3.13 Bean Events

You can hook into the creation of beans using one of the following interfaces:

• BeanInitializedEventListener - allows modifying or replacing of a bean after the properties have been set but prior to @PostConstruct event hooks.

• BeanCreatedEventListener - allows modifying or replacing of a bean after the bean is fully initialized and all @PostConstruct hooks called.

The BeanInitializedEventListener interface is commonly used in combination with Factory beans. Consider the following example:

public class V8Engine implements Engine {
    private final int cylinders = 8;
    private double rodLength; (1)
    public V8Engine(double rodLength) {
        this.rodLength = rodLength;
    }
    @Override
    public String start() {
        return "Starting V" + String.valueOf(getCylinders()) + " [rodLength=" + String.valueOf(getRodLength()) + "]";
    }
    @Override
    public final int getCylinders() {
        return cylinders;
    }
    public double getRodLength() {
        return rodLength;
    }
    public void setRodLength(double rodLength) {
        this.rodLength = rodLength;
    }
}
@Factory
public class EngineFactory {
    private V8Engine engine;
    private double rodLength = 5.7;
    @PostConstruct
    public void initialize() {
        engine = new V8Engine(rodLength); (2)
    }
    @Singleton
    public Engine v8Engine() {
        return engine;(3)
    }
    public void setRodLength(double rodLength) {
        this.rodLength = rodLength;
    }
}
@Singleton
public class EngineInitializer implements BeanInitializedEventListener<EngineFactory> { (4)
    @Override
    public EngineFactory onInitialized(BeanInitializingEvent<EngineFactory> event) {
        EngineFactory engineFactory = event.getBean();
        engineFactory.setRodLength(6.6);(5)
        return engineFactory;
    }
}

class V8Engine implements Engine {
    final int cylinders = 8
    double rodLength (1)
    @Override
    String start() {
        return "Starting V${cylinders} [rodLength=$rodLength]"
    }
}
@Factory
class EngineFactory {
    private V8Engine engine
    double rodLength = 5.7
    @PostConstruct
    void initialize() {
        engine = new V8Engine(rodLength: rodLength) (2)
    }
    @Singleton
    Engine v8Engine() {
        return engine (3)
    }
}
@Singleton
class EngineInitializer implements BeanInitializedEventListener<EngineFactory> { (4)
    @Override
    EngineFactory onInitialized(BeanInitializingEvent<EngineFactory> event) {
        EngineFactory engineFactory = event.bean
        engineFactory.rodLength = 6.6 (5)
        return event.bean
    }
}

class V8Engine(var rodLength: Double) : Engine {  (1)
    override val cylinders = 8
    override fun start(): String {
        return "Starting V$cylinders [rodLength=$rodLength]"
    }
}
@Factory
class EngineFactory {
    private var engine: V8Engine? = null
    private var rodLength = 5.7
    @PostConstruct
    fun initialize() {
        engine = V8Engine(rodLength) (2)
    }
    @Singleton
    fun v8Engine(): Engine? {
        return engine(3)
    }
    fun setRodLength(rodLength: Double) {
        this.rodLength = rodLength
    }
}
@Singleton
class EngineInitializer : BeanInitializedEventListener<EngineFactory> { (4)
    override fun onInitialized(event: BeanInitializingEvent<EngineFactory>): EngineFactory {
        val engineFactory = event.bean
        engineFactory.setRodLength(6.6)(5)
        return event.bean as EngineFactory
    }
}

The BeanCreatedEventListener interface is more typically used to decorate or enhance a fully initialized bean by creating a proxy for example.