Spring Boot and Application Context Hierarchy

Spring Boot supports a simple way of specifying a Spring application context hierarchy. 

This post is simply demonstrating this feature — I have yet to find a good use of it in the projects I have worked on. Spring Cloud uses this feature for creating a bootstrap context where properties are loaded up, if required, from an external configuration server, which is made available to the main application context later on.

To quickly take a step back — a Spring Application Context manages the lifecycle of all the beans registered with it. Application Context hierarchies provide a way to reuse beans — beans defined in the parent context are accessible in the child contexts.

Consider a contrived use-case of using multiple application contexts and the application context hierarchy — this is to provide two different ports with a different set of endpoints at each of these ports. 

Child1 and Child2 are typical Spring Boot Applications, along these lines:

package child1;   
import org.springframework.beans.factory.annotation.Value; 
import org.springframework.boot.autoconfigure.SpringBootApplication; 
import org.springframework.context.annotation.Bean; 
import org.springframework.context.annotation.PropertySource; 
import root.RootBean;   

@SpringBootApplication 
@PropertySource("classpath:/child1.properties") 
public class ChildContext1 {       
    @Bean     
    public ChildBean1 childBean(RootBean rootBean, 
    @Value("${root.property}") String someProperty) 
    {                        
        return new ChildBean1(rootBean, someProperty);     
    } 
}

Each of the applications resides in its own root package to avoid collisions when scanning for beans. Note that the bean in the child contexts depends on a bean that is expected to come from the root context. 

The port to listen on is provided as properties, since the two contexts are expected to listen on different ports I have explicitly specified the property file to load with a content along these lines:

server.port=8080
spring.application.name=child1

Given this setup, Spring Boot provides a fluid interface to load up the root context and the two child contexts:

SpringApplicationBuilder appBuilder =
       new
SpringApplicationBuilder()
               .parent(RootContext.class)
               .child(ChildContext1.class)
               .sibling(ChildContext2.class);


ConfigurableApplicationContext applicationContext  = appBuilder.run();

The application context returned by the SpringBootApplicationBuilder appears to be the final one in the chain, defined via ChildContext2 above.

If the application is now started up, there would be a root context with two different child contexts, each exposing an endpoint via a different port. A visualization via the /beans actuator endpoint shows this:

Not everything is clean, though. There are errors displayed in the console related to exporting JMX endpoints, however, these are informational and don't appear to affect the start-up.

Samples are available in my GitHub repo.