1. Introduction
Spring allows us to attach custom actions to bean creation and destruction. We can, for example, do it by implementing the InitializingBean and DisposableBean interfaces.
In this quick tutorial, we’ll look at a second possibility, the @PostConstruct and @PreDestroy annotations.
Spring @ComponentScan - 过滤器类型
In an earlier tutorial, we learned about the basics of Spring component scans.
2. @PostConstruct
Spring calls the methods annotated with @PostConstruct only once, just after the initialization of bean properties. Keep in mind that these methods will run even if there’s nothing to initialize.
The method annotated with @PostConstruct can have any access level, but it can’t be static.
One possible use of @PostConstruct is populating a database. For instance, during development, we might want to create some default users:
@Component
public class DbInit {
@Autowired
private UserRepository userRepository;
@PostConstruct
private void postConstruct() {
User admin = new User("admin", "admin password");
User normalUser = new User("user", "user password");
userRepository.save(admin, normalUser);
}
}
The above example will first initialize UserRepository and then run the @PostConstruct method.
3. @PreDestroy
A method annotated with @PreDestroy runs only once, just before Spring removes our bean from the application context.
Same as with @PostConstruct, the methods annotated with @PreDestroy can have any access level, but can’t be static.
@Component
public class UserRepository {
private DbConnection dbConnection;
@PreDestroy
public void preDestroy() {
dbConnection.close();
}
}
The purpose of this method should be to release resources or perform other cleanup tasks, such as closing a database connection, before the bean gets destroyed.
4. javax.annotation or jakarta.annotation
From JDK 6 to JDK 8, the @PostConstruct and @PreDestroy annotations were part of the standard Java libraries under the javax.annotation package. However, with JDK 9, the entire javax.annotation package was removed from the core Java modules and fully eliminated in JDK 11. In Jakarta EE 9, this package has been relocated to jakarta.annotation.
So, sometimes, we may ask, “Should I use the annotations from javax.annotation or jakarta annotation?” This depends on the Spring version we use. To quickly understand the compatibilities among Spring, JDK, javax, and jakarta namespaces, let’s summarize them in a table:
Spring Version
JDK Version
Java / Jakatar Namespace
6.1.x
JDK 17 to JDK 23
jakarta
6.0.x
JDK 17 to JDK 21
jakarta
5.3.x
JDK 8 to JDK 21
javax
As the table above shows, with Spring 6.0.x and 6.1.x, we should use the jakarta namespace. However, if our project uses Spring 5.3.x and JDK 9 or a later version, since Spring 5.3.x only supports the javax namespace, we must explicitly add the javax.annotation-api dependency to our project:
<dependency>
<groupId>javax.annotation</groupId>
<artifactId>javax.annotation-api</artifactId>
<version>1.3.2</version>
</dependency>
The above XML shows an example of adding the javax.annotation-api dependency to a Maven project’s pom.xml.
5. Conclusion
In this brief article, we learned how to use the @PostConstruct and @PreDestroy annotations.
As always, all the source code is available over on GitHub.