ResponseEntity | Image Source : tecoble.techcourse.co.kr

Importance of using ResponseEntity

SpringSeries#6 — Tailor Your API Responses and Improve Your RESTful Web Services in Spring Framework

What is a ResponseEntity?

ResponseEntity is an essential class in Spring Framework, particularly when building RESTful web services using Spring MVC or Spring Boot. It represents an HTTP response, allowing you to customize the response status, headers, and body that your API returns to the client. Here are some of the key reasons why ResponseEntity is important:

• HTTP Status and Headers Customization: With ResponseEntity, you can set custom HTTP status codes and headers for your API responses. This allows you to provide meaningful status codes and additional information in the headers to communicate the outcome of the request effectively.
• Handling Different Response Types: ResponseEntity is a generic class, allowing you to return various response types, such as JSON, XML, plain text, or even custom objects, as the body of the response. This flexibility makes it easier to cater to different client requirements.
• Error Handling: In case of errors or exceptions, ResponseEntity can be used to construct meaningful error responses with appropriate status codes and error messages. This is crucial for building robust and user-friendly APIs.

Check the below example :

//build GET employee by id
    @GetMapping("{id}")
    public ResponseEntity<Employee> getEmployeeById(@PathVariable long id){
        Employee employee = employeeRepository.findById(id)
                .orElseThrow(()-> new ResourceNotFoundException("Employee not exist with id: " +id));
        return ResponseEntity.ok(employee);

    }

//Explanation
    /*
    @GetMapping("{id}"): This annotation specifies that the method handles HTTP
 GET requests for a specific URL pattern. In this case, the 
URL pattern includes a path variable {id} which represents the unique 
identifier of the employee.

public ResponseEntity<Employee> getEmployeeById(@PathVariable long id): This 
method accepts a long parameter id annotated with @PathVariable. 
The @PathVariable annotation is used to bind the value of the path variable 
from the URL to the id parameter.

Employee employee = employeeRepository.findById(id).orElseThrow(() -> new ResourceNotFoundException("Employee not exist with id: " + id));: This line retrieves the employee from the employeeRepository by calling the findById() method, passing in the id parameter. If the employee with the given id is not found, it throws a custom exception ResourceNotFoundException with a specific error message.

return ResponseEntity.ok(employee);: If the employee is found, this 
line returns a response with the HTTP status code 200 (OK) and the 
employee object in the response body. The ResponseEntity.ok() method 
is used to create a ResponseEntity instance with the specified body and status.

In summary, this controller method handles a GET request to retrieve 
an employee by their unique identifier. It fetches the employee from 
the repository based on the provided id, throws an exception if the 
employee is not found, and returns the employee object in the response 
if it exists. The response is wrapped in a ResponseEntity object, allowing 
you to set additional response headers and status codes if needed.

The ()-> syntax you see in the code snippet is part of a lambda expression 
in Java.

The ()-> represents an anonymous function or lambda function 
with no parameters. It's shorthand for declaring a functional 
interface (an interface with a single abstract method) and 
implementing its method on the fly.

The lambda expression is used as an argument to the orElseThrow() method, 
which is a method provided by the Optional class in Java. 
The purpose of orElseThrow() is to either return the value contained 
within the Optional if it exists or throw an exception if it's empty.

In this case, if the findById(id) method returns an 
empty Optional (meaning no employee was found with the given id), the 
lambda expression () -> new ResourceNotFoundException("Employee not exist 
with id: " + id) is executed. It creates a new instance of the 
ResourceNotFoundException class with a specific error message. 
This exception is then thrown, terminating the execution and 
indicating that the requested resource (employee) was not found.

The lambda expression syntax ()-> allows you to define a block 
of code as an inline function without explicitly creating a 
separate method or implementing an interface. It provides a 
concise way to define and use simple functions in Java.

• Response Content Negotiation: Spring supports content negotiation based on the request’s Accept header. ResponseEntity enables you to return responses in the format requested by the client, making your API more versatile and easier to consume by various clients.
• Asynchronous Response: ResponseEntity can also be used in asynchronous scenarios, where you want to return a response at a later time or as a result of a non-blocking operation.
• Testing and Mocking: When writing tests for your API endpoints, ResponseEntity provides a convenient way to assert and verify the response status, headers, and body. It simplifies testing and allows you to validate the behavior of your endpoints accurately.

Example Usage of ResponseEntity | Image Source : miro.medium.com

Is it a Best Practice?

Yes, using ResponseEntity is considered a best practice in Spring when building RESTful APIs. It provides a flexible and powerful way to handle API responses, allowing you to customize status codes, headers, and response bodies based on various scenarios. By using ResponseEntity, you have greater control over how your API communicates with clients and can handle different situations more effectively, making it a recommended approach for developing robust and reliable RESTful services in Spring.