Generating client code from spec in Spring Boot

Introduction

Generating client code from a spec helps you quickly create code that talks to an API without writing it all by hand.

You want to call a web service described by an OpenAPI or Swagger file.

You need to avoid manual coding errors when working with APIs.

You want to save time by generating ready-to-use client classes.

You want your client code to stay in sync with API changes automatically.

Syntax

Spring Boot

openapi-generator-cli generate -i <spec-file.yaml> -g java -o <output-folder> --library spring-boot

Replace with your API specification file path.

The -g java option tells the tool to generate Java client code.

Use --library spring-boot to generate Spring Boot specific client code.

Examples

This command generates Spring Boot client code from api-spec.yaml into the ./client folder.

Spring Boot

openapi-generator-cli generate -i api-spec.yaml -g java -o ./client --library spring-boot

Generates client code directly from a remote API spec URL.

Spring Boot

openapi-generator-cli generate -i https://example.com/api.yaml -g java -o ./client --library spring-boot

Generates client code using Spring WebClient instead of RestTemplate.

Spring Boot

openapi-generator-cli generate -i api-spec.yaml -g java -o ./client --library webclient

Sample Program

This example shows how to generate client code from the petstore.yaml spec. Then it uses the generated PetApi class to get a pet by ID and print its name.

Spring Boot

openapi-generator-cli generate -i petstore.yaml -g java -o ./petstore-client --library spring-boot

// After generation, import the generated client in your Spring Boot app

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import com.example.petstore.client.api.PetApi;
import com.example.petstore.client.model.Pet;

@SpringBootApplication
public class DemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);

        PetApi petApi = new PetApi();
        Pet pet = petApi.getPetById(1L);
        System.out.println("Pet name: " + pet.getName());
    }
}

Important Notes

Make sure you have openapi-generator-cli installed before running commands.

Generated code may need some configuration like base URL or authentication setup.

Keep your spec file updated to regenerate client code when API changes.

Summary

Generating client code saves time and reduces errors when calling APIs.

Use openapi-generator-cli with your API spec and the java generator.

Generated code can be used directly in your Spring Boot projects.

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