Generating client code from spec in Spring Boot - Performance & Optimization

Performance: Generating client code from spec

MEDIUM IMPACT

This affects page load speed by increasing initial bundle size and can delay first contentful paint if client code is large or blocking.

Using generated client code from API spec in a Spring Boot frontend app

Spring Boot

public class ApiClient {
  // Split generated client code into modules
  // Load only needed endpoints on demand (lazy loading)
}

Lazy loading reduces initial bundle size and defers loading unused code until needed, improving load speed.

📈 Performance Gainreduces initial bundle by 70-90kb, improves LCP by 50-100ms

Using generated client code from API spec in a Spring Boot frontend app

Spring Boot

public class ApiClient {
  // Entire generated client code with all endpoints and models
  // Included and loaded eagerly in main bundle
}

Including full generated client code eagerly increases bundle size and blocks rendering until all code is parsed and executed.

📉 Performance Costadds 150kb+ to initial bundle, blocks rendering for 100-200ms on typical devices

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Eagerly load full generated client code	Minimal DOM impact	0 reflows	No direct paint cost	[X] Bad
Lazy load generated client code modules	Minimal DOM impact	0 reflows	No direct paint cost	[OK] Good

Rendering Pipeline

Generated client code is parsed and executed during the JavaScript engine phase after HTML and CSS are processed. Large code increases script parsing and execution time, delaying rendering.

→Script Parsing

→Script Execution

→Style Calculation

→Layout

→Paint

⚠️ BottleneckScript Parsing and Execution

Core Web Vital Affected

LCP

This affects page load speed by increasing initial bundle size and can delay first contentful paint if client code is large or blocking.

Optimization Tips

1Avoid including full generated client code eagerly to keep initial bundle small.

2Use lazy loading or code splitting to load generated client code only when needed.

3Minimize generated code size by excluding unused endpoints or models.

Performance Quiz - 3 Questions

What is the main performance drawback of including full generated client code eagerly in a Spring Boot frontend?

A It reduces network latency

B It increases initial bundle size and delays page rendering

C It causes excessive DOM reflows

D It improves CSS selector performance

DevTools: Performance

How to check: Record a performance profile during page load, then analyze the scripting section to see script parsing and execution times related to generated client code.

What to look for: Look for long scripting tasks blocking main thread and delaying first contentful paint; large script sizes indicate heavy generated code.

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