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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

M. Deinum
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chinmay
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5 Answers5

24

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

Moshe Katz
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virsha
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8

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.
Darshan Mehta
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  • I suspected that Spring uses Java reflection to load, but could not find the code that does this - would you point to the file or documentation that shows Spring uses reflection? Thanks. – flow2k Oct 22 '17 at 07:09
  • @flow2k have a look at [this](https://docs.spring.io/spring/docs/3.2.x/spring-framework-reference/html/aop.html#aop-introduction-proxies) documentation. – Darshan Mehta Oct 22 '17 at 07:50
7

Just know this: Spring Boot (specifically) auto configuration classes are always configured last. After all user beans have been created. Spring Boot auto configuration classes almost always use the @ConditionalXXXX annotations to make sure that any beans of the same type/name and other conditions that are configured in your application will take precedence over the Spring Boot auto-configured beans.

Strelok
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7

Can I give some precedence in similar manner to my beans

Yes.

A) To define a specific order your Configuration classes will be handled (by the way, a Configuration class does not have to be annotated with @Configuration (so-called full definition), but it's enough to be annotated with @Component, @ComponentScan, @Import, @ImportResource or just have a method annotated with @Bean - so-called lite definition), you should

1) add your Configuration Candidates to your SpringApplication's primarySource, for example, in your main method like that

SpringApplication.run(
   new Class[]{YourSpringBootApplication.class, Config1.class, Config2.class, ...},
   args);

2) and annotate each of your Configuration Candidates with @Order annotation, any other ordering means like Ordered interface, @DependsOn etc will be ignored by ConfigurationClassPostProcessor, the order in the primarySource array will also be ignored.

Then ConfigurationClassPostProcessor will sort your Configuration Candidates and handle them according the @Order annotation value you specified.

B) The precedence can also be achieved by defining your own AutoConfiguration classes. Although both Configuration and AutoConfiguration are handled by the same ConfigurationClassPostProcessor, they are essentially distinctive machineries. To do so

1) define in your classpath /META-INF/spring.factories file and put in the EnableAutoConfiguration section of it your AutoConfiguration classes like that

   org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
   your.package.AutoConfig1,your.package.AutoConfig2

2) and annotate your AutoConfiguration classes with @AutoConfigureOrder, @AutoConfigureAfter, or @AutoConfigureAfter annotations, any other ordering means again will be ignored.

Like @Strelok pointed out, AutoConfiguration classes, your own and provided e.g. by spring-boot-autoconfigure library alike, will be added to the end of the list of Configuration Candidates.

Remember, however, that the order the Configuration Candidates will be handled by ConfigurationClassPostProcessor does not necessarily coincide with the order the beans defined by the Configuration classes will be created. For example, you might define your Configuration class that overrides TomcatServletWebServerFactory to make your own customization of Tomcat web server like 

@Configuration
public class EmbeddedTomcatConfig {

@Bean
public TomcatServletWebServerFactory containerFactory() {
    ...
    return customizedTomcatWebServerFactory;
}

but this method will be called right at the moment when your Spring Boot application decides to create a Web server, regardless of how you defined the precedence for your EmbeddedTomcatConfig Configuration class.

Is Spring achieving this via some custom class loader

There is no need to. Although you could, as always with Spring, define your own ClassLoader for BeanFactory, standard ClassLoader is good enough if everything you need for Configuration in your application is available in the classpath. Please notice, that at first phase ConfigurationClassPostProcessor does not load (i.e. does not resolve) the Configuration candidates classes (otherwise, most of the classes in spring-boot-autoconfigure library will fail to load). Instead it analyzes their annotations with bytecode analyzer, ASM by default. For that purpose, it is just enough to get a binary form, a byte array, of a class to feed it to bytecode analyzer.

igor.zh
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1

If you want your @Component to take precedence over other @Component while scanning all the components by spring, use @Order(Ordered.LOWEST_PRECEDENCE) i.e. the max value to load your component over other.

@Primary is used to give your bean a default preference, we can override the default preference using @Qualifier

Shruti Gupta
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