SpringDoc OpenAPI setup in Spring Boot - Performance & Optimization

Performance: SpringDoc OpenAPI setup

MEDIUM IMPACT

This affects the initial page load speed of the API documentation UI and the backend response time for OpenAPI JSON generation.

Setting up SpringDoc OpenAPI for API documentation

Spring Boot

springdoc:
  packages-to-scan: com.example.api
  api-docs:
    path: /v3/api-docs
  swagger-ui:
    enabled: true

# Limits scanning to API packages only

Restricting package scanning reduces startup time and API docs size, speeding up UI load and backend response.

📈 Performance Gainreduces startup delay by 150-400ms; cuts API docs JSON size by 40-80kb

Setting up SpringDoc OpenAPI for API documentation

Spring Boot

springdoc:
  api-docs:
    path: /v3/api-docs
  swagger-ui:
    enabled: true

# No package filtering, scans entire application including unused classes

Scanning the entire application context causes slower startup and larger OpenAPI JSON, increasing UI load time.

📉 Performance Costblocks backend startup for 200-500ms extra; increases API docs JSON size by 50-100kb

Performance Comparison

Pattern	Backend Startup Delay	API Docs JSON Size	Frontend Load Impact	Verdict
Scan entire app packages	200-500ms extra	100-150kb	LCP +300-600ms	[X] Bad
Scan only API packages	50-150ms extra	20-50kb	LCP +50-150ms	[OK] Good
Swagger UI always enabled	N/A	N/A	Adds 200-300kb bundle, LCP +300-600ms	[X] Bad
Swagger UI enabled only in dev	N/A	N/A	Saves 200-300kb bundle, faster LCP	[OK] Good

Rendering Pipeline

SpringDoc generates OpenAPI JSON on backend startup or on-demand, which the Swagger UI frontend fetches and renders. Large JSON or unnecessary scanning delays backend response and increases frontend load time.

→Backend Startup

→Network Transfer

→Frontend Rendering

⚠️ BottleneckBackend Startup and Network Transfer of large OpenAPI JSON

Core Web Vital Affected

LCP

This affects the initial page load speed of the API documentation UI and the backend response time for OpenAPI JSON generation.

Optimization Tips

1Limit SpringDoc package scanning to only API packages to reduce startup delay.

2Disable Swagger UI in production to avoid loading heavy frontend assets.

3Monitor OpenAPI JSON size and backend response time to optimize LCP.

Performance Quiz - 3 Questions

How does scanning the entire application for OpenAPI documentation affect performance?

A Increases backend startup time and API docs size, slowing UI load

B Reduces backend startup time by caching all classes

C Has no impact on performance

D Improves frontend rendering speed

DevTools: Network and Performance panels

How to check: Open DevTools, reload Swagger UI page, check Network tab for /v3/api-docs JSON size and load time; use Performance tab to measure LCP and scripting time.

What to look for: Look for large JSON payloads and long backend response times; high LCP or long scripting times indicate performance issues.

Practice

1. What is the main purpose of adding springdoc-openapi-starter-webmvc-ui dependency in a Spring Boot project?

easy

A. To configure logging for the application

B. To enable database connectivity

C. To automatically generate live API documentation for the application

D. To add security features to the application

Solution

1. Step 1: Understand the role of springdoc-openapi-starter-webmvc-ui

   This dependency integrates SpringDoc OpenAPI with Spring Boot to generate live API docs automatically.

2. Step 2: Identify the purpose of the dependency

   It provides a user interface and backend support to display API documentation without manual setup.

3. Final Answer:

   To automatically generate live API documentation for the application -> Option C

4. Quick Check:

   SpringDoc dependency = live API docs [OK]

Hint: SpringDoc dependency creates live API docs automatically [OK]

Common Mistakes:

• Confusing it with database or security dependencies
• Thinking it configures logging
• Assuming manual doc writing is still needed

2. Which of the following is the correct way to define a basic OpenAPI bean in a Spring Boot application to customize API info?

easy

A. @Bean public OpenAPI customOpenAPI() { return new OpenAPI().info(new Info().name("API").ver("1.0")); }

B. @Bean public OpenAPI customOpenAPI() { return new OpenAPI().setTitle("API").setVersion("1.0"); }

C. @Bean public OpenAPI customOpenAPI() { return new OpenAPI().info("API", "1.0"); }

D. @Bean public OpenAPI customOpenAPI() { return new OpenAPI().info(new Info().title("API").version("1.0")); }

Solution

1. Step 1: Recall the OpenAPI bean syntax

   The OpenAPI bean requires an Info object with title and version set via methods like title() and version().

