Spring注解之@Import和@ImportResource

前言

为什么研究这两个注解，是因为在看Spring Boot源码的时候，对于其中大量的EnableXXX都使用了@Import注解，所以决定研究下这个注解，顺便写了一点关于@ImportRescource注解的东西，希望对大家有帮助。

@Import

简介：功能类似XML配置的，用来导入配置类，可以导入带有@Configuration注解的配置类或实现了ImportSelector/ImportBeanDefinitionRegistrar，或者导入普通的POJO（Spring会将其注册成Spring Bean，导入POJO需要使用Spring 4.2以上）。

下面注解源码，源码有比较详细的英文解释：

/**
 * Indicates one or more {@link Configuration @Configuration} classes to import.
 *
 * 
Provides functionality equivalent to the {@code } element in Spring XML.
 * Allows for importing {@code @Configuration} classes, {@link ImportSelector} and
 * {@link ImportBeanDefinitionRegistrar} implementations, as well as regular component
 * classes (as of 4.2; analogous to {@link AnnotationConfigApplicationContext#register}).
 *
 * 
{@code @Bean} definitions declared in imported {@code @Configuration} classes should be
 * accessed by using {@link org.springframework.beans.factory.annotation.Autowired @Autowired}
 * injection. Either the bean itself can be autowired, or the configuration class instance
 * declaring the bean can be autowired. The latter approach allows for explicit, IDE-friendly
 * navigation between {@code @Configuration} class methods.
 *
 * 
May be declared at the class level or as a meta-annotation.
 *
 * 
If XML or other non-{@code @Configuration} bean definition resources need to be
 * imported, use the {@link ImportResource @ImportResource} annotation instead.
 *
 * @author Chris Beams
 * @author Juergen Hoeller
 * @since 3.0
 * @see Configuration
 * @see ImportSelector
 * @see ImportResource
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Import {
   

    /**
     * {@link Configuration}, {@link ImportSelector}, {@link ImportBeanDefinitionRegistrar}
     * or regular component classes to import.
     */
    Class[] value();

}

关于Configuration和POJO，没什么好说的，就是简单的配置。这里说下ImportSelector和ImportBeanDefinitionRegistrar的作用（虽然开发过程中很少会使用到，但是Spring和Spring Boot的源码大面积使用了这个功能）。

ImportSelector

• 根据AnnotationMetadata（实质上是引入@Import注解的类的注解信息），来判断是否引入哪些配置类，通过字符串数组的方式返回配置类的全限定名。

DeferredImportSelector

• ImportSelector的子接口，区别是，他会在所有的@Configuration类加载完成之后再加载返回的配置类。ImportSelector会在当前Configuration类加载之前去加载返回的配置类。
• 可以使用@Order注解或者Ordered接口来指定DeferredImportSelector的加载顺序。
• 并且提供了新的方法getImportGroup()用来跨DeferredImportSelector实现自定义Configuration的加载顺序。

ImportBeanDefinitionRegistrar

• 根据AnnotationMetadata（实质上是引入@Import注解的类的注解信息）, 来注册BeanDenfinition,通过BeanDefinitionRegistry实例的register可以注册BeanDefinition。

三个接口的使用例子在文章最后面。

@ImportResource

和@Import类似，区别就是@ImportResource导入的是配置文件。

同样源码如下

/**
 * Indicates one or more resources containing bean definitions to import.
 *
 * 
Like {@link Import @Import}, this annotation provides functionality similar to
 * the {@code } element in Spring XML. It is typically used when designing
 * {@link Configuration @Configuration} classes to be bootstrapped by an
 * {@link AnnotationConfigApplicationContext}, but where some XML functionality such
 * as namespaces is still necessary.
 *
 * 
By default, arguments to the {@link #value} attribute will be processed using a
 * {@link org.springframework.beans.factory.groovy.GroovyBeanDefinitionReader GroovyBeanDefinitionReader}
 * if ending in {@code ".groovy"}; otherwise, an
 * {@link org.springframework.beans.factory.xml.XmlBeanDefinitionReader XmlBeanDefinitionReader}
 * will be used to parse Spring {@code } XML files. Optionally, the {@link #reader}
 * attribute may be declared, allowing the user to choose a custom {@link BeanDefinitionReader}
 * implementation.
 *
 * @author Chris Beams
 * @author Juergen Hoeller
 * @author Sam Brannen
 *