Customizing the Nature of a Bean

The Spring Framework provides a number of interfaces you can use to customize the nature of a bean. This section groups them as follows:

Lifecycle Callbacks

To interact with the container’s management of the bean lifecycle, you can implement the Spring InitializingBean and DisposableBean interfaces. The container calls afterPropertiesSet() for the former and destroy() for the latter to let the bean perform certain actions upon initialization and destruction of your beans.

The JSR-250 @PostConstruct and @PreDestroy annotations are generally considered best practice for receiving lifecycle callbacks in a modern Spring application. Using these annotations means that your beans are not coupled to Spring-specific interfaces. For details, see Using @PostConstruct and @PreDestroy.

If you do not want to use the JSR-250 annotations but you still want to remove coupling, consider init-method and destroy-method bean definition metadata.

Internally, the Spring Framework uses BeanPostProcessor implementations to process any callback interfaces it can find and call the appropriate methods. If you need custom features or other lifecycle behavior Spring does not by default offer, you can implement a BeanPostProcessor yourself. For more information, see Container Extension Points.

In addition to the initialization and destruction callbacks, Spring-managed objects may also implement the Lifecycle interface so that those objects can participate in the startup and shutdown process, as driven by the container’s own lifecycle.

The lifecycle callback interfaces are described in this section.

Initialization Callbacks

The org.springframework.beans.factory.InitializingBean interface lets a bean perform initialization work after the container has set all necessary properties on the bean. The InitializingBean interface specifies a single method:

void afterPropertiesSet() throws Exception;

We recommend that you do not use the InitializingBean interface, because it unnecessarily couples the code to Spring. Alternatively, we suggest using the @PostConstruct annotation or specifying a POJO initialization method. In the case of XML-based configuration metadata, you can use the init-method attribute to specify the name of the method that has a void no-argument signature. With Java configuration, you can use the initMethod attribute of @Bean. See Receiving Lifecycle Callbacks. Consider the following example:

<bean id="exampleInitBean" class="examples.ExampleBean" init-method="init"/>

  • Java

  • Kotlin

public class ExampleBean {

	public void init() {
		// do some initialization work
	}
}
class ExampleBean {

	fun init() {
		// do some initialization work
	}
}

The preceding example has almost exactly the same effect as the following example (which consists of two listings):

<bean id="exampleInitBean" class="examples.AnotherExampleBean"/>

  • Java

  • Kotlin

public class AnotherExampleBean implements InitializingBean {

	@Override
	public void afterPropertiesSet() {
		// do some initialization work
	}
}
class AnotherExampleBean : InitializingBean {

	override fun afterPropertiesSet() {
		// do some initialization work
	}
}

However, the first of the two preceding examples does not couple the code to Spring.

Be aware that @PostConstruct and initialization methods in general are executed within the container’s singleton creation lock. The bean instance is only considered as fully initialized and ready to be published to others after returning from the @PostConstruct method. Such individual initialization methods are only meant for validating the configuration state and possibly preparing some data structures based on the given configuration but no further activity with external bean access. Otherwise there is a risk for an initialization deadlock.

For a scenario where expensive post-initialization activity is to be triggered, for example, asynchronous database preparation steps, your bean should either implement SmartInitializingSingleton.afterSingletonsInstantiated() or rely on the context refresh event: implementing ApplicationListener<ContextRefreshedEvent> or declaring its annotation equivalent @EventListener(ContextRefreshedEvent.class). Those variants come after all regular singleton initialization and therefore outside of any singleton creation lock.

Alternatively, you may implement the (Smart)Lifecycle interface and integrate with the container’s overall lifecycle management, including an auto-startup mechanism, a pre-destroy stop step, and potential stop/restart callbacks (see below).

