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.

Practice

1. What is the main benefit of generating client code from an API specification in Spring Boot projects?

easy

A. It automatically deploys the API to the cloud.

B. It saves time and reduces errors when calling APIs.

C. It replaces the need for a backend server.

D. It creates database schemas from the API.

Solution

1. Step 1: Understand the purpose of client code generation

   Generating client code means creating ready-to-use code to call APIs, which saves manual coding effort.

2. Step 2: Identify the main benefit in Spring Boot context

   This process reduces human errors and speeds up development by automating client creation.

3. Final Answer:

   It saves time and reduces errors when calling APIs. -> Option B

4. Quick Check:

   Client code generation = saves time and reduces errors [OK]

Hint: Remember: generated client code automates API calls [OK]

Common Mistakes:

• Thinking it deploys APIs automatically
• Confusing client code with backend code
• Assuming it creates database schemas

2. Which command correctly generates Spring client code using openapi-generator-cli from a spec file named api.yaml?

easy

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

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

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

D. openapi-generator-cli build -i api.yaml -g spring -o ./client

Solution

1. Step 1: Identify the correct command syntax for code generation

   The command uses 'generate' to create code, '-i' for input spec, '-g' for generator type, and '-o' for output folder.

2. Step 2: Match the correct generator for Spring client code

   The generator name for Spring client code is 'spring', not 'springboot' or 'java'.

3. Final Answer:

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

4. Quick Check:

   Correct command = generate + -g spring [OK]

Hint: Use 'generate' and '-g spring' for Spring client code [OK]

Common Mistakes:

• Using 'create' or 'build' instead of 'generate'
• Using wrong generator name like 'springboot'
• Using '-g java' which is for Java server, not Spring client

3. Given the command openapi-generator-cli generate -i petstore.yaml -g spring -o ./client, what will be the main output?

medium

A. A Spring Boot server application implementing the API

B. A database migration script

C. A JavaScript client library for the API

D. A Spring client library to call the API endpoints

Solution

1. Step 1: Analyze the generator option '-g spring'

   The 'spring' generator creates Spring client code, not server code or JavaScript libraries.

2. Step 2: Understand the output folder './client'

   The generated code will be placed in './client' as a client library to call API endpoints.

3. Final Answer:

   A Spring client library to call the API endpoints -> Option D

4. Quick Check:

   Generator 'spring' = Spring client library [OK]

Hint: '-g spring' generates client code, not server or JS [OK]

Common Mistakes:

• Confusing client code with server implementation
• Expecting JavaScript code from Spring generator
• Thinking it creates database scripts

4. You ran openapi-generator-cli generate -i api.yaml -g springboot -o ./client but got an error: Unknown generator: springboot. What is the likely cause?

medium

A. You need to install Java separately.

B. The input file 'api.yaml' is missing.

C. You used the wrong generator name 'springboot' instead of 'spring'.

D. The output folder './client' already exists.

Solution

1. Step 1: Check the error message details

   The error says 'Unknown generator: springboot', indicating the generator name is invalid.

2. Step 2: Verify correct generator name for Spring client

   The correct generator is 'spring', not 'springboot'. Using 'springboot' causes this error.

3. Final Answer:

   You used the wrong generator name 'springboot' instead of 'spring'. -> Option C

4. Quick Check:

   Error 'Unknown generator' = wrong generator name [OK]

Hint: Check generator name spelling carefully [OK]

Common Mistakes:

• Typing 'springboot' instead of 'spring'
• Ignoring error details
• Assuming missing input file causes this error

5. You want to generate a Spring client from an OpenAPI spec but only want to include APIs tagged with 'user'. Which approach should you take?

hard

A. Modify the OpenAPI spec to only include 'user' tagged APIs before generation.

B. Manually delete unwanted APIs from the generated code after generation.

C. Use --ignore-file-override to exclude APIs by tag.

D. Use the --global-property apis=user option with openapi-generator-cli.

Solution

1. Step 1: Understand filtering options in openapi-generator-cli

   The CLI does not support filtering APIs by tag directly during generation.

2. Step 2: Consider practical ways to generate only 'user' APIs

   Modifying the OpenAPI spec to include only desired tags is the cleanest and recommended approach.

3. Final Answer:

   Modify the OpenAPI spec to only include 'user' tagged APIs before generation. -> Option A

4. Quick Check:

   Filter APIs by editing spec before generation [OK]

Hint: Filter APIs by editing spec before generating client [OK]

Common Mistakes:

• Expecting CLI to filter APIs by tag automatically
• Deleting code manually after generation (error-prone)
• Using unsupported CLI options for filtering