JSTL Tag Library Detailed information

HTML clipboard STL Tag Library detailed information preface

starting with the JSP 1.1 specification, JSP supports the use of custom tags in jsp. The widespread use of custom labels has created a duplication of definitions for programmers, which has contributed to the birth of Jstl (JavaServer Pages Standard Tag Library). Author: Dingling (http:///www.DingL.com/)

jstl Introduction

Jstl is a constantly improving open source JSP tag library that is maintained by the Apache Jakarta team. Jstl can only be run on containers that support JSP1.2 and Servlet2.3 specifications, such as Tomcat 4.x. However, the upcoming JSP 2.0 is supported as a standard. The latest version of the
Jstl is 1.02, and the final release is 1.0. The jstl contains two parts: the tag library and the El (Expression Language expression language) language. The tag library currently supports four kinds of tags:

Label	Uri	Prefix	Example
Core	Http://java.sun.com/jstl/core	C	<c:tagname ...>
XML processing	Http://java.sun.com/jstl/xml	X	<x:tagname ...>
i18n capable formatting	Http://java.sun.com/jstl/fmt	Fmt	<fmt:tagname ...>
Database Access (SQL)	Http://java.sun.com/jstl/sql	Sql	<sql:tagname ...>

Core supports some basic operations in JSP;
XML processing supports the processing of XML documents;
I18N capable formatting supports the internationalization of JSP pages;
Database access (SQL) supports JSP operations on databases.

Because of my limited level, this article only introduces the core tag, if interested, can explore the other three kinds of labels for use and expansion.

El Language Introduction

The El language is the Jstl output (input) representation of a Java expression.
In Jstl, the El language can only be used in attribute values. The El language can only be invoked by establishing an expression ${EXP1}. There are three ways to use an expression in an attribute value.

1, the Value property contains an expression
<some:tag value= "${expr}"/>
In this case, the expression value is computed and assigned to the Value property based on the type conversion rule. For example: <c:out value= "${username}"/> ${username} is an EL, which is equivalent to the JSP statement <%=request.getattribute ("username")%> or & Lt;%=session.getattribute ("username")%>

2. The Value property contains one or more properties that are split by text or surround
<some:tag value= "some${expr}${expr}text${expr}"/>
In this case, the expression is evaluated from left to right and the result is converted to a string type (based on the type conversion rule) and the result is assigned to the Value property

3, the Value property contains only text
<some:tag value= "Sometext"/>
In this case, the string property value is converted to the type that the label expects based on the type conversion rule.

operator for El language
Gets the value of an attribute in an object or collection
To get the properties in the collection, El supports the following two actions
1. Use the. operator to obtain a named attribute. For example, an expression ${user.username} indicates the Username property of the object user
2. Use the [] operator to obtain a name or numeric attribute.
Expression ${user["username"]} has the same meaning as expression ${user. Username}
Expression ${row[0]} indicates the first entry of the row collection.
Here the user is an object of a class whose attribute username must conform to the standard JavaBean specification, that is, the appropriate getter and setter method must be defined for the Username property.

Empty operator (null value check)

Use the empty operator to determine whether an object, collection, or string variable is null or NULL. For example:
${empty Param.username}
If the username value in the parameter list for the request is null, the value of the expression is true. El can also be compared directly with NULL using the comparison operator. such as ${param.firstname = = null}.
comparison operator

Operator	Describe
= = or EQ	Equality check
!= or NE	Unequal check
< or LT	Less than check
> or GT	Greater than check
<= or Le	Less than equals check
>= or GE	Greater than equals check

Numeric operators and logical operators are the same as the Java language and are no longer listed.

Core Tag Library

1. General label

<c:out>
The <c:out> tab is used to display data in the JSP, which has the following properties

Property	Description	Whether you must	Default value
Value	The output information, which can be an El expression or a constant	Is	No
Default	Display information when value is empty	Whether	No
EscapeXML	True to avoid special XML character sets	Whether	True

Example:

Your username is: <c:out value= "${user.username}" default= "Guest"/>

Displays the user's user name, if empty, displays the guest

<c:out value= "${sessionscope.username}"/>

Specifies that the value of the username is displayed from the session;

<c:out value= "${username}"/>

Displays the value of the username, by default, from request (page), if no object named username is taken from the session, and no session is from application (ServletContext) and is not displayed if no value is taken.

