Generating client code from spec in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of the generated client method call?

Given a Spring Boot generated client from an OpenAPI spec, what will be the output when calling getUserById(5) if the server returns {"id":5,"name":"Alice"}?

Spring Boot

public User getUserById(int id) {
    // Generated client code
    ResponseEntity<User> response = restTemplate.getForEntity("/users/{id}", User.class, id);
    return response.getBody();
}

// Assume User class has fields id and name with getters

A Null because response body is not parsed

B User object with id=5 and name='Alice'

C Exception thrown due to missing headers

D User object with id=0 and name=null

💡 Hint

Think about how restTemplate.getForEntity works and what getBody() returns.

The generated client uses RestTemplate to call the API and maps the JSON response to a User object. The getBody() method returns the deserialized User instance with the correct fields.

📝 Syntax

intermediate

Which option correctly configures OpenAPI Generator for Spring Boot client code?

You want to generate a Spring Boot client from an OpenAPI spec using OpenAPI Generator CLI. Which command is correct?

A openapi-generator-cli generate -i api.yaml -g java-spring -o ./client

B openapi-generator-cli generate -i api.yaml -g springboot -o ./client

C openapi-generator-cli generate -i api.yaml -g spring -o ./client

D openapi-generator-cli generate -i api.yaml -g springboot-client -o ./client

💡 Hint

Check the official generator names for Spring Boot clients.

The correct generator name for Spring Boot client code is java-spring. Other options are invalid generator names and will cause errors.

🔧 Debug

advanced

Why does the generated Spring Boot client fail with 404 on valid endpoint?

You generated a Spring Boot client from an OpenAPI spec. Calling getUserById(10) returns 404, but the server endpoint exists and works via curl. What is the likely cause?

A The server does not support GET requests

B The client is missing authentication headers

C The base URL in the client is incorrect or missing

D The User class is not annotated with @Entity

💡 Hint

Think about what differs between the client and curl calls.

A common cause is the client not having the correct base URL configured, so requests go to the wrong address and get 404. Authentication or entity annotations do not cause 404 here.

❓ state_output

advanced

What is the state of the client after calling an asynchronous generated method?

A Spring Boot client generated method getUserByIdAsync(5) returns a CompletableFuture<User>. After calling it, what is the state of the future immediately?

Spring Boot

CompletableFuture<User> future = client.getUserByIdAsync(5);
// Immediately after this line

A The future is not done and will complete later with the User

B The future is already completed with the User

C The future is completed exceptionally with an error

D The future is null

💡 Hint

Consider how asynchronous calls behave in Java.

Immediately after calling the async method, the CompletableFuture is not done yet because the HTTP call is still in progress. It will complete later when the response arrives.

🧠 Conceptual

expert

Which statement about generating Spring Boot clients from OpenAPI specs is true?

Choose the correct statement about generating Spring Boot client code from OpenAPI specifications.

A Generated clients do not support authentication mechanisms

B Generated clients always include UI components for API testing

C OpenAPI Generator only supports server code generation, not clients

D The generated client code can be customized by modifying templates before generation

💡 Hint

Think about the flexibility of OpenAPI Generator.

OpenAPI Generator allows customizing client code by editing or creating templates. It supports both server and client generation, and clients can include authentication support. UI components are not generated by default.