JSP EL expression

EL namely Expression Language (expression language) is a simple data access language.

EL and output expression <% =%> similar function, the output value of the expression is, but they use different subjects in both.

EL expression syntax:

} // {$ expression after an expression not semicolon.

Access constants:

${1} //int

${"ok"} //String

${false} //boolean

$ {Null} // null

Access variables:

$ {Name = 1} // 1. EL variable in expressions without first statement | defined, can be used directly. For example, here is not declared int name.

$ {Name} // access variable value

Java to get the value if the variable does not exist or value is null ,, will be reported null pointer exception. El expression is not, EL expression outputs an empty string.

Arithmetic operators:

$ {A + b} // pressurized

$ {Ab} // Save

$ {A * b} // multiply

$ {A / b} or $ {a div b} // addition

$ {A% b} or $ {a mod b} // modulo

Comparison operators:

Returns a boolean value, to avoid the html tag <> conflict, the English abbreviation may be used.

${a==b} 或 ${a eq b} equals

${a!=b} 或 ${a nq b} not equals

${a<b} 或 ${a lt b} less than

${a<=b} 或 ${a le b} less equals

${a>b} 或 ${a gt b} greater than

${a>=b} 或 ${a ge b} greater equals

Logical Operators:

${a && b} 或 ${a and b}

${a || b} 或 ${a or b}

${!a} 或 ${not a}

Ternary operator:

${a?b:c}

the empty operator:

$ {Empty a} // determines whether a variable is empty, returns a boolean value

If a variable is not present, it returns true;

If a variable is present, but the value is null, return true;

If the variable is a collection type (List, Set, Map), the value is not null, but the collection is empty, returns true.

When the priority uncertain operators, plus ().

EL attribute takes object (member variables)

Object name $ {.} // attribute name attribute name without quotes. Properties are generally private, can not be accessed directly, in real terms, or call the getter () method.
{$ Object name .getXxx ()} // use getters () method
{Object name $ [ "attribute name"]} // attribute names in quotes

EL array elements take

{$ Array name [index]}

EL expressions output expression <% =%> of the similarities and differences

Use the same position. Html tag placement and the same, in addition also on the properties of html tags, such as <form action = "$ {}"> </ form>.
JSP output expression can access the built-in objects, you can visit <!%%> Variable <%%> defined, you can call <%!> Defined in the method, you can access the <jsp: useBean /> object definition .
JSP EL expression can not access the built-in objects, can not access the <!%%> Variable <%> definition, can not call <%!> Defined in the method, access <jsp: useBean /> defined object .

EL expressions can access so much data, why is the data access language?

EL expression has its own built-in objects, in general, an EL expression using its own built-in objects to access data.

EL expressions of the 11 built-in objects | implicit object

pageScope
requestScope
sessionScope
applicationScope

pageContext // equivalent to JSP built pageContext, commonly used in the acquisition of other objects

param // all request parameters (form data, url in? later sections)
paramValues

All fields header // HTTP request header
headerValues

cookie
initParam // initialization parameters entire web application

These objects are built using Map to store data.

Although not access EL expression variable <%> defined, such as <% = Student student new new Student ();%> , student can access objects in an EL expression.

However Servlet, JSP may be used setAttribute () data (variables) into four domains, EL expression data extracted from the domain.

 1  <%
 2       pageContext.setAttribute("id","01");   //page域
 3       request.setAttribute("name","chy");  //request域
 4       session.setAttribute("age",20);  //session域
 5       application.setAttribute("score",100);  //application域
 6   %>
 7 
 8 <%--  2种写法等效--%>
 9   ${pageScope.get("id")}
10   ${pageContext.getAttribute("id")}
11 
12   ${requestScope.get("name")}
13   ${pageContext.request.getAttribute("name")}
14   
15   ${sessionScope.get("age")}
16   ${pageContext.session.getAttribute("age")}
17   
18   ${applicationScope.get("score")}
19   ${pageContext.servletContext.getAttribute("score")}

(1) pageScope, requestScope, sessionScope, applicationScope four common method domain objects

get (key) // Gets the property value
getOrDefault (key, defaultValue) // If no attribute domain name, the default value is returned
clear () // clear the entire domain
remove (key) // remove a property
remove(key,value)
replace (key, oldValue, newValue) // replace the value of a property
containsKey (key) // contains the specified key, returns a boolean
keySet () // Gets a collection of key domains, return Set
values () // set values acquired return Collection
entrySet () // get entry (a key-value pairs) is set to return Set

The method takes only four data fields, the data is not provided setXxx () method.

Conventional method (2) pageContext of

.servletContext
.servletConfig
.out
.session
.request
.response
.exception

These objects get other ways, ostensibly to get through member variables, actually still call getXxx () to get.

pageContext the equivalent of a large Map, which also installed some small Map.

EL built 4 domain data objects setXxx not set () method. Data can be set with a few member variables to achieve the above pageContext:

${pageContext.request|session|servletContext.set|remove|getAttribute()   }

With only three fields, page fields it?

pageContext page domain itself: pageContext.set | GET | removeAttribute ()

In addition there are two common methods pageContext:

forward (String url) // forward the request
include (String url) // request comprising

Are internal server act, only the browser sends an HTTP request, you can see the address bar is not changed.

Methods EL expression summary data from the field

${ pageScope.get("age") } //xxxScope.get("xxx")
$ {PageScope.age} // get directly to attribute names, attribute names are not quoted. Very convenient, it is recommended.
$ {Age} // get directly to attribute names. To this domain is not specified, the default from the page, request, session, applicaiotn in order to find, find a return, no longer continue to look down.
$ {PageContext.request.getAttribute ( "age")} // Get first domain specified by pageContext, data reacquisition
$ {PageContext.getAttribute ( "age")} // Special: pageContext page itself represents domain

(3) the ultimate, paramValues

param, it is stored paramValues request parameters (form data, url in? later sections).

Request parameters are stored as 2 parts:

param. key is a String (field name), value is a String. Because using Map storage, key can not be repeated, so have the same field names, for example? Hobby = "running" & hobby = "swimming", only the first store, behind the same is not stored.
paramValues. key is a String (field name), value is a String []. With or without the same field names, values are stored as a String [] in the form of, for example? Hobby = "running", is stored as { "running"}.

<form method="post" action="handle.jsp">
     姓名：<input type="text" name="name" /><br />
     爱好：<input type="checkbox" name="hobby" value="running" >跑步
     <input type="checkbox" name="hobby" value="swimming" >游泳<br />
     <button type="submit">提交</button>
 </form>

<%
    request.setCharacterEncoding("utf-8");
    response.setContentType("text/html;charset=utf-8");
%>

<p>姓名：${param.name}</p>
<p>爱好：${paramValues.hobby[0]}、${paramValues.hobby[1]}</p>

param is Map <Stirng, String> type, is paramValues Map <Stirng, String []> type, these two objects can be built using the EL Map method.

(4) cookie objects

Map storage request cookie object represents a collection of all the Cookie header.

$ {Cookie.JSESSIONID} // Gets the Cookie JSESSIONID

$ {Cookie.JSESSIONID.name} // get JSESSIONID this Cookie's name

$ {Cookie.JSESSIONID.value} // get the value JSESSIONID this Cookie