Custom auto-configuration in Spring Boot - Step-by-Step Execution

Concept Flow - Custom auto-configuration

Start Application

↓

Spring Boot scans classpath

↓

Find @EnableAutoConfiguration

↓

Load auto-config classes

↓

Check @Conditional annotations

↓

Apply config

↓

Register beans

↓

Application context ready

Spring Boot starts, scans for auto-configuration classes, checks conditions, and applies custom configurations if conditions are met.

Execution Sample

Spring Boot

@Configuration
@ConditionalOnClass(DataSource.class)
public class MyDataSourceAutoConfig {
  @Bean
  public DataSource dataSource() {
    return new HikariDataSource();
  }
}

Defines a custom auto-configuration that creates a DataSource bean only if DataSource class is on the classpath.

Execution Table

Step	Action	Condition Check	Result	Beans Registered
1	Start Spring Boot app	N/A	Begin scanning	None
2	Find @EnableAutoConfiguration	N/A	Load auto-config classes	None
3	Check MyDataSourceAutoConfig @ConditionalOnClass	Is DataSource class present?	Yes	None
4	Apply MyDataSourceAutoConfig	N/A	Create DataSource bean	DataSource bean
5	Finish context setup	N/A	Context ready	DataSource bean

💡 All auto-configurations processed; application context is ready with registered beans.

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
beansRegistered	0	0	1 (DataSource)	1 (DataSource)
autoConfigApplied	false	true	true	true

Key Moments - 2 Insights

Why does the DataSource bean only register if the DataSource class is on the classpath?

Because of the @ConditionalOnClass annotation checked at Step 3 in the execution_table, which ensures the configuration applies only if the class exists.

What happens if the condition in @ConditionalOnClass is false?

The configuration is skipped (see Step 3 result 'No'), so no beans from that config are registered.

Visual Quiz - 3 Questions

Look at the execution_table, at which step is the DataSource bean registered?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Check the 'Beans Registered' column in the execution_table at each step.

According to variable_tracker, what is the value of beansRegistered after Step 4?

A 0

B 2

C 1

D Undefined

💡 Hint

Look at the 'beansRegistered' row under 'After Step 4' in variable_tracker.

If the DataSource class was missing, what would happen at Step 3 in the execution_table?

A Beans would be registered anyway

B Condition check would be No and config skipped

C Condition check would be Yes

D Application would crash

💡 Hint

Refer to the 'Condition Check' and 'Result' columns at Step 3 in execution_table.

Concept Snapshot

Custom auto-configuration in Spring Boot:
- Create @Configuration class with @Conditional annotations
- Spring Boot loads it if conditions match
- Beans inside are registered automatically
- Use @ConditionalOnClass to check class presence
- Enables modular, flexible setup without manual config

Full Transcript

When a Spring Boot application starts, it scans the classpath for auto-configuration classes marked with @EnableAutoConfiguration. Each auto-configuration class is checked against conditions like @ConditionalOnClass to decide if it should apply. If conditions are met, Spring Boot registers the beans defined in that configuration. For example, a custom DataSource bean is only created if the DataSource class is available. This process allows automatic setup of components without manual configuration. The execution table shows each step from app start to context ready, tracking condition checks and bean registration. Variable tracking shows how the number of registered beans changes. Key moments clarify why conditions matter and what happens if they fail. The visual quiz tests understanding of when beans register and how conditions affect the flow.