SPRING BOOT EXCEPTION HANDLING

Source: Dev.to

1. What is Exception?

Exception = unwanted situation that breaks normal flow of program.

Goal of exception handling

• Prevent program crash
• Control the error
• Provide proper messages

2. Java Exception Hierarchy

Throwable
├── Error
└── Exception
    ├── RuntimeException (Unchecked)
    └── Other Exceptions (Checked)

3. Types of Exception (Java Level)

(A) Checked Exception

• Checked at compile time
• Must be handled (try‑catch or throws)
• Extends Exception (but not RuntimeException)

Examples: IOException, SQLException, ClassNotFoundException, FileNotFoundException, InterruptedException

(B) Unchecked Exception

• Occur at runtime
• Handling is optional
• Extends RuntimeException

Examples: NullPointerException, ArithmeticException, ArrayIndexOutOfBoundsException, NumberFormatException, IllegalArgumentException

(C) Error

• JVM / system‑level problems
• Normally not handled; creating custom Error is bad practice

Examples: OutOfMemoryError, StackOverflowError

4. Checked vs Unchecked Rule

extends Exception        → Checked Exception
extends RuntimeException → Unchecked Exception
extends Error           → Error (avoid)

5. Extending Other Exceptions

Technically possible, but best practice is to extend only:

• Exception for checked exceptions
• RuntimeException for unchecked exceptions

Avoid extending specific exceptions such as NullPointerException, IOException, SQLException, etc.

6. try‑catch vs throws

try‑catch

Handle the exception at the point where it occurs.

try {
    FileReader fr = new FileReader("a.txt");
} catch (IOException e) {
    System.out.println("File error");
}

throws

Delegate responsibility to the caller.

public void readFile() throws IOException {
    FileReader fr = new FileReader("a.txt");
}

Rule: Checked exceptions must be either caught or declared with throws.

7. throw vs throws

• throw – manually throw an exception.

throw new IOException("File not found");

• throws – declare that a method may throw an exception.

public void read() throws IOException { }

8. Common Exception Meanings

• ClassNotFoundException – class not found at runtime.
• FileNotFoundException – file missing or incorrect path.
• IOException – problems with file, network, or streams.
• SQLException – SQL database errors (MySQL, PostgreSQL, Oracle). Not used with MongoDB.
• MongoDB equivalents: MongoException, DuplicateKeyException.
• InterruptedException – thread was interrupted during sleep, wait, or join.

9. Custom Exceptions

Checked Custom Exception

class MyCheckedException extends Exception {
}

Unchecked Custom Exception

class MyUncheckedException extends RuntimeException {
}

10. Java vs Spring Boot Exception Handling

Java level

• try‑catch
• throws
• Method‑level handling

Spring Boot level

• Controller/API‑level handling
• Return JSON responses to clients
• Annotations: @ExceptionHandler, @ControllerAdvice / @RestControllerAdvice

11. Spring Boot Exception Handling Example

Custom Exception

class UserNotFoundException extends RuntimeException {
    public UserNotFoundException(String msg) {
        super(msg);
    }
}

Global Handler

@RestControllerAdvice
class GlobalExceptionHandler {

    @ExceptionHandler(UserNotFoundException.class)
    public ResponseEntity handle(UserNotFoundException e) {
        return ResponseEntity.status(404).body(e.getMessage());
    }

    @ExceptionHandler(Exception.class)
    public ResponseEntity handleAll(Exception e) {
        return ResponseEntity.status(500).body("Server Error");
    }
}

Controller

@GetMapping("/user/{id}")
public User getUser(@PathVariable int id) {
    return repo.findById(id)
               .orElseThrow(() -> new UserNotFoundException("User not found"));
}

12. Key Interview Points

• Checked exception: compile‑time, must be handled.
• Unchecked exception: runtime, handling optional.
• SQLException applies only to relational databases, not MongoDB.
• Custom exception: extend Exception for checked, RuntimeException for unchecked.
• Spring Boot uses @ControllerAdvice (or @RestControllerAdvice) to handle API errors globally.

13. Summary