Spring Framework 基础 —— 第四章 Spring 相关 API

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1. ApplicationContext 的继承体系

applicationContext：接口类型，代表应用上下文，可以通过其实例获得 Spring 容器中的 Bean 对象

2. ApplicationContext 的实现类

1. ClassPathApplicationContext

   它是从类的根路径下加载配置文件，通常情况下推荐使用这种方式。

2. FileSystemApplicationContext

   它是从磁盘路径上加载配置文件，配置文件可以在磁盘的任意位置。

3. AnnotationConfigApplicationContext

   当使用注解配置容器对象的时候，需要使用此类来创建 Spring 容器，它用来读取注解。

3. getBean() 方法使用

getBean() 的源码如下：

/**
 * Return an instance, which may be shared or independent, of the specified bean.
 * <p>This method allows a Spring BeanFactory to be used as a replacement for the
 * Singleton or Prototype design pattern. Callers may retain references to
 * returned objects in the case of Singleton beans.
 * <p>Translates aliases back to the corresponding canonical bean name.
 * <p>Will ask the parent factory if the bean cannot be found in this factory instance.
 * @param name the name of the bean to retrieve
 * @return an instance of the bean
 * @throws NoSuchBeanDefinitionException if there is no bean with the specified name
 * @throws BeansException if the bean could not be obtained
 */
public Object getBean(String name) throws BeansException &#123;
    this.assertBeanFactoryActive();
    return this.getBeanFactory().getBean(name);
&#125;

/**
 * Return an instance, which may be shared or independent, of the specified bean.
 * <p>Behaves the same as &#123;@link #getBean(String)&#125;, but provides a measure of type
 * safety by throwing a BeanNotOfRequiredTypeException if the bean is not of the
 * required type. This means that ClassCastException can't be thrown on casting
 * the result correctly, as can happen with &#123;@link #getBean(String)&#125;.
 * <p>Translates aliases back to the corresponding canonical bean name.
 * <p>Will ask the parent factory if the bean cannot be found in this factory instance.
 * @param name the name of the bean to retrieve
 * @param requiredType type the bean must match; can be an interface or superclass
 * @return an instance of the bean
 * @throws NoSuchBeanDefinitionException if there is no such bean definition
 * @throws BeanNotOfRequiredTypeException if the bean is not of the required type
 * @throws BeansException if the bean could not be created
 */
public <T> T getBean(String name, Class<T> requiredType) throws BeansException &#123;
    this.assertBeanFactoryActive();
    return this.getBeanFactory().getBean(name, requiredType);
&#125;

其中，当参的数据类型是字符串时，标识根据 Bean 的 id 从容器中获得的 Bean 实例，返回是 Object，需要强制转换。当参数的数据类型时，表示根据类型从容器中匹配的 Bean 实例，当容器中相同类型的 Bean 有多个时，用此方法会报错。

使用示例：

ApplicationContext app = new ClassPathXmlApplicationContext("applicationContext.xml");
UserService userService1 = (UserService) app.getBean("userService");
UserService userService2 = app.getBean(UserService.class);

4. Spring 的重点 API （重点）

ApplicationContext app = new ClassPathXmlApplicationContext("xml file");
app.getBean("id");
app.getBean(Class);

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