Destruction Callbacks

Implementing the org.springframework.beans.factory.DisposableBean interface lets a bean get a callback when the container that contains it is destroyed. The DisposableBean interface specifies a single method:

void destroy() throws Exception;

We recommend that you do not use the DisposableBean callback interface, because it unnecessarily couples the code to Spring. Alternatively, we suggest using the @PreDestroy annotation or specifying a generic method that is supported by bean definitions. With XML-based configuration metadata, you can use the destroy-method attribute on the <bean/>. With Java configuration, you can use the destroyMethod attribute of @Bean. See Receiving Lifecycle Callbacks. Consider the following definition:

<bean id="exampleDestructionBean" class="examples.ExampleBean" destroy-method="cleanup"/>

  • Java

  • Kotlin

public class ExampleBean {

	public void cleanup() {
		// do some destruction work (like releasing pooled connections)
	}
}
class ExampleBean {

	fun cleanup() {
		// do some destruction work (like releasing pooled connections)
	}
}

The preceding definition has almost exactly the same effect as the following definition:

<bean id="exampleDestructionBean" class="examples.AnotherExampleBean"/>

  • Java

  • Kotlin

public class AnotherExampleBean implements DisposableBean {

	@Override
	public void destroy() {
		// do some destruction work (like releasing pooled connections)
	}
}
class AnotherExampleBean : DisposableBean {

	override fun destroy() {
		// do some destruction work (like releasing pooled connections)
	}
}

However, the first of the two preceding definitions does not couple the code to Spring.

Note that Spring also supports inference of destroy methods, detecting a public close or shutdown method. This is the default behavior for @Bean methods in Java configuration classes and automatically matches java.lang.AutoCloseable or java.io.Closeable implementations, not coupling the destruction logic to Spring either.

For destroy method inference with XML, you may assign the destroy-method attribute of a <bean> element a special (inferred) value, which instructs Spring to automatically detect a public close or shutdown method on the bean class for a specific bean definition. You can also set this special (inferred) value on the default-destroy-method attribute of a <beans> element to apply this behavior to an entire set of bean definitions (see Default Initialization and Destroy Methods).

For extended shutdown phases, you may implement the Lifecycle interface and receive an early stop signal before the destroy methods of any singleton beans are called. You may also implement SmartLifecycle for a time-bound stop step where the container will wait for all such stop processing to complete before moving on to destroy methods.

Default Initialization and Destroy Methods

When you write initialization and destroy method callbacks that do not use the Spring-specific InitializingBean and DisposableBean callback interfaces, you typically write methods with names such as init(), initialize(), dispose(), and so on. Ideally, the names of such lifecycle callback methods are standardized across a project so that all developers use the same method names and ensure consistency.

You can configure the Spring container to “look” for named initialization and destroy callback method names on every bean. This means that you, as an application developer, can write your application classes and use an initialization callback called init(), without having to configure an init-method="init" attribute with each bean definition. The Spring IoC container calls that method when the bean is created (and in accordance with the standard lifecycle callback contract described previously). This feature also enforces a consistent naming convention for initialization and destroy method callbacks.

Suppose that your initialization callback methods are named init() and your destroy callback methods are named destroy(). Your class then resembles the class in the following example:

  • Java

  • Kotlin

public class DefaultBlogService implements BlogService {

	private BlogDao blogDao;

	public void setBlogDao(BlogDao blogDao) {
		this.blogDao = blogDao;
	}

	// this is (unsurprisingly) the initialization callback method
	public void init() {
		if (this.blogDao == null) {
			throw new IllegalStateException("The [blogDao] property must be set.");
		}
	}
}
class DefaultBlogService : BlogService {

	private var blogDao: BlogDao? = null

	// this is (unsurprisingly) the initialization callback method
	fun init() {
		if (blogDao == null) {
			throw IllegalStateException("The [blogDao] property must be set.")
		}
	}
}

You could then use that class in a bean resembling the following:

<beans default-init-method="init">

	<bean id="blogService" class="com.something.DefaultBlogService">
		<property name="blogDao" ref="blogDao" />
	</bean>

</beans>

The presence of the default-init-method attribute on the top-level <beans/> element attribute causes the Spring IoC container to recognize a method called init on the bean class as the initialization method callback. When a bean is created and assembled, if the bean class has such a method, it is invoked at the appropriate time.

