Using depends-on

At the end of this section, you should know

• How to explicitly force certain beans to be initialized before the required bean?
• What’s the bean destruction order when depends-on or @DependsOn is used?
• How does depends-on influence the initialization and shutdown order?

---

Sometimes dependencies between beans are less direct. An example is when a static initializer in a class needs to be triggered, such as for database driver registration. The depends-on attribute or @DependsOn annotation can explicitly force one or more beans to be initialized before the bean using this element is initialized.

XML Configuration

<bean id="beanOne" class="ExampleBean" depends-on="manager,accountDao">
  <property name="manager" ref="manager" />
</bean>

<bean id="manager" class="ManagerBean" />
<bean id="accountDao" class="x.y.jdbc.JdbcAccountDao" />

Annotation Configuration

@Configuration
public class AppConfig {

    @Bean
    @DependsOn({"manager", "accountDao"})
    public ExampleBean beanOne(@Autowired ManagerBean manager, @Autowired JdbcAccountDao accountDao) {
        ExampleBean exampleBean = new ExampleBean();
        exampleBean.setManager(manager);
        return exampleBean;
    }

    @Bean
    public ManagerBean manager() {
        return new ManagerBean();
    }

    @Bean
    public JdbcAccountDao accountDao() {
        return new JdbcAccountDao();
    }
}

This annotation ensures that the specified beans (manager and accountDao) are initialized before beanOne.

In addition to controlling initialization order, the depends-on attribute (or @DependsOn in annotation-based configuration) can also influence the shutdown order of beans. This is only relevant for singleton beans, as these are typically destroyed when the application context is closed.

@DependsOn (or depends-on) ensures that beans that declare a dependency on others are destroyed after the beans they depend on. This can be important for resource management or cleanup processes. For example, a bean that manages resources (like a connection pool or file stream) may need to be destroyed before beans that rely on those resources to avoid resource leaks.

In above example, beanOne will be destroyed after manager and accountDao when the application context is shut down.

---