How Does Spring Boot Auto-Configuration Work?
In this article, we take a look at how Spring Boot auto-configuration makes it easier than ever to have your beans configured for you.
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What Is It?
As its name implies, "autoconfiguration" helps you to configure your Spring Boot application automatically. More precisely, it automatically create beans for you! Strangely, it is a cool feature, but not applicable outside of Spring Boot.
The Purpose and Method
The purpose is simple: to save you time when you create beans, which is usually done by yourself. It sounds like magic, but behind the scene, it's nothing fancy actually.
It is simply a bunch of normal Spring configuration classes (like others with the same annotation
The difference is that these configuration classes are using some different annotations:
@ConditionalOnClass: acts only if a given class is on a classpath
@ConditionalOnMissingBean: acts only if a given bean is not created
@Conditional: acts only if given condition is met
In one sentence, generally, if a class is found in a classpath (
@ConditionalOnClass) , and a bean is requested (
@Autowired), but no others is creating it (
@ConditionalOnMissingBean), it'll create that bean.
Let's have a look at one example:
What Does the Process Look?
You are asking for a bean, by using an annotation like
@Autowire, Spring BeanFactory tries to find the bean for you.
If you have declared a bean by yourself (i.e.
@Service, etc.), Spring will pick that up and instantiate it for you. Otherwise, Spring will look into those auto-configuration classes (still
@Configuration but with a
@Conditional annotation), to find the suitable bean and instantiate it for you.
If still nothing is found, an exception is thrown by Spring, like
How Do I Use This Feature, and Is There Anything I Can Do?
It is used via the annotation
@EnableAutoConfiguration, which is included in
Spring does, however, leave options for you to have some control:
- You can tell Spring to disable certain auto-configuration classes. (i.e.
- You can tell Spring to include your own auto-configuration class.
- You can declare your own bean, which will be respected by Spring and auto-configuration class will be ignored. This is done via
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