You can configure destroy method callbacks similarly (in XML, that is) by using the default-destroy-method attribute on the top-level <beans/> element.

Where existing bean classes already have callback methods that are named at variance with the convention, you can override the default by specifying (in XML, that is) the method name by using the init-method and destroy-method attributes of the <bean/> itself.

The Spring container guarantees that a configured initialization callback is called immediately after a bean is supplied with all dependencies. Thus, the initialization callback is called on the raw bean reference, which means that AOP interceptors and so forth are not yet applied to the bean. A target bean is fully created first and then an AOP proxy (for example) with its interceptor chain is applied. If the target bean and the proxy are defined separately, your code can even interact with the raw target bean, bypassing the proxy. Hence, it would be inconsistent to apply the interceptors to the init method, because doing so would couple the lifecycle of the target bean to its proxy or interceptors and leave strange semantics when your code interacts directly with the raw target bean.

Combining Lifecycle Mechanisms

As of Spring 2.5, you have three options for controlling bean lifecycle behavior:

If multiple lifecycle mechanisms are configured for a bean and each mechanism is configured with a different method name, then each configured method is run in the order listed after this note. However, if the same method name is configured — for example, init() for an initialization method — for more than one of these lifecycle mechanisms, that method is run once, as explained in the preceding section.

Multiple lifecycle mechanisms configured for the same bean, with different initialization methods, are called as follows:

  1. Methods annotated with @PostConstruct

  2. afterPropertiesSet() as defined by the InitializingBean callback interface

  3. A custom configured init() method

Destroy methods are called in the same order:

  1. Methods annotated with @PreDestroy

  2. destroy() as defined by the DisposableBean callback interface

  3. A custom configured destroy() method

Startup and Shutdown Callbacks

The Lifecycle interface defines the essential methods for any object that has its own lifecycle requirements (such as starting and stopping some background process):

public interface Lifecycle {

	void start();

	void stop();

	boolean isRunning();
}

Any Spring-managed object may implement the Lifecycle interface. Then, when the ApplicationContext itself receives start and stop signals (for example, for a stop/restart scenario at runtime), it cascades those calls to all Lifecycle implementations defined within that context. It does this by delegating to a LifecycleProcessor, shown in the following listing:

public interface LifecycleProcessor extends Lifecycle {

	void onRefresh();

	void onClose();
}

Notice that the LifecycleProcessor is itself an extension of the Lifecycle interface. It also adds two other methods for reacting to the context being refreshed and closed.

Note that the regular org.springframework.context.Lifecycle interface is a plain contract for explicit start and stop notifications and does not imply auto-startup at context refresh time. For fine-grained control over auto-startup and for graceful stopping of a specific bean (including startup and stop phases), consider implementing the extended org.springframework.context.SmartLifecycle interface instead.

Also, please note that stop notifications are not guaranteed to come before destruction. On regular shutdown, all Lifecycle beans first receive a stop notification before the general destruction callbacks are being propagated. However, on hot refresh during a context’s lifetime or on stopped refresh attempts, only destroy methods are called.

The order of startup and shutdown invocations can be important. If a “depends-on” relationship exists between any two objects, the dependent side starts after its dependency, and it stops before its dependency. However, at times, the direct dependencies are unknown. You may only know that objects of a certain type should start prior to objects of another type. In those cases, the SmartLifecycle interface defines another option, namely the getPhase() method as defined on its super-interface, Phased. The following listing shows the definition of the Phased interface:

public interface Phased {

	int getPhase();
}

The following listing shows the definition of the SmartLifecycle interface:

public interface SmartLifecycle extends Lifecycle, Phased {

	boolean isAutoStartup();

	void stop(Runnable callback);
}

When starting, the objects with the lowest phase start first. When stopping, the reverse order is followed. Therefore, an object that implements SmartLifecycle and whose getPhase() method returns Integer.MIN_VALUE would be among the first to start and the last to stop. At the other end of the spectrum, a phase value of Integer.MAX_VALUE would indicate that the object should be started last and stopped first (likely because it depends on other processes to be running). When considering the phase value, it is also important to know that the default phase for any “normal” Lifecycle object that does not implement SmartLifecycle is 0. Therefore, any negative phase value indicates that an object should start before those standard components (and stop after them). The reverse is true for any positive phase value.

