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how to use @PostConstruct Spring

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Spring Beans lifecycle – PostConstruct and PreDestroy

Spring Beans are Java Objects or Instances which will be created and managed by Spring IOC/DI container.

Below is the lifecycle of Spring Beans

  • 1.  Bean Definition
    Spring Bean will be defined using stereotype annotations or XML Bean configurations.
  • 2.  Bean Creation and Instantiate
    As soon as bean created and It will be instantiated and loaded into ApplicationContext and JVM memory.
  • 3. Populating Bean properties
    Spring container will create a bean id, scope, default values based on the bean definition.
  • 4. Post-initialization
    Spring provides Aware interfaces to access application bean meta-data details and callback methods to hook into the bean life cycle to execute custom application-specific logic.
  • 5. Ready to Serve
    Now, Bean is created and injected all the dependencies and should be executed all the Aware and callback methods implementation. Bean is ready to serve.
  • 6. Pre-destroy
    Spring provides callback methods to execute custom application-specific logic and clean-ups before destroying a bean from ApplicationContext.
  • 7. Bean Destroyed
    Bean will be removed or destroyed from and JVM memory.

Using Spring @PostConstruct and @PreDestroy annotations

The @PostConstruct annotation is used on a method that needs to be executed after dependency injection is done to perform any initialization. You can use annotation @PostConstruct for setup configuration before initializing bean.

The @PreDestroy annotation is used on methods as a callback notification to signal that the instance is in the process of being removed by the container. You can use @PreDestroy annotation for release memory after bean has finished processing.

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package com.learncode24h.lifecycle.spring;
 
import java.util.HashMap;
import java.util.Map;
 
import javax.annotation.PostConstruct;
import javax.annotation.PreDestroy;
 
import org.springframework.stereotype.Component;
 
@Component
public class MailService {
 
   private Map<String, String> map=null;
    
   public MailService() {
      map=new HashMap<>();
   }
 
   public void send(String mailTo){
      System.out.println("Send email to - " + mailTo + "success");
   }
    
   @PostConstruct
   public void init() {
      map.put("host", "mail.example.com");
      map.put("port", "25");
      map.put("from", "admin@abc.com");
      System.out.println("Init config email " + map);
   }
 
   @PreDestroy
   public void destroy() {
      map.clear();
      System.out.println("Release memory");
   }
}
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package com.learncode24h.lifecycle.spring;
 
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
 
@Configuration
@ComponentScan(basePackages = "com.learncode24h.lifecycle.spring")
public class AppConfig {
    
}

@ComponentScan annotation scans all beans, whose class is annotated by the @Componentannotation in a package, specified bybasePackages attribute.

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package com.learncode24h.lifecycle.spring;
 
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
 
public class MainApp {
   public static void main(String[] args) {
      AnnotationConfigApplicationContext context =
            new AnnotationConfigApplicationContext(AppConfig.class);
       
      // Send mail 1
      MailService mailService1 = context.getBean(MailService.class);
      mailService1.send("contact@learncode24h.com);
 
      // Send mail 2
      MailService mailService2 = context.getBean(MailService.class);
      mailService2.send("admin@learncode24h.com");
 
      context.close();
   }
}

Result printed

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Init config mail- {port=25, host=mail.example.com, from=admin@abc.com}
Send email to - contact@learncode24h.com
Send email to - admin@learncode24h.com
Release memory
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