EXCEPTION HANDLING IN JSP

The exception is normally an object that is thrown at runtime. Exception Handling is the process to handle the runtime errors. There may occur exception any time in your web application. So handling exceptions is a safer side for the web developer. In JSP, there are two ways to perform exception handling:

1. By errorPage and isErrorPage attributes of page directive
2. By <error-page> element in web.xml file

---

Example of exception handling in jsp by the elements of page directive

In this case, you must define and create a page to handle the exceptions, as in the error.jsp page. The pages where may occur exception, define the errorPage attribute of page directive, as in the process.jsp page.

There are 3 files:

• index.jsp for input values
• process.jsp for dividing the two numbers and displaying the result
• error.jsp for handling the exception

index.jsp

1. <form action="process.jsp">  
2. No1:<input type="text" name="n1" /><br/><br/>  
3. No1:<input type="text" name="n2" /><br/><br/>  
4. <input type="submit" value="divide"/>  
5. </form>  

process.jsp

1. <%@ page errorPage="error.jsp" %>  
2. <%  
3.   
4. String num1=request.getParameter("n1");  
5. String num2=request.getParameter("n2");  
6.   
7. int a=Integer.parseInt(num1);  
8. int b=Integer.parseInt(num2);  
9. int c=a/b;  
10. out.print("division of numbers is: "+c);  
11.   
12. %>  

error.jsp

1. <%@ page isErrorPage="true" %>  
2.   
3. <h3>Sorry an exception occured!</h3>  
4.   
5. Exception is: <%= exception %>  

download this example

Output of this example:

   

Expression Language (EL) in JSP

The Expression Language (EL) simplifies the accessibility of data stored in the Java Bean component, and other objects like request, session, application etc.

There are many implicit objects, operators and reserve words in EL.

It is the newly added feature in JSP technology version 2.0.

Syntax for Expression Language (EL)

1. ${ expression }  

Implicit Objects in Expression Language (EL)

There are many implicit objects in the Expression Language. They are as follows:

Implicit Objects	Usage
pageScope	it maps the given attribute name with the value set in the page scope
requestScope	it maps the given attribute name with the value set in the request scope
sessionScope	it maps the given attribute name with the value set in the session scope
applicationScope	it maps the given attribute name with the value set in the application scope
param	it maps the request parameter to the single value
paramValues	it maps the request parameter to an array of values
header	it maps the request header name to the single value
headerValues	it maps the request header name to an array of values
cookie	it maps the given cookie name to the cookie value
initParam	it maps the initialization parameter
pageContext	it provides access to many objects request, session etc.

Simple example of Expression Language that prints the name of the user

In this example, we have created two files index.jsp and process.jsp. The index.jsp file gets input from the user and sends the request to the process.jsp which in turn prints the name of the user using EL.

index.jsp

1. <form action="process.jsp">  
2. Enter Name:<input type="text" name="name" /><br/><br/>  
3. <input type="submit" value="go"/>  
4. </form>  

process.jsp

1. Welcome, ${ param.name }  

download this example

---

Example of Expression Language that prints the value set in the session scope

In this example, we printing the data stored in the session scope using EL. For this purpose, we have used sessionScope object.

index.jsp

1. <h3>welcome to index page</h3>  
2. <%  
3. session.setAttribute("user","sonoo");  
4. %>  
5.   
6. <a href="process.jsp">visit</a>  

process.jsp

1. Value is ${ sessionScope.user }  

download this example

---

Precedence of Operators in EL

There are many operators that have been provided in the Expression Language. Their precedence are as follows:

[] .

()

-(unary) not ! empty

* / div % mod

+ - (binary)

< <= > >= lt le gt ge

== != eq ne

&& and

|| or

?:

Reserve words in EL

There are many reserve words in the Expression Language. They are as follows:

lt	le	gt	ge
eq	ne	true	false
and	or	not	instanceof
div	mod	empty	null