Core library in Java

Making QR-Code in Java Core library

The Core tag library provides general functionality that page developers commonly need most The convention is to reference all tags in the Core library with the prefix c

The general-purpose tags consists of four tags that you use to manipulate scoped variables (variables that exist in one of the four scopes page, request, session, or application) They are

the current JSPWriter (responsible for writing out the page contents) The following example outputs the value of the vendorName property found in the po object stored in the session:

of a target object For example, use the following to set a scoped vari able named currentResident :

To set the city property of the customeraddress target object, use the following:

<c:remove>: The remove tag removes a scoped variable from the scope To remove the currentResident variable from the page scope, use

the following:

<c:catch>: The catch tag catches an exception (javalang Throwable) in a nested block For example, to catch an exception in a

set of tags, use the following:

<c:catch var= theException > <c:out value= ${popoNumber} /> other tags </c:catch> <c:if test= ${theException != null} > Oops! An error occurred </c:if>

Conditional tags support the conditional execution of various enclosed page elements based on the evaluation of one or more expressions Two basic tags are available, a simple conditional tag and a mutually exclusive tag:

<c:if>: The simple conditional is the if tag If the expression evaluates to true the body of the if tag is executed For example:

<c:if test= ${customeraccesses == 1} > This is your first access Welcome to the Blah-Blah Web site </c:if>

<c:choose>, <c:when>, and <c:otherwise>: In the body of choose you define the mutually exclusive conditions by using the when tag If none of the when conditions are true, the optional otherwise tag is

executed For example:

<c:choose> <c:when test= ${potag == save } > </c:when> <c:when test= ${potag == update } > </c:when> <c:when test= ${potag == delete } >

The iterator tags provides a looping mechanism to iterate over a wide variety of collections of objects Two very flexible tags are available to accomplish this:

element in the collection of objects it is iterating over The collection of objects can be any implementation of javautilCollection, java utilMap, javautilIterator, javautilEnumeration, an array, and even a String of comma-separated values Here is an example iter ating over a collection of purchase orders and writing out the purchase order number of each purchase order:

<table> <c:forEach var= po items= ${purchaseorders} > <tr><td><c:out value= ${popoNumber} /></td></tr> </c:forEach> </table>

Here is a similar iteration, but this time the collection of objects is of the type javautilMap Each item from the map will be of type javautil MapEntry and have two properties, key and value The example out puts the value property:

<table> <c:forEach var= ponumber items= ${ponumbers} > <tr><td><c:out value= ${ponumbervalue} /></td></tr> </c:forEach> </table>

You may also use the forEach tag to iterate over a particular range of numbers, with a starting and ending number as well as a number to increment by Here is an example that iterates 11 times, from number 100 to 110

Finally, you may need to know the particular iteration number you are currently on You may define the varStatus attribute and retrieve the count property from that variable Here is an example:

<table> <c:forEach var= purchaseorder items= ${purchaseorders} varStatus= status > <tr> <td><c:out value= ${statuscount} /></td> <td><c:out value= ${purchaseorderpoNumber} /></td> </tr> </c:forEach> </table>

<c:forTokens>: The forTokens tag is similar to the forEach tag except forTokens iterates over a set of tokens (user-defined entities) that are separated by the delimiters supplied as an attribute The set of tokens are enclosed in a String Here is an example of using forTokens with a ; delimiter:

<table> <c:forTokens var= alphavalue items= a;b;c;r,s;w;z delims= ; > <tr><td><c:out value= ${alphavalue} /></td></tr> </c:forEach> </table>