@Conditional and Custom Conditions

Every time Spring Boot automatically sets up a DataSource because H2 is on the classpath, or skips configuring JdbcTemplate because you already defined one, the mechanism is @Conditional. It is the decision engine behind all of Spring Boot's auto-configuration.

The core interface

• Condition has one method: matches(ConditionContext, AnnotatedTypeMetadata) returns true or false
• If false, Spring skips the annotated @Bean or @Configuration entirely — no bean definition, no instantiation, no error
• Evaluation happens at bean definition time, before any bean is created — not at runtime

The built-in conditions

• @ConditionalOnClass — checks if a class is on the classpath; how Spring Boot knows to configure JPA only when Hibernate is present
• @ConditionalOnMissingBean — registers a bean only if no bean of that type already exists; user configurations are processed before auto-configurations, so your custom bean wins
• @ConditionalOnProperty — evaluates a property value at startup

Custom conditions

• Implement Condition, use ConditionContext to check the Environment, ClassLoader, BeanDefinitionRegistry, or ResourceLoader
• Compose into a custom annotation: @ConditionalOnFeature("dark-mode") backed by a condition that reads feature.dark-mode.enabled

@Profile is just @Conditional with a built-in condition that checks active profiles. Anything you want to conditionally enable — a feature flag, an environment-specific service, a paid-tier bean graph — can be driven by a custom @Conditional.