Handle exceptions in spring boot globally

In Spring Boot, global exception handling is typically done using @ControllerAdvice and @ExceptionHandler annotations. This approach helps you centralize exception handling logic for all controllers in your application, promoting cleaner code and consistent error responses.

✅ Steps to Handle Exceptions Globally in Spring Boot

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1. Create a Custom Exception (optional)

If your application has specific business rules, define custom exceptions:

package com.example.demo.exception;

public class ResourceNotFoundException extends RuntimeException {
    public ResourceNotFoundException(String message) {
        super(message);
    }
}

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2. Create a Global Exception Handler Class

Use @ControllerAdvice to handle exceptions across all controllers.

package com.example.demo.exception;

import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.MethodArgumentNotValidException;
import org.springframework.web.bind.annotation.ControllerAdvice;
import org.springframework.web.bind.annotation.ExceptionHandler;

import java.time.LocalDateTime;
import java.util.HashMap;
import java.util.Map;

@ControllerAdvice
public class GlobalExceptionHandler {

    // Handle custom exceptions
    @ExceptionHandler(ResourceNotFoundException.class)
    public ResponseEntity<Object> handleResourceNotFound(ResourceNotFoundException ex) {
        Map<String, Object> error = new HashMap<>();
        error.put("timestamp", LocalDateTime.now());
        error.put("message", ex.getMessage());
        error.put("status", HttpStatus.NOT_FOUND.value());
        return new ResponseEntity<>(error, HttpStatus.NOT_FOUND);
    }

    // Handle validation errors
    @ExceptionHandler(MethodArgumentNotValidException.class)
    public ResponseEntity<Object> handleValidationErrors(MethodArgumentNotValidException ex) {
        Map<String, Object> error = new HashMap<>();
        Map<String, String> fieldErrors = new HashMap<>();

        ex.getBindingResult().getFieldErrors().forEach(err -> {
            fieldErrors.put(err.getField(), err.getDefaultMessage());
        });

        error.put("timestamp", LocalDateTime.now());
        error.put("status", HttpStatus.BAD_REQUEST.value());
        error.put("message", "Validation Failed");
        error.put("errors", fieldErrors);

        return new ResponseEntity<>(error, HttpStatus.BAD_REQUEST);
    }

    // Handle generic exceptions
    @ExceptionHandler(Exception.class)
    public ResponseEntity<Object> handleGenericException(Exception ex) {
        Map<String, Object> error = new HashMap<>();
        error.put("timestamp", LocalDateTime.now());
        error.put("message", "An unexpected error occurred");
        error.put("details", ex.getMessage());
        error.put("status", HttpStatus.INTERNAL_SERVER_ERROR.value());
        return new ResponseEntity<>(error, HttpStatus.INTERNAL_SERVER_ERROR);
    }
}

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✅ Optional: Create a Standard Error Response DTO

package com.example.demo.exception;

import java.time.LocalDateTime;

public class ErrorResponse {
    private LocalDateTime timestamp;
    private int status;
    private String message;
    private Object details;

    // Constructors, getters, and setters
}

Then, return ErrorResponse objects instead of Map<String, Object> in the handler.

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🧪 Example: When a Department is Not Found

If a controller throws:

throw new ResourceNotFoundException("Department not found with ID: " + id);

Then you’ll get a JSON response like:

{
  "timestamp": "2025-07-27T11:35:00",
  "message": "Department not found with ID: 5",
  "status": 404
}

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✅ Summary

Component	Purpose
@ControllerAdvice	Declares a global exception handler class
@ExceptionHandler	Handles specific exception types
@ResponseStatus (opt.)	Set HTTP status code for exceptions
MethodArgumentNotValidException	Handles validation errors (e.g. @Valid)