml version = "1.0" encoding = "GBK"?> <Taglib xmlns = "http://java.sun.com/xml/ns/j2ee" xmlns: xsi = "http://www.w3.org/2001/XMLSchema-instance" xsi: schemaLocation = "http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-jsptaglibrary_2_0.xsd" version = "2.0"> <! -- Define the function version --> <tlib-version> 1.0 </tlib-version> <! -- Define the function name --> <short-name> el </short-name> <! -- Define the first function --> <function> <! -- Define a function --> <name> hello </name> <! -- Fixed function processing class --> <function-class> com. lp. els. ElFun </function-class> <! -- Define the corresponding method of the function --> <function-signature> java. lang. string getHelloWorld (java. lang. string) </function-signature> </function> </taglib>
Note: The Defined Function Name is the el expression in your jsp to be referenced.
Then we can reference
<body><%@ taglib uri="/WEB-INF/tlds/el.tld" prefix="el"%>${el:hello("Hello World")}</body>
Here we can see that hello after el is not getHelloWorld in the class. Also, do not habitually Add a number after the el expression, which may lead to errors.
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