Spring参考文档翻译09--IOC容器07

 1.7. Bean Definition Inheritance

Bean定义继承

A bean definition can contain a lot of configuration information, including constructor arguments, property values, and container-specific information, such as the initialization method, a static factory method name, and so on. A child bean definition inherits configuration data from a parent definition. The child definition can override some values or add others as needed. Using parent and child bean definitions can save a lot of typing. Effectively, this is a form of templating.

一个 bean 定义可以包含很多配置信息，包括构造函数参数、属性值和特定于容器的信息，例如初始化方法、静态工厂方法名称等。子 bean 定义从父定义继承配置数据。子定义可以根据需要覆盖某些值或添加其他值。使用父子bean定义可以节省大量输入。实际上，这是一种模板形式。

If you work with an ApplicationContext interface programmatically, child bean definitions are represented by the ChildBeanDefinition class. Most users do not work with them on this level. Instead, they configure bean definitions declaratively in a class such as the ClassPathXmlApplicationContext. When you use XML-based configuration metadata, you can indicate a child bean definition by using the parent attribute, specifying the parent bean as the value of this attribute. The following example shows how to do so:

如果您以ApplicationContext编程方式使用接口，则子 bean 定义由ChildBeanDefinition类表示。大多数用户不在此级别上与他们合作。相反，他们在诸如ClassPathXmlApplicationContext. 当您使用基于 XML 的配置元数据时，您可以通过使用属性来指示子 bean 定义parent，将父 bean 指定为该属性的值。以下示例显示了如何执行此操作：

<bean id="inheritedTestBean" abstract="true"
        class="org.springframework.beans.TestBean">
    <property name="name" value="parent"/>
    <property name="age" value="1"/>
</bean>
<bean id="inheritsWithDifferentClass"
        class="org.springframework.beans.DerivedTestBean"
        parent="inheritedTestBean" init-method="initialize">  
    <property name="name" value="override"/>
    <!-- the age property value of 1 will be inherited from parent -->
</bean>

	Note the parent attribute.
	注意parent属性。

A child bean definition uses the bean class from the parent definition if none is specified but can also override it. In the latter case, the child bean class must be compatible with the parent (that is, it must accept the parent’s property values).

如果没有指定子 bean 定义，则使用父定义中的 bean 类，但也可以覆盖它。在后一种情况下，子 bean 类必须与父类兼容（即，它必须接受父类的属性值）。

A child bean definition inherits scope, constructor argument values, property values, and method overrides from the parent, with the option to add new values. Any scope, initialization method, destroy method, or static factory method settings that you specify override the corresponding parent settings.

子 bean 定义从父 bean 继承范围、构造函数参数值、属性值和方法覆盖，并可选择添加新值。您指定的任何范围、初始化方法、销毁方法或static工厂方法设置都会覆盖相应的父设置。

The remaining settings are always taken from the child definition: depends on, autowire mode, dependency check, singleton, and lazy init.

其余的设置总是取自子定义：依赖、自动装配模式、依赖检查、单例和惰性初始化。

The preceding example explicitly marks the parent bean definition as abstract by using the abstract attribute. If the parent definition does not specify a class, explicitly marking the parent bean definition as abstract is required, as the following example shows:

abstract前面的示例通过使用属性将父 bean 定义显式标记为抽象。如果父定义未指定类，则将父 bean 定义显式标记abstract为必需，如以下示例所示：

<bean id="inheritedTestBeanWithoutClass" abstract="true">
    <property name="name" value="parent"/>
    <property name="age" value="1"/>
</bean>
<bean id="inheritsWithClass" class="org.springframework.beans.DerivedTestBean"
        parent="inheritedTestBeanWithoutClass" init-method="initialize">
    <property name="name" value="override"/>
    <!-- age will inherit the value of 1 from the parent bean definition-->
</bean>

The parent bean cannot be instantiated on its own because it is incomplete, and it is also explicitly marked as abstract. When a definition is abstract, it is usable only as a pure template bean definition that serves as a parent definition for child definitions. Trying to use such an abstract parent bean on its own, by referring to it as a ref property of another bean or doing an explicit getBean() call with the parent bean ID returns an error. Similarly, the container’s internal preInstantiateSingletons() method ignores bean definitions that are defined as abstract.

父 bean 不能自己实例化，因为它不完整，而且它也显式标记为abstract. 当定义为abstract时，它只能用作纯模板 bean 定义，用作子定义的父定义。尝试单独使用这样的abstract父 bean，通过将其引用为另一个 bean 的 ref 属性或getBean()使用父 bean ID 进行显式调用会返回错误。同样，容器的内部 preInstantiateSingletons()方法会忽略定义为抽象的 bean 定义。

ApplicationContext pre-instantiates all singletons by default. Therefore, it is important (at least for singleton beans) that if you have a (parent) bean definition which you intend to use only as a template, and this definition specifies a class, you must make sure to set the abstract attribute to true, otherwise the application context will actually (attempt to) pre-instantiate the abstract bean.

ApplicationContext默认情况下预实例化所有单例。因此，重要的是（至少对于单例 bean），如果您有一个（父）bean 定义，您打算仅将其用作模板，并且此定义指定了一个类，则必须确保将抽象属性设置为true，否则应用程序上下文将实际（尝试）预实例化abstractbean。