Introduction: The development of El expression is very much, now let's review about if you want to manipulate El expressions
1:el expression SyntaxAll El expressions are terminated by {start}, and expressions are used with the. and [] operator to access the data analogy ${user.username}, ${user["UserName"]} is equivalent.
2: Several important operatorsArithmetic, logic, and relational operators are the same as our Java syntax operations.
2.1:empty operatorThe empty operation is to determine whether a value is null or "", such as ${!empty user}
2.2: Three mesh operatorThe EL expression also supports the three-mesh operator ${a? result 1: Result 2} For example: ${1+1>2?1:2} The result of the output is 2
3: Hidden ObjectsThere are 11 hidden objects in the El expression, and I'll enumerate them.
3.1:pagecontextPageContext is an instance of Javax.servlet.jsp.PageContext that can access objects such as servletcontext,request,response,session through PageContext.
3.2:pagescopeWe have seen the scope of JSP before, and this is the same as the JSP scope, from the meaning we can see that this is mainly to get the page range of properties. Let's look at an example and we'll start by creating a JSP file
<%UserBean User1=new UserBean (); user1.setage(N); User1.setemail ("[Email Protected]User1.setusername ("John Doe");p Agecontext.setattribute ("user1", user1); %>Then we started to visit ${pagescope.user1.username} and I also dropped the first PageContext app with ${pagecontext.request.requesturi}
<body>${pagecontext.request.requesturi}${pagescope.user1.username}</body>Primarily get the value of the property within the request scope, modify the code in the above to the following code
<%UserBean User1=new UserBean (); user1.setage(N); User1.setemail ("[Email Protected]User1.setusername ("John Doe"); Request.setattribute ("user", user1) ; %>The El in JSP is ${requestscope.user1.username}
3.4:sessionscopeThis primarily gets the value of the properties in the session scope. Let's start by creating a servlet class and assigning a value to the session
protected void throws servletexception, IOException { UserBean user=new UserBean (); User.setage (+); User.setemail ("[email protected]"); User.setusername ("Zhang San"); HttpSession session=request.getsession (); Session.setattribute ("user", user); }Then in el.jsp to get the value of the session property ${sessionscope.user.username}, remember to run the Servlet class first and then run the el.jsp effect as follows
3.5:applicationscopeThis is the value that gets the object in the global scope, which is the same as the say and.
3.6:paramIf we want to get the parameter value of the URL (http://localhost:8080/taglib-test/el.jsp?userName= Zhao four) request, we can use ${param.username} to get
3.7:paramvaluesThe key to get the parameter request parameter information is an array, with not many
3.8:headerThe header information used to get the request, such as ${header["User-agent"} to get the value of the parameter in the header information
3.9:headervaluesUsed primarily to obtain the value of the request header information.
3.10:cookieNeedless to say too much
3.11:initparamGets the value of the Web application initialization parameter primarily
4: Named variableIn the El expression, the value of the named variable is found through the Pagecontext.findattribute () method, the analogy above if we do not write their scope, then will follow Page,request,session, Application to find, so we try to write, so as to achieve the purpose of optimizing the program.
5:el Expressions using FunctionsThis is like a custom tag, and we can also use a function
First we define a class (I create a very simple)
Public class Elfun { publicstatic string Gethelloworld (String str) { return str; }}Then we are creating a TLD file
<?xml version= "1.0" encoding= "GBK"?> <taglib xmlns= "HTTP://JAVA.SUN.COM/XML/NS/J2EE"Xmlns:xsi= "Http://www.w3.org/2001/XMLSchema-instance"xsi:schemalocation= "Http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-jsptaglibrary_2_0.xsd "version= "2.0" > <!--define function versions--<tlib-version>1.0</tlib-version> <!--define function names-- > < Short-name>el</ Short-name> <!--define the first function--<function> <!--define a function--<name>hello< ;/name> <!--defining function processing class-<function-class>com.lp.els.elfun</function-class> <!--the corresponding method of defining functions---<function-signature>java.lang.String Gethelloworld (java.lang.String)</function-signature> </function> </taglib>
One thing to note: This is the definition of the function name that you want the EL expression in your JSP to refer to. Don't be mistaken.
Then we can reference it on the JSP page
<body><%@ taglib uri= "/web-inf/tlds/el.tld" prefix= "el"%>${el:hello ("Hello World")} </body>From here we see that the hello behind El is not gethelloworld in our class. There is no habitual in the El expression after the addition of the number, will bring an error.
Javaweb review of the nineth El expression
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