4.5 Customizing the nature of a bean

4.5.1 Lifecycle callbacks

The Spring Framework provides several callback interfaces to change the behavior of your bean in the container; they include InitializingBean and DisposableBean. Implementing these interfaces will result in the container calling afterPropertiesSet() for the former and destroy() for the latter to allow the bean to perform certain actions upon initialization and destruction.

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 doesn't offer out-of-the-box, you can implement a BeanPostProcessor yourself. More information about this can be found in the section entitled Section 4.7, “Container extension points”.

All the different lifecycle callback interfaces are described below. In one of the appendices, you can find diagrams that show how Spring manages beans, how those lifecycle features change the nature of your beans, and how they are managed.

4.5.1.1 Initialization callbacks

Implementing the org.springframework.beans.factory.InitializingBean interface allows a bean to perform initialization work after all necessary properties on the bean have been set by the container. The InitializingBean interface specifies exactly one method:

void afterPropertiesSet() throws Exception;

Generally, the use of the InitializingBean interface can be avoided and is actually discouraged since it unnecessarily couples the code to Spring. As an alternative, bean definitions provide support for a generic initialization method to be specified. In the case of XML-based configuration metadata, this is done using the 'init-method' attribute. For example, the following definition:

<bean id="exampleInitBean" class="examples.ExampleBean" init-method="init"/>
public class ExampleBean {
    
    public void init() {
        // do some initialization work
    }
}

...is exactly the same as...

<bean id="exampleInitBean" class="examples.AnotherExampleBean"/>
public class AnotherExampleBean implements InitializingBean {
    
    public void afterPropertiesSet() {
        // do some initialization work
    }
}

... but does not couple the code to Spring.

4.5.1.2 Destruction callbacks

Implementing the org.springframework.beans.factory.DisposableBean interface allows a bean to get a callback when the container containing it is destroyed. The DisposableBean interface specifies a single method:

void destroy() throws Exception;

Generally, the use of the DisposableBean callback interface can be avoided and is actually discouraged since it unnecessarily couples the code to Spring. As an alternative, bean definitions provide support for a generic destroy method to be specified. When using XML-based configuration metadata this is done via the 'destroy-method' attribute on the <bean/>. For example, the following definition:

<bean id="exampleInitBean" class="examples.ExampleBean" destroy-method="cleanup"/>
public class ExampleBean {

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

...is exactly the same as...

<bean id="exampleInitBean" class="examples.AnotherExampleBean"/>
public class AnotherExampleBean implements DisposableBean {

    public void destroy() {
        // do some destruction work (like releasing pooled connections)
    }
}

... but does not couple the code to Spring.

4.5.1.3 Default initialization & destroy methods

When writing initialization and destroy method callbacks that do not use the Spring-specific InitializingBean and DisposableBean callback interfaces, one typically finds oneself writing methods with names such as init(), initialize(), dispose(), etc. The names of such lifecycle callback methods are (hopefully!) standardized across a project so that all developers on a team use the same method names and thus ensure some level of consistency.

The Spring container can be configured to 'look' for named initialization and destroy callback method names on every bean. This means that you, as an application developer, can simply write your application classes, use a convention of having an initialization callback called init(), and then (without having to configure each and every bean with, in the case of XML-based configuration, an 'init-method="init"' attribute) be safe in the knowledge that the Spring IoC container will call that method when the bean is being created (and in accordance with the standard lifecycle callback contract described previously).

Let's look at an example to make the use of this feature completely clear. For the sake of the example, let us say that one of the coding conventions on a project is that all initialization callback methods are to be named init() and that destroy callback methods are to be called destroy(). This leads to classes like so...

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.");
        }
    }
}
<beans default-init-method="init">

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

</beans>

Notice the use of the 'default-init-method' attribute on the top-level <beans/> element. The presence of this attribute means that the Spring IoC container will recognize a method called 'init' on beans as being the initialization method callback, and when a bean is being created and assembled, if the bean's class has such a method, it will be invoked at the appropriate time.

