Order of Configuration in SpringBoot

Question

I am trying to understand how beans that we make using @Configuration tends to override the beans that are generated by SpringBoot by default. I have been working on a project where in many cases we create beans for things like ZuulConfigs and the assumption is, whatever we are making shall take precedence over the default generated bean. I have been trying to figure this out but can't. Basically,

1. Is Spring achieving this via some custom class loader
2. If not how is this precedence working. Can I give some precedence in similar manner to my beans
3. Can I generate similar hierarchy in my project,if so how

The help is highly appreciated

Answer 1

Spring AutoConfiguration is used to provide a basic configuration if certain classes are in the classpath or not.

If you want to configure the order in which beans are instantiated by spring you can use

@DependsOn("A") 
 public class B {
 ...    
}

This would create bean "A", then "B". Hence you can order the configuration depending upon the beans need first to be done. Anyways Spring automatically detects the dependencies by analyzing the bean classes. for more help check this question Spring Boot AutoConfiguration Order

Alternative : There is also "@AutoConfigureOrder" annotation(where you can prioritise the configuration), you can have a look in the code for deeper understanding.

Documentation of AutoConfiguration is here

Answer 2

First of all, class loading and bean creation are two different things. We don't need to create a bean to load a class, however, a class has to be loaded in order to create a bean.

Now, coming back to Spring's example, Spring looks into all the packages configured by @componentScan and creates beans of all the classes annotated with @Bean, @Configuration and/or @Component. Spring's container keeps track of all the beans created and hence, when it encounters user defined bean with same name and class type as default bean, it replaces the original definition with user defined one (e.g. we can create our custom @ObjectMapper to override Spring boot's own instance). You can also use @Primary annotation to make you bean take precedence if another definition with same class exists (documentation here).

Below are the answers for your questions:

1. Spring uses reflection to load the classes and create instances. Although you can load the classes with your custom class loader (more on that here), you don't need to worry about it for @Configuration.
2. Yes, you can use @Primary annotation to give your bean a precedence. You can also use @Order(here) to define the creation order for your beans.
3. With @Primary, @Order and @Qualifier annotation you can define your own hierarchy for bean creation.