Spring的@Import注解四种使用方式

来介绍@Import注解的使用之前，我们先看源码：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Import {

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

}
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从注释中，我们可以看到@Import注解可以将@Configuration、ImportSelector、ImportBeanDefinitionRegistrar和普通的component类导入。

准备Spring环境

Spring依赖

<dependency>
  <groupId>org.springframework</groupId>
  <artifactId>spring-context</artifactId>
  <version>5.3.7</version>
</dependency>
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导入@Configuration类

需要导入的bean：Hello

public class Hello {

    public void print() {
        System.out.println("hello word");
    }
}
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配置类：HelloConfiguration

@Configuration
public class HelloConfiguration {

    @Bean
    public Hello createHello() {
        return new Hello();
    }
}
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启动类：SpringTestApplication

@Import(HelloConfiguration.class)
public class SpringTestApplication {

    public static void main(String[] args) {
        ApplicationContext applicationContext = new AnnotationConfigApplicationContext(SpringTestApplication.class);
        Hello hello = applicationContext.getBean(Hello.class);
        hello.print();
    }
}
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启动之后，控制台输出如下：

hello word

Process finished with exit code 0
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说明Hello对象确实通过@Import注解注入到了容器中。

导入ImportSelector

ImportSelector实现类：HelloImportSelector

ImportSelector是一个接口，实现这个接口需要重写selectImports方法。

selectImports方法会返回一个String数组。这个数组包含的元素是需要被导入容器中的类的全限定名。

下面我们实现ImportSelector接口并重写selectImports方法，将Hello类的全限定名返回。

public class HelloImportSelector implements ImportSelector {
    @Override
    public String[] selectImports(AnnotationMetadata importingClassMetadata) {
        List<String> classNameList = new ArrayList<>();
        classNameList.add("com.xgc.entity.Hello");
        return StringUtils.toStringArray(classNameList);
    }
}
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启动类：SpringTestApplication

@Import(HelloImportSelector.class)
public class SpringTestApplication {

    public static void main(String[] args) {
        ApplicationContext applicationContext = new AnnotationConfigApplicationContext(SpringTestApplication.class);
        Hello hello = applicationContext.getBean(Hello.class);
        hello.print();
    }
}
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启动之后，控制台输出如下：

hello word

Process finished with exit code 0
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导入ImportBeanDefinitionRegistrar

ImportBeanDefinitionRegistrar实现类：HelloImportDefinitionRegistrar

public class HelloImportBeanDefinitionRegistrar implements ImportBeanDefinitionRegistrar {

    @Override
    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
        RootBeanDefinition rootBeanDefinition = new RootBeanDefinition(Hello.class);
        registry.registerBeanDefinition("hello", rootBeanDefinition);
    }
}
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启动类：SpringTestApplication

@Import(HelloImportBeanDefinitionRegistrar.class)
public class SpringTestApplication {

    public static void main(String[] args) {
        ApplicationContext applicationContext = new AnnotationConfigApplicationContext(SpringTestApplication.class);
        Hello hello = applicationContext.getBean(Hello.class);
        hello.print();
    }
}
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启动之后，控制台输出如下：

hello word

Process finished with exit code 0
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直接导入Bean

启动类：SpringTestApplication

@Import(Hello.class)
public class SpringTestApplication {

    public static void main(String[] args) {
        ApplicationContext applicationContext = new AnnotationConfigApplicationContext(SpringTestApplication.class);
        Hello hello = applicationContext.getBean(Hello.class);
        hello.print();
    }
}
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启动之后，控制台输出如下：

hello word

Process finished with exit code 0
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