Home > Articles > Programming > Java

Understanding Basic Tag Logic in JSTL

Using the JSP Standard Tag Library (JSTL), learn how to control the flow of your program and some basic error-handling methods.
This chapter is from the book

The Core tag library of JSTL provides many rudimentary tags for performing basic flow control. Flow control refers to a program's ability to selectively execute portions of itself. Most programming languages achieve this flow control through if statements and other logical tags. A program's flow of execution can also be affected by errors.

Error handling is an important part of any computer program. JSP provides tags that let you perform basic error handling. By making use of JSTL's built-in exception-handling capabilities, you can create programs that are capable of handling common runtime errors.

In this chapter, we present a sample application that shows how to tie all these concepts together. A simple forms-based chat application will show you how to use scope and basic tag logic to allow two users to chat. Let's begin by examining JSTL's error-handling capabilities.

Exception Processing

Java programs handle errors through the use of exceptions. When a section of code is susceptible to throwing an exception, you can enclose that section of code in a try block. For example, when you're creating a new URL object, it is possible that an invalid URL will be provided to the object. If this happens, the URL object will throw a MalformedURLException. To properly handle this exception, the URL instantiation must be enclosed in a try block, and a catch must be provided that will catch the MalformedURLException. The following code illustrates this:

try
{
 URL = new URL("http://www.sams.com");
}
catch(MalformedURLException e)
{
}

Exception handling allows your program to react to errors that are not a result of bad program design. For example, exceptions could be trapped when the user enters invalid information, a URL fails to load, or when some other noncritical error occurs. By trapping these exceptions, you design your program to handle these errors as they happen so they don't result in a server error. This ensures that your Web pages present a consistent and robust interface.

JSTL provides the <c:catch> tag to handle this case. Before we examine how to catch an exception in JSTL, let's see what causes an exception to be thrown.

Generating Exceptions

Many of the JSTL tags are capable of throwing exceptions, yet JSTL is designed to throw exceptions as little as possible. Many errors that would generate an exception in Java or other programming languages are ignored in JSTL. Consider division by zero. Nearly any programming language ever produced will return some sort of an error if a division is specified with zero in the denominator. Yet in JSTL, this case does not generate an exception. Listing 3.1 demonstrates this.

Listing 3.1 Division by Zero (exception.jsp)

<%@ taglib uri="http://java.sun.com/jstl/core" prefix="c" %><html>
 <head>
 <title>Throw an Exception</title>
 </head>

 <body>10 divided by 0 is 
 <c:out value="${10/0}" />

 <br />
 </body>
</html>

As you can see, Listing 3.1 attempts to throw an exception by dividing 10 by 0. Under most circumstances, this would throw an exception. However, JSTL simply ignores this error and continues processing. As you learn more about JSTL error handling, you will see that in most cases, JSTL attempts to continue processing after an error has occurred. If you run the JSP code in Listing 3.1, you get the following output:

10 divided by 0 is infinity

While JSTL attempts to handle most exceptions as they occur, certain things can go wrong that will still cause JSTL to throw an exception. Listing 3.2 shows an example of an error that will throw an exception.

Listing 3.2 An Uncaught Exception (exception2.jsp)

<%@ taglib uri="http://java.sun.com/jstl/core" prefix="c" %>
<html>
 <head>
 <title>Throw an Exception</title>
 </head>

 <body>
 <c:set var="x" value="10" scope="page" />

 <c:set var="y" value="five" scope="page" />

 x divided by y is 
 <c:out value="${x/y}" />

 <br />
 </body>
</html>

Listing 3.2 attempts to throw an exception by causing a type mismatch. The value 10 is stored in the scoped variable x. The value five, the literal string, is stored in the scoped variable y. The program then attempts to divide x by y. While it is permissible to divide by zero in JSTL, it is not permissible to divide a string. When executed, Listing 3.2 will throw an exception. Because there is nothing set up to catch this exception, it will be thrown to the Web browser as a server error. Figure 3.1 shows this error message.

