Bean Definition Inheritance

1.7. Bean Definition Inheritance

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.

bean定义可以包含很多配置信息，包括构造函数参数、属性值和容器特定的信息，比如初始化方法、静态工厂方法名，等等。子bean定义从父bean定义继承配置数据。子定义可以根据需要重写一些值或添加其他值。使用父bean和子bean定义可以节省大量的输入。实际上，这是模板的一种形式。

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:

如果您以编程的方式使用ApplicationContext接口，子bean定义将由ChildBeanDefinition类表示。大多数用户不会在这个级别上使用它们。相反，它们在类(比如ClassPathXmlApplicationContext)中声明性地配置bean定义。当您使用基于xml的配置元数据时，您可以通过使用父属性来指示子bean定义，并指定父bean作为该属性的值。下面的例子展示了如何这样做:

<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>

1. Note the parent attribute.

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，则子bean定义使用父定义中的bean类，但也可以覆盖它。在后一种情况下，子bean类必须与父bean类兼容(也就是说，它必须接受父bean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继承作用域、构造函数参数值、属性值和方法重写，并可以添加新值。您指定的任何范围、初始化方法、销毁方法或静态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.

其余的设置总是从子定义中获取:depends, autotowire模式，dependency check, singleton和lazy init。

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:

前面的示例通过使用abstract属性显式地将父bean定义标记为抽象。如果父定义没有指定类，则需要显式地将父bean定义标记为抽象，如下例所示:

<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不能单独实例化，因为它是不完整的，而且它还显式地被标记为抽象的。当定义是抽象的时候，它只能作为作为子定义的父定义的纯模板bean定义使用。试图单独使用这样一个抽象的父bean，通过将其作为另一个bean的ref属性引用，或者使用父bean ID执行显式getBean()调用，将返回错误。类似地，容器内部的preInstantiateSingletons()方法忽略被定义为抽象的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默认预实例化所有的单例。因此，重要的是(至少对于单例bean来说)，如果您有一个(父)bean定义，您打算只将其用作模板，并且这个定义指定了一个类，那么您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)预实例化抽象bean。