The simplest technique for modularizing configurations is to split up a single @Configuration class into multiple smaller classes:

// monolithic configuration
@Configuration
public class AppConfig {
    @Bean
    public ServiceA serviceA() {
       // ...
    }

    @Bean
    public ServiceB serviceB() {
       // ...
    }

   // assume many bean definitions follow
}
            

The above configuration class might be supplied as a parameter to JavaConfigApplicationContext:

JavaConfigApplicationContext context = new JavaConfigApplicationContext(AppConfig.class);
ServiceA serviceA = context.getBean(ServiceA.class);
ServiceB serviceB = context.getBean(ServiceB.class);
            

We can easily partition this configuration such that bean definitions are spread across two classes, instead of one:

// partitioned configuration
@Configuration
public class AppConfigA {
    @Bean
    public ServiceA serviceA() {
       // ...
    }
}

@Configuration
public class AppConfigB {
    @Bean
    public ServiceB serviceB() {
       // ...
    }
}
            

Now simply supply both configuration classes to the constructor of JavaConfigApplicationContext:

JavaConfigApplicationContext context =
    new JavaConfigApplicationContext(AppConfigA.class, AppConfigB.class);
// both beans are still available in the resulting application context
ServiceA serviceA = context.getBean(ServiceA.class);
ServiceB serviceB = context.getBean(ServiceB.class);