Using depends-on

如果一个 Bean 是另一个 Bean 的依赖项，则通常表示一个 Bean 被设置为另一个 Bean 的属性。通常，您可以使用基于 XML 的配置元数据中的 <<`<ref/>`元素，beans-ref-element>> 来实现此目的。然而，有时 Bean 之间的依赖关系不那么直接。一个示例是当需要触发类中的静态初始化程序时，例如，数据库驱动程序注册。depends-on 属性可以显式地强制一个或多个 Bean 在使用此元素初始化 Bean 之前进行初始化。以下示例使用 depends-on 属性来表示对单个 Bean 的依赖：

If a bean is a dependency of another bean, that usually means that one bean is set as a property of another. Typically you accomplish this with the <<`<ref/>` element,beans-ref-element>> in XML-based configuration metadata. However, 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 can explicitly force one or more beans to be initialized before the bean using this element is initialized. The following example uses the depends-on attribute to express a dependency on a single bean:

<bean id="beanOne" class="ExampleBean" depends-on="manager"/>
<bean id="manager" class="ManagerBean" />

要表示对多个 Bean 的依赖，请提供一个 Bean 名称列表作为 depends-on 属性的值（逗号、空格和分号是有效的定界符）：

To express a dependency on multiple beans, supply a list of bean names as the value of the depends-on attribute (commas, whitespace, and semicolons are valid delimiters):

<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" />

depends-on 属性可以指定初始化时依赖项，在仅限 singleton bean 的情况下，可以指定相应的销毁时依赖项。定义了具有给定 bean 的 depends-on 关系的依赖 bean 在销毁给定 bean 本身之前首先销毁。因此，depends-on 还可以控制关闭顺序。

The depends-on attribute can specify both an initialization-time dependency and, in the case of singleton beans only, a corresponding destruction-time dependency. Dependent beans that define a depends-on relationship with a given bean are destroyed first, prior to the given bean itself being destroyed. Thus, depends-on can also control shutdown order.