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# 1.4.4. 延迟初始化的 Bean

默认情况下，作为初始化过程的一部分，`ApplicationContext`实现会急切地创建和配置所有 [单例bean。](https://docs.spring.io/spring-framework/docs/current/reference/html/core.html#beans-factory-scopes-singleton)通常，这种预实例化是可取的，因为配置或周围环境中的错误会立即发现，而不是几小时甚至几天之后。当这种行为不可取时，您可以通过将 bean 定义标记为延迟初始化来防止单例 bean 的预实例化。延迟初始化的 bean 告诉 IoC 容器在第一次被请求时创建一个 bean 实例，而不是在启动时。

在 XML 中，此行为由`<bean/>` 元素上的属性`lazy-init`控制，如以下示例所示：

```xml
<bean id="lazy" class="com.something.ExpensiveToCreateBean" lazy-init="true"/>
<bean name="not.lazy" class="com.something.AnotherBean"/>
```

当前面的配置被 `ApplicationContext`使用时，`lazy`bean 不会在`ApplicationContext`启动时急切地预实例化，而`not.lazy` bean 是急切地预实例化的。

但是，当延迟初始化的 bean 是未延迟初始化的单例 bean 的依赖项时，`ApplicationContext`将在启动时创建延迟初始化的 bean，因为它必须满足单例的依赖项。延迟初始化的 bean 被注入到没有延迟初始化的其他地方的单例 bean 中。

您还可以通过使用元素上的 `default-lazy-init`属性在容器级别控制延迟初始化`<beans/>`，如以下示例所示：

```xml
<beans default-lazy-init="true">
    <!-- no beans will be pre-instantiated... -->
</beans>
```


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