Spring in Depth 2

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Component Scan

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By default, when we annotate a class with @SpringBootApplication, it performs a component scan because it’s a combination of @ComponentScan, @EnableAutoConfiguration and @Configuration.

@ComponentScan by default scans the package where the class is and the sub-packages of the package where the class is present. So what if you want to access beans not present in the same package.

By default, @ComponentScan means @ComponentScan("<package its present>"). So, if a specific package needs to be scanned, then we need to specify the package.

@ComponentScan(basePackages = "\<package to scan>")

If the bean is not present in the same package or @ComponentScan is missed, then it results in bean not found error.

Like how we are including packages, we can exclude them as well,

@ComponentScan(excludeFilters = 
  @ComponentScan.Filter(type=FilterType.REGEX,
    pattern="me\\.vignesh\\.springapp\\.exclude\\..*"))

One thing to note would be to not put the @Configuration in the default package, which would lead Spring to scan all the packages present in classpath and fail to start up.

Life Cycle of a Bean

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The major life cycle stages of a bean in Spring IOC Container are as follows,

1. Bean Creation and Instantiate

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Bean created and loaded in application context.

2. Populating Bean properties

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Spring container will create a bean id, scope, default values based on the bean definition.

3. Post-initialization

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Spring provides Aware interfaces to access application bean details and callback methods to hook into the bean life cycle to execute custom logic.

4. Ready to Serve

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Bean is created and injected all the dependencies, all the Aware and callback methods implementation should be done. Bean is ready to serve.

5. Pre-destroy

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Spring provides callback methods to execute custom logic and clean-ups before destroying a bean from ApplicationContext.

6. Bean Destroyed

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Bean will be removed or destroyed from memory.

We can interface with lifecycle methods in 2 ways,

Spring Aware Interface

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Just a list of the Spring Aware interfaces available, generally not used much in practice.

@PostConstruct and @PreDestroy annotations

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To do something just after the bean is created, we can use the @PostConstruct.

@PostConstruct
public void preDestroy() {
  //Initialize something if required
}

To do something, just before a bean is deleted.

@PreDestroy
public void preDestroy() {
  //Clean up code, like closing a connection maybe
}