@Conditional Annotation in Spring

Direct Answer

@Conditional registers a configuration class or @Bean only when its Condition matches while the context is being built. Spring Boot provides focused conditions such as @ConditionalOnClass, @ConditionalOnMissingBean, and @ConditionalOnProperty. These are startup/configuration-time decisions for the ApplicationContext, not live per-request checks.

Code Example

// Custom condition checking if running in production
class ProductionCondition implements Condition {
    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        var env = context.getEnvironment();
        return "production".equals(env.getProperty("spring.profiles.active"));
    }
}

@Configuration
class CacheConfiguration {
    
    @Bean
    @Conditional(ProductionCondition.class)  // Only in production
    public CacheManager redisCacheManager() {
        return new RedisCacheManager();
    }
    
    @Bean
    @ConditionalOnMissingBean(CacheManager.class)  // Fallback if no other CacheManager
    public CacheManager simpleCacheManager() {
        return new SimpleCacheManager();
    }
    
    @Bean
    @ConditionalOnClass(name = "com.fasterxml.jackson.databind.ObjectMapper")  // If Jackson on classpath
    @ConditionalOnProperty(name = "app.json.enabled", havingValue = "true")  // AND property is true
    public ObjectMapper objectMapper() {
        return new ObjectMapper();
    }
}

Common Mistakes & Pitfalls

• Condition Evaluation Timing: Freshers think @Conditional checks happen every time you @Autowire the bean. Actually, the condition runs once during application startup when Spring builds the context. If matches returns false, the bean is never created at all.
• Public Condition Classes: Writing the Condition implementation as a private inner class fails because Spring can't instantiate it. Always make your Condition class public and ensure it has a no-arg constructor.
• Overusing Custom Conditions: Creating complex custom Condition classes when Spring Boot already provides @ConditionalOnProperty or @ConditionalOnBean. Check the 20+ built-in @Conditional variants before writing your own — @ConditionalOnWebApplication, @ConditionalOnExpression, @ConditionalOnResource all solve common cases.