<c:set>
<c:set> tags are used to save data, which has the following properties

Property	Description	Whether you must	Default value
Value	The information to be saved can be an El expression or a constant	Whether
Target	Variable name that needs to be modified for the property, typically an instance of JavaBean	Whether	No
Property	JavaBean properties that need to be modified	Whether	No
Var	Variables that require information to be saved	Whether	No
Scope	The scope of the variable that holds the information	Whether	Page

If the target attribute is specified, the property attribute must also be specified.
Example:

<c:set value= "${test.testinfo}" var= "Test2" scope= "Session"/>

Saves the value of the Test.testinfo to the test2 of the session, where test is an instance of JavaBean, and TestInfo is the property of the test object.

<c:set target= "${cust.address}" property= "City" value= "${city}"/>

To save the city property value of an object cust.address in the variable city

<c:remove>
<c:remove> tags are used to delete data, which has the following properties

Property	Description	Whether you must	Default value
Var	The variable to delete	Is	No
Scope	The scope of the variable being deleted	Whether	All ranges, including page, request, session, application, etc.

Example:

<c:remove var= "Test2" scope= "Session"/>

Deletes the TEST2 variable from the session.

2. Flow control label

<c:if>

The <c:if> label has the following properties

Property	Description	Whether you must	Default value
Test	Conditions to be evaluated, equivalent to if (...) The condition in the {} statement	Is	No
Var	Variable name that requires the condition result to be saved	Whether	No
Scope	To save a variable range of conditional results	Whether	Page

<c:choose>
This label does not accept any attributes

<c:when>
The <c:when> label has the following properties

Property	Description	Whether you must	Default value
Test	Conditions that need to be evaluated	Is	No

<c:otherwise>
This label also does not accept any attributes

Example:

<c:if test= "${user.wealthy}" > User.wealthy is true. </c:if>

Displays User.wealthy is true if the User.wealthy value is true.

<c:choose> <c:when test= "${user.generous}" > User.generous is true. </c:when> <c:when test= "${user.stingy}" > User.stingy is true. </c:when> <c:otherwise> User.generous and User.stingy are false. </c:otherwise> </c:choose>

The user.generous is true only appears if the condition user.generous return value is true.
The user.stingy is true only appears if the condition User.stingy return value is true.
All other cases (that is, user.generous and user.stingy values are not true) show all user.generous and User.stingy are false.

Because Jstl has no shape like if () {...}} else {...} , so this form of statement can only be done in conjunction with the <c:choose>, <c:when> and <c:otherwise> tags.

3. Cyclic control label

<c:forEach>
<c:forEach> tags are used for common data, which has the following properties

Property	Description	Whether you must	Default value
Items	The project to cycle through	Whether	No
Begin	Start condition	Whether	0
End	End condition	Whether	The last item in the collection
Step	Step	Whether	1
Var	Represents the variable name of the current project	Whether	No
Varstatus	Variable showing the loop state	Whether	No

Example:

<c:foreach items= "${vectors}" var= "vector" > <c:out value= "${vector}"/> </c:forEach>

Equivalent to Java statements

for (int i=0;i<vectors.size (); i++) { Out.println (Vectors.get (i)); }

Here vectors is a Java.util.Vector object that contains string data, and vector is the string object under the current loop condition. In fact, the vectors here can be any object that implements the Java.util. Collection interface.

<c:foreach begin= "0" end= "var=" "I" step= "1" > Count=<c:out value= "${i}"/><br> </c:forEach>

Output:
Count=0
...
count=100

<c:forTokens>
The <c:forTokens> label has the following properties

Property	Description	Whether you must	Default value
Items	The project to cycle through	Is	No
Delims	Split character	Is	No
Begin	Start condition	Whether	0
End	End condition	Whether	The last item in the collection
Step	Step	Whether	1
Var	Represents the variable name of the current project	Whether	No
Varstatus	Variable showing the loop state	Whether	No

Example

<c:fortokens items= "A:b:c:d" delims= ":" var= "token" > <c:out value= "${token}"/> </c:forTokens>

The use of this label is equivalent to the Java.util.StringTokenizer class. Here, a:b:c:d the string to: loop four times, token is looping to the currently split string.

4. Import files and URLs

The JSTL core tag library supports using <c:import> to include files, using <c:url> to print and format URLs, and using <c:redirect> to redirect URLs.

<c:import>
The <c:import> tag contains another page code to the current page, which has the following properties Td>scope

Property	Description	must	default
URL	the URL where the page needs to be imported	Yes	none
context	/followed by the name of the local Web application	no	The current application
charencoding	The character set used to import data	no	iso-8859-1
var	accept the imported text variable name	no	page
The variable scope of the variable that accepts the imported text	no	1
varreader	java.io.Reader variable name to accept imported text	no	no
varstatus	Variable	that shows the loop state no	no