Destroy method callbacks are configured similarly (in XML that is) using the 'default-destroy-method' attribute on the top-level <beans/> element.

The use of this feature can save you the (small) housekeeping chore of specifying an initialization and destroy method callback on each and every bean, and it is great for enforcing a consistent naming convention for initialization and destroy method callbacks, as consistency is something that should always be aimed for.

Consider the case where you have some existing beans where the underlying classes already have initialization callback methods that are named at variance with the convention. You can always override the default by specifying (in XML that is) the method name using the 'init-method' and 'destroy-method' attributes on the <bean/> element itself.

Finally, please be aware that the Spring container guarantees that a configured initialization callback is called immediately after a bean has been supplied with all of its dependencies. This means that the initialization callback will be called on the raw bean reference, which means that any AOP interceptors or suchlike that will ultimately be applied to the bean will not yet be in place. A target bean is fully created first, then an AOP proxy (for example) with its interceptor chain is applied. Note that, 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 very inconsistent to apply the interceptors to the init method, since that would couple the lifecycle of the target bean with its proxy/interceptors and leave strange semantics when talking to the raw target bean directly.

4.5.1.4 Combining lifecycle mechanisms

As of Spring 2.5, there are three options for controlling bean lifecycle behavior: the InitializingBean and DisposableBean callback interfaces; custom init() and destroy() methods; and the @PostConstruct and @PreDestroy annotations.

When combining different lifecycle mechanisms - for example, in a class hierarchy in which various lifecycle mechanisms are in use - developers should be aware of the order in which these mechanisms are applied. The following is the ordering for initialization methods:

  • Methods annotated with @PostConstruct

  • afterPropertiesSet() as defined by the InitializingBean callback interface

  • A custom configured init() method

Destroy methods are called in the same order:

  • Methods annotated with @PreDestroy

  • destroy() as defined by the DisposableBean callback interface

  • A custom configured destroy() method

[Note]Note

If multiple lifecycle mechanisms are configured for a given bean, and each mechanism is configured with a different method name, then each configured method will be executed in the order listed above; however, if the same method name is configured - for example, init() for an initialization method - for more than one of the aforementioned lifecycle mechanisms, that method will only be executed once.

4.5.1.5 Shutting down the Spring IoC container gracefully in non-web applications

[Note]Note

This next section does not apply to web applications (in case the title of this section did not make that abundantly clear). Spring's web-based ApplicationContext implementations already have code in place to handle shutting down the Spring IoC container gracefully when the relevant web application is being shutdown.

If you are using Spring's IoC container in a non-web application environment, for example in a rich client desktop environment, and you want the container to shutdown gracefully and call the relevant destroy callbacks on your singleton beans, you will need to register a shutdown hook with the JVM. This is quite easy to do (see below), and will ensure that your Spring IoC container shuts down gracefully and that all resources held by your singletons are released. Of course it is still up to you to both configure the destroy callbacks for your singletons and implement such destroy callbacks correctly.

So to register a shutdown hook that enables the graceful shutdown of the relevant Spring IoC container, you simply need to call the registerShutdownHook() method that is declared on the AbstractApplicationContext class. To wit...

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

public final class Boot {

    public static void main(final String[] args) throws Exception {
        AbstractApplicationContext ctx
            = new ClassPathXmlApplicationContext(new String []{"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...
    }
}

4.5.2 Knowing who you are

4.5.2.1 BeanFactoryAware

A class which implements the org.springframework.beans.factory.BeanFactoryAware interface is provided with a reference to the BeanFactory that created it, when it is created by that BeanFactory.

public interface BeanFactoryAware {

    void setBeanFactory(BeanFactory beanFactory) throws BeansException;
}

This allows beans to manipulate the BeanFactory that created them programmatically, through the BeanFactory interface, or by casting the reference to a known subclass of this which exposes additional functionality. Primarily this would consist of programmatic retrieval of other beans. While there are cases when this capability is useful, it should generally be avoided, since it couples the code to Spring and does not follow the Inversion of Control style, where collaborators are provided to beans as properties.

