Overview

JSP Expression Language (EL) facilitates the simplification of accessing data stored in JavaBeans components. By allowing the easy integration of application data into JSP pages, EL reduces the need for extensive Java code, making the development process more efficient and manageable. Its usage is critical for creating dynamic web applications with cleaner, more readable code.

Key Concepts

1. Syntax and Operators: EL provides a simple syntax to access data and supports a variety of operators for arithmetic, logical, and relational operations.
2. Implicit Objects: EL offers implicit objects for direct access to request, session, application scopes, and more without Java code.
3. Custom Functions: EL allows the creation and use of custom functions, increasing its versatility and power in JSP pages.

Common Interview Questions

Basic Level

1. What is JSP Expression Language (EL), and why is it used?
2. How do you access an array or a list using EL?

Intermediate Level

3. How can you disable EL processing on a JSP page?

Advanced Level

4. Discuss the use of custom functions in EL. How do you define and use them?

Detailed Answers

1. What is JSP Expression Language (EL), and why is it used?

Answer: JSP Expression Language (EL) is a language designed to simplify the process of accessing and manipulating data within JSP pages. It was introduced to eliminate the need for Java code in accessing application data stored in JavaBeans. The use of EL makes JSP pages more maintainable and readable by providing a straightforward way to bind objects and their properties to the UI components.

Key Points:
- Simplifies data access in JSP pages.
- Reduces the need for Java code within JSP pages.
- Enhances page readability and maintainability.

Example:

${employee.name} // Accesses the name property of an employee object

2. How do you access an array or a list using EL?

Answer: In EL, arrays or lists can be accessed using square bracket notation or dot notation for indices. This feature makes it straightforward to access elements within these collections without resorting to Java code.

Key Points:
- Use dot notation or square brackets to access elements.
- Indices are zero-based.
- Can access properties of objects within lists directly.

Example:

${myArray[0]} // Accesses the first element of the array
${myList[2].name} // Accesses the name property of the third element in a list

3. How can you disable EL processing on a JSP page?

Answer: EL processing can be disabled at the page level using the isELIgnored attribute of the <%@page%> directive. Setting this attribute to true prevents the interpretation of EL expressions within the JSP page, treating them as plain text.

Key Points:
- Disabling EL helps in pages where expressions should not be evaluated.
- Must be declared at the beginning of the JSP page.
- Useful in specific scenarios where dynamic expression processing is not desired.

Example:

<%@ page isELIgnored="true" %>

4. Discuss the use of custom functions in EL. How do you define and use them?

Answer: Custom functions in EL allow developers to extend the capabilities of EL by defining reusable functions that can be called within EL expressions. These functions are typically housed in a separate Java class and are mapped through a TLD (Tag Library Descriptor) file.

Key Points:
- Increases the power and flexibility of EL.
- Requires defining the function in a Java class and mapping it in a TLD file.
- Enhances reusability across JSP pages.

Example:
1. Define the function in a Java class:

public class CustomFunctions {
    public static String toLowerCase(String input) {
        return input.toLowerCase();
    }
}

2. Map the function in a TLD file:

<function>
    <name>toLowerCase</name>
    <function-class>com.example.CustomFunctions</function-class>
    <function-signature>java.lang.String toLowerCase(java.lang.String)</function-signature>
</function>

3. Use the function in a JSP page:

${myCustom:toLowerCase(employee.name)}