How do you create custom exception classes in Spring?

Table of Contents

• Introduction
• Creating Custom Exception Classes in Spring
  • 1. Basic Custom Exception Class
    • Example: Creating a Simple Custom Exception
    • Key Points:
  • 2. Custom Exception with Error Codes
    • Example: Custom Exception with Error Codes
  • 3. Using Custom Exception with **@ControllerAdvice** and **@ExceptionHandler**
    • Example: Handling Custom Exceptions Globally
    • Key Points:
  • 4. Custom Exception Handling for REST APIs
    • Example: Returning JSON for Custom Exceptions
    • Key Points:
  • 5. Throwing Custom Exceptions in Controllers
    • Example: Throwing Custom Exceptions in a Controller
    • Key Points:
• Conclusion

Introduction

In Spring applications, handling exceptions is a critical part of ensuring the system behaves predictably and provides meaningful feedback to users. While Spring provides a number of built-in exceptions (such as ResourceNotFoundException or DataAccessException), you often need to create your own custom exception classes for specific application logic.

Custom exceptions allow you to:

• Represent domain-specific error conditions.
• Provide meaningful error messages to users.
• Integrate custom error handling across your Spring-based application.

In this guide, we will explore how to create and handle custom exception classes in Spring, including integrating them with global exception handling.

Creating Custom Exception Classes in Spring

1. Basic Custom Exception Class

To create a custom exception class in Spring, you typically extend RuntimeException (for unchecked exceptions) or Exception (for checked exceptions). A custom exception can have additional properties like error codes, messages, and more, which can be useful for logging or displaying error details.

Example: Creating a Simple Custom Exception

In this example:

• The exception ResourceNotFoundException is a custom exception that represents the situation where a resource is not found.
• The class extends RuntimeException, meaning it is an unchecked exception (does not require explicit handling with try-catch blocks).
• The constructor takes additional parameters (resourceName, fieldName, and fieldValue) to provide more context when the exception is thrown.

Key Points:

• Unchecked exceptions extend RuntimeException and do not need to be explicitly handled.
• Checked exceptions extend Exception and must be declared with throws or handled with try-catch.
• Custom exceptions should be descriptive and provide useful information for debugging or user feedback.

2. Custom Exception with Error Codes

For more structured exception handling, you can add error codes to your custom exceptions. This can be helpful if you need to standardize error responses or support internationalization (i18n).

Example: Custom Exception with Error Codes

In this example:

• The exception includes an errorCode that can be used to categorize the error.
• The getErrorCode() method allows easy access to the error code.

3. Using Custom Exception with **@ControllerAdvice** and **@ExceptionHandler**

Once you have defined your custom exception class, you can use Spring’s @ExceptionHandler and @ControllerAdvice to handle it in a centralized manner.

Example: Handling Custom Exceptions Globally

1. Create a custom exception class:

2. Define a global exception handler using **@ControllerAdvice**:

In this example:

• @ExceptionHandler is used to handle ResourceNotFoundException globally.
• @ResponseStatus is used to set the HTTP response status (404 for resource not found).
• The error message is added to the model and displayed on a custom error page.

Key Points:

• **@ExceptionHandler** allows you to handle exceptions globally or at the controller level.
• **@ControllerAdvice** enables global exception handling across all controllers.

4. Custom Exception Handling for REST APIs

In RESTful APIs, it’s common to return error responses in JSON format instead of redirecting to a view. You can handle custom exceptions in a similar way and return structured JSON error messages.

Example: Returning JSON for Custom Exceptions

1. Create the custom exception:

2. Define a custom error response class:

3. Handle exceptions and return JSON responses:

In this example:

• Custom exceptions like ResourceNotFoundException are caught in the @ControllerAdvice.
• The exception handler returns a ResponseEntity containing a structured JSON response (ErrorResponse), with an appropriate HTTP status code (e.g., 404 for not found).

Key Points:

• Custom exceptions are typically handled in @ControllerAdvice for REST APIs.
• JSON error responses can be standardized using a custom ErrorResponse class.
• You can return different status codes based on the exception type.

5. Throwing Custom Exceptions in Controllers

Once your custom exceptions are defined, you can throw them from your controller methods based on specific business logic or conditions.

Example: Throwing Custom Exceptions in a Controller

In this example:

• If the id parameter is invalid (less than or equal to 0), a ResourceNotFoundException is thrown.
• This exception is then caught by the global exception handler, which returns a JSON error response.

Key Points:

• Throw custom exceptions in controller methods based on specific logic.
• Exception handling can be centralized using @ControllerAdvice for consistency.

Conclusion

Creating custom exception classes in Spring is a powerful way to handle application-specific errors gracefully. By defining custom exceptions, you can:

• Represent domain-specific error conditions.
• Add context to errors, such as error codes and messages.
• Handle exceptions centrally using @ExceptionHandler and @ControllerAdvice.
• Return structured error responses, especially for REST APIs.

Using custom exceptions along with Spring’s built-in exception handling mechanisms allows for a cleaner, more organized way of managing errors in your applications, providing meaningful feedback to users and developers alike.