The order in which Spring container loads beans cannot be predicted. There's no specific ordering logic specification given by Spring framework. But Spring guarantees if a bean A has dependency of B (e.g. bean A has an instance variable @Autowired B b;) then B will be initialized first. But what if bean A doesn't have direct dependency of B and we still want B to initialize first?
When we want to control beans initializing order
There might be scenarios where A is depending on B indirectly. For example assume A is some kind of events publisher and B is listening to those events. This is a typical scenario of observer pattern. We don't want B to miss any events and would like to have B being initialize before A.
@DependsOn annotation
Spring provides @DependsOn indicating which bean should be initialized before this bean:
Example
Following example shows a very basic observer pattern.
EventManager, a facility to register listeners and publishing events
package com.logicbig.example.bean;
import javax.annotation.PostConstruct;
import java.util.ArrayList;
import java.util.List;
import java.util.function.Consumer;
public class EventManager {
private final List<Consumer<String>> listeners = new ArrayList<>();
@PostConstruct
public void initialize() {
System.out.println("initializing: "+this.getClass().getSimpleName());
}
public void publish(final String message) {
listeners.forEach(l -> l.accept(message));
}
public void addListener(Consumer<String> eventConsumer) {
listeners.add(eventConsumer);
}
}
EventPublisher
package com.logicbig.example.bean;
import org.springframework.beans.factory.annotation.Autowired;
import javax.annotation.PostConstruct;
public class EventPublisher {
@Autowired
private EventManager eventManager;
@PostConstruct
public void initialize() {
System.out.println("initializing: "+this.getClass().getSimpleName());
eventManager.publish("event published from EventPublisherBean");
}
}
EventListener
package com.logicbig.example.bean;
import org.springframework.beans.factory.annotation.Autowired;
import javax.annotation.PostConstruct;
public class EventListener {
@Autowired
private EventManager eventManager;
@PostConstruct
private void initialize() {
System.out.println("initializing: "+this.getClass().getSimpleName());
eventManager.addListener(s ->
System.out.println("event received in EventListenerBean : " + s));
}
}
Defining beans and running main class
package com.logicbig.example;
import com.logicbig.example.bean.EventListener;
import com.logicbig.example.bean.EventManager;
import com.logicbig.example.bean.EventPublisher;
import org.springframework.context.annotation.*;
@Configuration
@ComponentScan("com.logicbig.example")
public class AppConfig {
@Bean
@DependsOn("eventListenerBean")
public EventPublisher eventPublisherBean() {
return new EventPublisher();
}
@Bean
public EventListener eventListenerBean() {
return new EventListener();
}
@Bean
public EventManager eventManagerBean() {
return new EventManager();
}
public static void main(String... strings) {
AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(AppConfig.class);
context.close();
}
}
Outputinitializing: EventManager
initializing: EventListener
initializing: EventPublisher
event received in EventListenerBean : event published from EventPublisherBean
If we don't use @DependsOn, there's no guarantee that EventListener will initialize first:
@Configuration
@ComponentScan("com.logicbig.example")
public class AppConfig {
@Bean
// @DependsOn("eventListenerBean")
public EventPublisher eventPublisherBean() {
return new EventPublisher();
}
@Bean
public EventListener eventListenerBean() {
return new EventListener();
}
@Bean
public EventManager eventManagerBean() {
return new EventManager();
}
public static void main(String... strings) {
AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(AppConfig.class);
context.close();
}
}
Output
initializing: EventManager
initializing: EventPublisher
initializing: EventListener
Example ProjectDependencies and Technologies Used: - spring-context 5.3.8: Spring Context. Version Compatibility: 3.2.9.RELEASE - 5.3.8
- JDK 8
- Maven 3.8.1
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