How to handle exceptions in JSP

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Last Updated on 21 July 2019   |   Print  Email


Generally, there are three ways to handle exceptions in a JSP page:

Let’s take closer look at each approach in details with their own benefit and drawback.

 

1. Catching exceptions directly in JSP page

This is the simplest way, as we place a try-catch block directly into the JSP page, for example:

<%@ page language="java" %>
<html>
<body>
	<%
		try {
			int x = Integer.parseInt(request.getParameter("x"))	;

			%>
			<h2>You pass the number: <%=x%></h2>
			<%

		} catch (NumberFormatException ex) {
			%>
			<h2>Error: x must be a number</h2>
			<%
		}
	%>
</body>
</html>
The above code snippet tries to parse a number from a parameter which is passed from URL query string. If the parameter is not a number (indicated by the NumberFormatException is thrown), shows an error message.

Following is a more meaningful example which is a JSP page that calculates sum of two numeric parameters passed from query string:

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" 
	"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Sum calculation</title>
</head>
<body>
	<%! 
		int x;
		int y;
	%>
	<%
		try {
			x = Integer.parseInt(request.getParameter("x"))	;
		} catch (NumberFormatException ex) {
			%>
			<h2>Error: x must be a number</h2>
			<%
			return;
		}
	
		try {
			y = Integer.parseInt(request.getParameter("y"))	;
		} catch (NumberFormatException ex) {
			%>
			<h2>Error: y must be a number</h2>
			<%
			return;
		}	
		
	%>
	<h1>
		Sum of <%=x%> and <%=y%> is <%=(x + y) %>
	</h1>
	
</body>
</html>
If the two parameters are numeric, the page displays sum of them:

Test catch exception in JSP

If either a number is not numeric, an appropriate error message is displayed:



Test catch exception in JSP - error message

Benefits:

Drawbacks:

 

2. Page-level exception handling

In this way, we use JSP page directives to specify a separate error page which will be redirected in case exceptions occurred in the normal JSP page. The following diagram depicts this approach:

jsp page-level exception handling diagram

With this approach, there are two attributes of the page directive we have to specify for handling exceptions:

Throwable class diagram

Let’s see an example. Consider the following JSP page:

<%@ page language="java" contentType="text/html; charset=UTF-8"
	errorPage="error_jstl.jsp"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" 
	"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Calculate division of two numbers</title>
</head>
<body>
<%
	int x = Integer.parseInt(request.getParameter("x"));
	int y = Integer.parseInt(request.getParameter("y"));
	
	int div = x / y;
	
%>
<h2>Division of <%=x%> and <%=y%> is <%=div%></h2>
</body>
</html>

This page calculates division of two numbers passed from parameters query string. There are two possible exceptions here:

And here is code of the error handling page (error.jsp):

<%@ page language="java" contentType="text/html; charset=UTF-8"
	isErrorPage="true"
	pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" 
	"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Error</title>
</head>
<body>
<h2>
	Error: <%=exception.getClass().getName() %><br/>
	<%=exception.getMessage() %><br/>	
</h2>
</body>
</html>
Here are some outputs when running this example:

Test JSP page-level exception handling - exception 2

Apart from accessing the exception object using JSP scriptlet, we can use JSTL to achieve the same goal in a more convenient way. For example:

Error: ${pageContext.exception}
To print the exception stack traces using JSTL:

<c:forEach var="trace" items="${pageContext.exception.stackTrace}">
	${trace}<br/>
</c:forEach>
Here is a better version of the error.jsp page using JSTL:

<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<%@ page language="java" contentType="text/html; charset=UTF-8"
	isErrorPage="true"
	pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" 
	"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Error</title>
</head>
<body>
	<h2>
		Error: ${pageContext.exception}  <br/>
	</h2>	

	Exception stack trace:<br/>
	
	<c:forEach var="trace" items="${pageContext.exception.stackTrace}">
		${trace}<br/>
	</c:forEach>	

</body>
</html>
Output:

Test JSP page-level exception handling - stack trace

Benefits:

Drawbacks:                                    

3. Application-level exception handling

In this way, we can configure error handling page per exception type for all JSPs and servlets of the web application, by adding some entries in the web.xml file. Here is the syntax for configuring an error page for a specific exception type:

<error-page>
	<exception-type>fully_qualified_name_of_exception_class</exception-type>
	<location>path_to_error_jsp_page</location>
</error-page>
For example:

<error-page>
	<exception-type>java.sql.SQLException</exception-type>
	<location>/dbError.jsp</location>
</error-page>
That will tell the server to redirect the clients to the dbError.jsp page whenever any JSP/Servlet throws an exception of type java.sql.SQLException. The dbError.jsp page does not have to declare the attribute isErrorPage.

Using this approach we can also specify a custom error handling page for specific HTTP error code like 404, 500… The syntax is:

<error-page>
	<error-code>HTTP_error_code</error-code>
	<location>path_to_error_jsp_page</location>
</error-page>
For example:

<error-page>
	<error-code>404</error-code>
	<location>/404error.jsp</location>
</error-page>
That will tell the server redirects all requests causing HTTP 404 error to the custom page 404error.jsp, instead of showing the default 404 error. Following is example code of the 404error.jsp page:

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" 
	"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>404 Error</title>
</head>
<body>
<center>
	<h2>
		Apologize, we could not find the page you were looking for:
	</h2>
	${pageContext.errorData.requestURI}
</center>	
</body>
</html>
The errorData object provides detailed information about the error such as the request URI, status code, servlet name, and the Throwable object that caused the error:

ErrorData class diagram

Here is the output when trying to access non-exist URL on the server:

Test 404 error code handling

Benefit:

Drawback:

 

Related Java Exception Handling Tutorials:

 

Other JSP Tutorials:


About the Author:

is certified Java programmer (SCJP and SCWCD). He started programming with Java in the time of Java 1.4 and has been falling in love with Java since then. Make friend with him on Facebook and watch his Java videos you YouTube.



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