The stop method defined by SmartLifecycle accepts a callback. Any implementation must invoke that callback’s run() method after that implementation’s shutdown process is complete. That enables asynchronous shutdown where necessary, since the default implementation of the LifecycleProcessor interface, DefaultLifecycleProcessor, waits up to its timeout value for the group of objects within each phase to invoke that callback. The default per-phase timeout is 30 seconds. You can override the default lifecycle processor instance by defining a bean named lifecycleProcessor within the context. If you want only to modify the timeout, defining the following would suffice:

<bean id="lifecycleProcessor" class="org.springframework.context.support.DefaultLifecycleProcessor">
	<!-- timeout value in milliseconds -->
	<property name="timeoutPerShutdownPhase" value="10000"/>
</bean>

As mentioned earlier, the LifecycleProcessor interface defines callback methods for the refreshing and closing of the context as well. The latter drives the shutdown process as if stop() had been called explicitly, but it happens when the context is closing. The 'refresh' callback, on the other hand, enables another feature of SmartLifecycle beans. When the context is refreshed (after all objects have been instantiated and initialized), that callback is invoked. At that point, the default lifecycle processor checks the boolean value returned by each SmartLifecycle object’s isAutoStartup() method. If true, that object is started at that point rather than waiting for an explicit invocation of the context’s or its own start() method (unlike the context refresh, the context start does not happen automatically for a standard context implementation). The phase value and any “depends-on” relationships determine the startup order as described earlier.

Shutting Down the Spring IoC Container Gracefully in Non-Web Applications

This section applies only to non-web applications. Spring’s web-based ApplicationContext implementations already have code in place to gracefully shut down the Spring IoC container when the relevant web application is shut down.

If you use Spring’s IoC container in a non-web application environment (for example, in a rich client desktop environment), register a shutdown hook with the JVM. Doing so ensures a graceful shutdown and calls the relevant destroy methods on your singleton beans so that all resources are released. You must still configure and implement these destroy callbacks correctly.

To register a shutdown hook, call the registerShutdownHook() method that is declared on the ConfigurableApplicationContext interface, as the following example shows:

  • Java

  • Kotlin

import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public final class Boot {

	public static void main(final String[] args) throws Exception {
		ConfigurableApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml");

		// add a shutdown hook for the above context...
		ctx.registerShutdownHook();

		// app runs here...

		// main method exits, hook is called prior to the app shutting down...
	}
}
import org.springframework.context.support.ClassPathXmlApplicationContext

fun main() {
	val ctx = ClassPathXmlApplicationContext("beans.xml")

	// add a shutdown hook for the above context...
	ctx.registerShutdownHook()

	// app runs here...

	// main method exits, hook is called prior to the app shutting down...
}

Thread Safety and Visibility

The Spring core container publishes created singleton instances in a thread-safe manner, guarding access through a singleton lock and guaranteeing visibility in other threads.

As a consequence, application-provided bean classes do not have to be concerned about the visibility of their initialization state. Regular configuration fields do not have to be marked as volatile as long as they are only mutated during the initialization phase, providing visibility guarantees similar to final even for setter-based configuration state that is mutable during that initial phase. If such fields get changed after the bean creation phase and its subsequent initial publication, they need to be declared as volatile or guarded by a common lock whenever accessed.

Note that concurrent access to such configuration state in singleton bean instances, for example, for controller instances or repository instances, is perfectly thread-safe after such safe initial publication from the container side. This includes common singleton FactoryBean instances which are processed within the general singleton lock as well.

For destruction callbacks, the configuration state remains thread-safe but any runtime state accumulated between initialization and destruction should be kept in thread-safe structures (or in volatile fields for simple cases) as per common Java guidelines.