2. Step 2: Compare options with correct method names

   Only @Bean public OpenAPI customOpenAPI() { return new OpenAPI().info(new Info().title("API").version("1.0")); } uses correct chaining: new OpenAPI().info(new Info().title("API").version("1.0"))

3. Final Answer:

   @Bean public OpenAPI customOpenAPI() { return new OpenAPI().info(new Info().title("API").version("1.0")); } -> Option D

4. Quick Check:

   OpenAPI bean uses info(new Info().title().version()) [OK]

Hint: Use Info().title() and .version() methods inside OpenAPI bean [OK]

Common Mistakes:

• Using incorrect method names like setTitle or info(String, String)
• Passing strings directly instead of Info object
• Using non-existent methods like name() or ver()

3. Given this Spring Boot configuration snippet, what will be the URL path to access the Swagger UI documentation?

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

Assuming springdoc-openapi-starter-webmvc-ui is added as dependency and no custom properties are set.

medium

A. /swagger-ui.html

B. /swagger-ui/index.html

C. /api-docs

D. /openapi-ui

Solution

1. Step 1: Recall default Swagger UI path with SpringDoc

   SpringDoc by default serves Swagger UI at /swagger-ui/index.html

2. Step 2: Eliminate other options

   /swagger-ui.html is old Swagger path, /api-docs is for JSON docs, /openapi-ui is invalid default

3. Final Answer:

   /swagger-ui/index.html -> Option B

4. Quick Check:

   Default Swagger UI path = /swagger-ui/index.html [OK]

Hint: Swagger UI default path ends with /swagger-ui/index.html [OK]

Common Mistakes:

• Using old Swagger UI path /swagger-ui.html
• Confusing JSON docs path with UI path
• Assuming custom paths without configuration

4. You added the springdoc-openapi-starter-webmvc-ui dependency but when you run your Spring Boot app, the Swagger UI page shows a 404 error. What is the most likely cause?

medium

A. You are trying to access the Swagger UI at the wrong URL

B. You did not include the springdoc-openapi-starter-webmvc-ui dependency correctly

C. You have not defined an OpenAPI bean in your configuration

D. You forgot to add the @SpringBootApplication annotation

Solution

1. Step 1: Check common reasons for 404 on Swagger UI

   Missing dependency or wrong annotation usually cause app startup errors, not 404 on UI page.

2. Step 2: Verify URL correctness

   Swagger UI default URL is /swagger-ui/index.html, accessing other URLs like /swagger-ui.html causes 404.

3. Final Answer:

   You are trying to access the Swagger UI at the wrong URL -> Option A

4. Quick Check:

   404 on Swagger UI usually means wrong URL [OK]

Hint: Check Swagger UI URL carefully if 404 occurs [OK]

Common Mistakes:

• Assuming missing OpenAPI bean causes 404
• Thinking missing @SpringBootApplication causes 404 instead of startup failure
• Ignoring URL path correctness

5. You want to customize your Spring Boot API documentation with a title "My API" and version "2.0" using SpringDoc. Which code snippet correctly achieves this and ensures the OpenAPI bean is recognized by Spring?

hard

A. @Bean public OpenAPI customOpenAPI() { return new OpenAPI().info(new Info().title("My API").version("2.0")); }

B. @Component public OpenAPI customOpenAPI() { return new OpenAPI().info(new Info().title("My API").version("2.0")); }

C. public OpenAPI customOpenAPI() { return new OpenAPI().info(new Info().title("My API").version("2.0")); }

D. @Configuration public OpenAPI customOpenAPI() { return new OpenAPI().info(new Info().title("My API").version("2.0")); }

Solution

1. Step 1: Ensure the method is a Spring bean

   To register the OpenAPI object, the method must be annotated with @Bean inside a configuration class.

2. Step 2: Check the code snippet correctness

   @Bean public OpenAPI customOpenAPI() { return new OpenAPI().info(new Info().title("My API").version("2.0")); } correctly uses @Bean annotation and returns new OpenAPI().info(new Info().title("My API").version("2.0"))

3. Final Answer:

   @Bean public OpenAPI customOpenAPI() { return new OpenAPI().info(new Info().title("My API").version("2.0")); } -> Option A

4. Quick Check:

   Use @Bean annotation to register OpenAPI bean [OK]

Hint: Use @Bean annotation on method returning OpenAPI [OK]

Common Mistakes:

• Omitting @Bean annotation so Spring ignores the method
• Using @Component or @Configuration on method instead of class
• Not returning an OpenAPI object with info set