Figure 3.1 An uncaught JSTL exception is thrown.

It is important to understand which errors in JSTL will result in an exception and which errors will be ignored. In general, the following errors will cause an exception to be thrown.

  • Specification of an invalid scope

  • An empty var attribute in a tag

  • An invalid type

  • An invalid value

  • Invalid expressions

JSTL is very forgiving of most other errors. For example, null values, which often cause errors in other languages, are simply treated as empty strings.

Now that you have seen what can cause an exception, it is important to see how to handle exceptions. The next section covers how to catch exceptions in JSTL.

Using the <c:catch> Tag

JSTL provides the <c:catch> tag for catching exceptions. Any tags that are within the body of the <c:catch> tag will be handled. The <c:catch> tag is capable of catching any exception that is a subclass of java.lang.Throwable; this includes all Java exceptions. The following shows the structure of a <c:catch> tag:

<c:catch var="e">
 ... Program code that may thrown an exception ...
</c:catch>

One attribute is accepted by the <c:catch> tag:

Attribute

Required

Purpose

var

Y

Specifies the scoped variable to receive the exception.


Our previous sample would catch an exception and store it in the e variable. If an exception occurs inside the body of the <c:catch> tag, JSTL immediately continues executing with the line just after the ending </c:catch> tag. The variable specified in the var attribute, in this case e, will receive a copy of the exception that was thrown. If no exception is thrown, then the exception variable will receive a null value.

When inserting a <c:catch> tag in your code, make sure that the code handles both an exception being thrown and an exception not being thrown. Listing 3.3 shows the catch division-by-zero example with a <c:catch> tag ready to handle any errors.

Listing 3.3 Our Catch Division-by-Zero example (catch.jsp)

<%@ taglib uri="http://java.sun.com/jstl/core" prefix="c" %>
<html>
 <head>
 <title>Catch an Exception?</title>
 </head>

 <body>
 <c:catch var="e">

 10 divided by 0 is 
 <c:out value="${10/0}" />

 <br />
 </c:catch>

 <c:if test="${e!=null}">The caught exception is:
 <c:out value="${e}" />

 <br />
 </c:if>

 <c:if test="${e==null}">No exception was thrown
 <br />
 </c:if>
 </body>
</html>

As you can see in the following code snippet, the division by zero is now wrapped in a <c:catch> tag. If any error occurs, the program will immediately exit the <c:catch> tag:

<c:catch var="e">
10 divided by 0 is 
<c:out value="${10/0}" />
<br />
</c:catch>

Because the division by zero does not cause an exception, this program will glide through the catch and continue on with a null value assigned to the variable e. The program continues by checking this variable and displaying the error if it was set:

<c:if test="${e!=null}">The caught exception is:
<c:out value="${e}" />
<br />
</c:if>

If an exception does occur, the program stores a value in the variable specified to the <c:catch> tag. Listing 3.4 shows an example of a program that will throw an exception. The line that generates the exception is protected by a <c:catch> tag. As a result, the exception will be handled internally by the JSP page.

Listing 3.4 Catching an Exception (catch2.jsp)

<%@ taglib uri="http://java.sun.com/jstl/core" prefix="c" %>
<html>
 <head>
 <title>Catch an Exception</title>
 </head>

 <body>
 <c:catch var="e">
 <c:set var="x" value="10" scope="page" />

 <c:set var="y" value="five" scope="page" />

 x divided by y is 
 <c:out value="${x/y}" />

 <br />
 </c:catch>

 <br />
 <c:if test="${e!=null}">The caught exception is:
 <c:out value="${e}" />

 <br />
 </c:if>

 <c:if test="${e==null}">No exception was thrown
 <br />
 </c:if>
 </body>
</html>

This program performs in exactly the same way as the division-by-zero example. However, in this case, the code following the <c:catch> tag actually has something to print. This sample program will display the following:

The caught exception is: An exception occurred trying to convert String "five" to type java.lang.Double.

This text string identifies the exception that was thrown, and includes a message that describes why the error occurred.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020