Deeper Lifecycle integration as shown above involves runtime-mutable state such as a runnable field which will have to be declared as volatile. While the common lifecycle callbacks follow a certain order, for example, a start callback is guaranteed to only happen after full initialization and a stop callback only after an initial start, there is a special case with the common stop before destroy arrangement: It is strongly recommended that the internal state in any such bean also allows for an immediate destroy callback without a preceding stop since this may happen during an extraordinary shutdown after a cancelled bootstrap or in case of a stop timeout caused by another bean.

ApplicationContextAware and BeanNameAware

When an ApplicationContext creates an object instance that implements the org.springframework.context.ApplicationContextAware interface, the instance is provided with a reference to that ApplicationContext. The following listing shows the definition of the ApplicationContextAware interface:

public interface ApplicationContextAware {

	void setApplicationContext(ApplicationContext applicationContext) throws BeansException;
}

Thus, beans can programmatically manipulate the ApplicationContext that created them, through the ApplicationContext interface or by casting the reference to a known subclass of this interface (such as ConfigurableApplicationContext, which exposes additional functionality). One use would be the programmatic retrieval of other beans. Sometimes this capability is useful. However, in general, you should avoid it, because it couples the code to Spring and does not follow the Inversion of Control style, where collaborators are provided to beans as properties. Other methods of the ApplicationContext provide access to file resources, publishing application events, and accessing a MessageSource. These additional features are described in Additional Capabilities of the ApplicationContext.

Autowiring is another alternative to obtain a reference to the ApplicationContext. The traditional constructor and byType autowiring modes (as described in Autowiring Collaborators) can provide a dependency of type ApplicationContext for a constructor argument or a setter method parameter, respectively. For more flexibility, including the ability to autowire fields and multiple parameter methods, use the annotation-based autowiring features. If you do, the ApplicationContext is autowired into a field, constructor argument, or method parameter that expects the ApplicationContext type if the field, constructor, or method in question carries the @Autowired annotation. For more information, see Using @Autowired.

When an ApplicationContext creates a class that implements the org.springframework.beans.factory.BeanNameAware interface, the class is provided with a reference to the name defined in its associated object definition. The following listing shows the definition of the BeanNameAware interface:

public interface BeanNameAware {

	void setBeanName(String name) throws BeansException;
}

The callback is invoked after population of normal bean properties but before an initialization callback such as InitializingBean.afterPropertiesSet() or a custom init-method.

Other Aware Interfaces

Besides ApplicationContextAware and BeanNameAware (discussed earlier), Spring offers a wide range of Aware callback interfaces that let beans indicate to the container that they require a certain infrastructure dependency. As a general rule, the name indicates the dependency type. The following table summarizes the most important Aware interfaces:

Table 1. Aware interfaces
Name Injected Dependency Explained in…​

ApplicationContextAware

Declaring ApplicationContext.

ApplicationContextAware and BeanNameAware

ApplicationEventPublisherAware

Event publisher of the enclosing ApplicationContext.

Additional Capabilities of the ApplicationContext

BeanClassLoaderAware

Class loader used to load the bean classes.

Instantiating Beans

BeanFactoryAware

Declaring BeanFactory.

The BeanFactory API

BeanNameAware

Name of the declaring bean.

ApplicationContextAware and BeanNameAware

LoadTimeWeaverAware

Defined weaver for processing class definition at load time.

Load-time Weaving with AspectJ in the Spring Framework

MessageSourceAware

Configured strategy for resolving messages (with support for parameterization and internationalization).

Additional Capabilities of the ApplicationContext

NotificationPublisherAware

Spring JMX notification publisher.

Notifications

ResourceLoaderAware

Configured loader for low-level access to resources.

Resources

ServletConfigAware

Current ServletConfig the container runs in. Valid only in a web-aware Spring ApplicationContext.

Spring MVC

ServletContextAware

Current ServletContext the container runs in. Valid only in a web-aware Spring ApplicationContext.

Spring MVC

Note again that using these interfaces ties your code to the Spring API and does not follow the Inversion of Control style. As a result, we recommend them for infrastructure beans that require programmatic access to the container.