An alternative option that is equivalent in effect to the BeanFactoryAware-based approach is to use the org.springframework.beans.factory.config.ObjectFactoryCreatingFactoryBean. (It should be noted that this approach still does not reduce the coupling to Spring, but it does not violate the central principle of IoC as much as the BeanFactoryAware-based approach.)

The ObjectFactoryCreatingFactoryBean is a FactoryBean implementation that returns a reference to an object (factory) that can in turn be used to effect a bean lookup. The ObjectFactoryCreatingFactoryBean class does itself implement the BeanFactoryAware interface; what client beans are actually injected with is an instance of the ObjectFactory interface. This is a Spring-specific interface (and hence there is still no total decoupling from Spring), but clients can then use the ObjectFactory's getObject() method to effect the bean lookup (under the hood the ObjectFactory implementation instance that is returned simply delegates down to a BeanFactory to actually lookup a bean by name). All that you need to do is supply the ObjectFactoryCreatingFactoryBean with the name of the bean that is to be looked up. Let's look at an example:

package x.y;

public class NewsFeed {
    
    private String news;

    public void setNews(String news) {
        this.news = news;
    }

    public String getNews() {
        return this.toString() + ": '" + news + "'";
    }
}
package x.y;

import org.springframework.beans.factory.ObjectFactory;

public class NewsFeedManager {

    private ObjectFactory factory;

    public void setFactory(ObjectFactory factory) {
        this.factory = factory;
    }

    public void printNews() {
        // here is where the lookup is performed; note that there is no
        // need to hard code the name of the bean that is being looked up...
        NewsFeed news = (NewsFeed) factory.getObject();
        System.out.println(news.getNews());
    }
}

Find below the XML configuration to wire together the above classes using the ObjectFactoryCreatingFactoryBean approach.

<beans>
    <bean id="newsFeedManager" class="x.y.NewsFeedManager">
        <property name="factory">
            <bean
class="org.springframework.beans.factory.config.ObjectFactoryCreatingFactoryBean">
                <property name="targetBeanName">
                    <idref local="newsFeed" />
                </property>
            </bean>
        </property>
    </bean>
    <bean id="newsFeed" class="x.y.NewsFeed" scope="prototype">
        <property name="news" value="... that's fit to print!" />
    </bean>
</beans>

And here is a small driver program to test the fact that new (prototype) instances of the newsFeed bean are actually being returned for each call to the injected ObjectFactory inside the NewsFeedManager's printNews() method.

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import x.y.NewsFeedManager;

public class Main {

    public static void main(String[] args) throws Exception {

        ApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml");
        NewsFeedManager manager = (NewsFeedManager) ctx.getBean("newsFeedManager");
        manager.printNews();
        manager.printNews();
    }
}

The output from running the above program will look like so (results will of course vary on your machine).

[email protected]: '... that's fit to print!'
[email protected]: '... that's fit to print!'

As of Spring 2.5, you can rely upon autowiring of the BeanFactory as yet another alternative to implementing the BeanFactoryAware interface. The "traditional" constructor and byType autowiring modes (as described in the section entitled Section 4.3.5, “Autowiring collaborators”) are now capable of providing a dependency of type BeanFactory for either a constructor argument or setter method parameter respectively. For more flexibility (including the ability to autowire fields and multiple parameter methods), consider using the new annotation-based autowiring features. In that case, the BeanFactory will be autowired into a field, constructor argument, or method parameter that is expecting the BeanFactory type as long as the field, constructor, or method in question carries the @Autowired annotation. For more information, see the section entitled Section 4.11.2, “@Autowired”.

4.5.2.2 BeanNameAware

If a bean implements the org.springframework.beans.factory.BeanNameAware interface and is deployed in a BeanFactory, the BeanFactory will call the bean through this interface to inform the bean of the name it was deployed under. The callback will be invoked after population of normal bean properties but before an initialization callback like InitializingBean's afterPropertiesSet or a custom init-method.