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.
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. Java 9+
Note that both the @PostConstruct and @PreDestroy annotations are part of Java EE. Since Java EE was deprecated in Java 9, and removed in Java 11, we have to add an additional dependency to use these annotations:
<dependency>
<groupId>javax.annotation</groupId>
<artifactId>javax.annotation-api</artifactId>
<version>1.3.2</version>
</dependency>
5. Conclusion
In this brief article, we learned how to use the @PostConstruct and @PreDestroy annotations.
As always, all the source code is available on GitHub.