浅谈Spring中@Import注解的作用和使用

@Import用来导入@Configuration注解的配置类、声明@Bean注解的bean方法、导入ImportSelector的实现类或导入ImportBeanDefinitionRegistrar的实现类。

@Import注解的作用

查看Import注解源码

/** * Indicates one or more {@link Configuration @Configuration} classes to import. * * <p>Provides functionality equivalent to the {@code <import/>} element in Spring XML. * Only supported for classes annotated with {@code @Configuration} or declaring at least * one {@link Bean @Bean} method, as well as {@link ImportSelector} and * {@link ImportBeanDefinitionRegistrar} implementations. * * <p>{@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. * * <p>May be declared at the class level or as a meta-annotation. * * <p>If XML or other non-{@code @Configuration} bean definition resources need to be * imported, use {@link ImportResource @ImportResource} * * @author Chris Beams * @since 3.0 * @see Configuration * @see ImportSelector * @see ImportResource */@Target(ElementType.TYPE)@Retention(RetentionPolicy.RUNTIME)@Documentedpublic @interface Import { /** * The @{@link Configuration}, {@link ImportSelector} and/or * {@link ImportBeanDefinitionRegistrar} classes to import. */ Class<?>[] value();}

分析类注释得出结论：

声明一个bean 导入@Configuration注解的配置类 导入ImportSelector的实现类 导入ImportBeanDefinitionRegistrar的实现类

@Import注解的使用

声明一个bean

package com.example.demo.bean;public class TestBean1 {}package com.example.demo;import com.example.demo.bean.TestBean1;import org.springframework.context.annotation.Configuration;import org.springframework.context.annotation.Import;@Import({TestBean1.class})@Configurationpublic class AppConfig {}

导入@Configuration注解的配置类

package com.example.demo.bean;public class TestBean2 {}package com.example.demo.bean;import org.springframework.context.annotation.Bean;import org.springframework.context.annotation.Configuration;@Configurationpublic class TestConfig { @Bean public TestBean2 getTestBean2(){ return new TestBean2(); }}package com.example.demo;import com.example.demo.bean.TestBean1;import com.example.demo.bean.TestConfig;import org.springframework.context.annotation.Configuration;import org.springframework.context.annotation.Import;@Import({TestBean1.class,TestConfig.class})@Configurationpublic class AppConfig {}

导入ImportSelector的实现类

package com.example.demo.bean;public class TestBean3 {}package com.example.demo.bean;import org.springframework.context.annotation.ImportSelector;import org.springframework.core.type.AnnotationMetadata;public class TestImportSelector implements ImportSelector { @Override public String[] selectImports(AnnotationMetadata importingClassMetadata) { return new String[]{'com.example.demo.bean.TestBean3'}; }}package com.example.demo;import com.example.demo.bean.TestBean1;import com.example.demo.bean.TestConfig;import com.example.demo.bean.TestImportSelector;import org.springframework.context.annotation.Configuration;import org.springframework.context.annotation.Import;@Import({TestBean1.class,TestConfig.class,TestImportSelector.class})@Configurationpublic class AppConfig {}

导入ImportBeanDefinitionRegistrar的实现类

package com.example.demo.bean;public class TestBean4 {}package com.example.demo.bean;import org.springframework.beans.factory.support.BeanDefinitionRegistry;import org.springframework.beans.factory.support.RootBeanDefinition;import org.springframework.context.annotation.ImportBeanDefinitionRegistrar;import org.springframework.core.type.AnnotationMetadata;public class TestImportBeanDefinitionRegistrar implements ImportBeanDefinitionRegistrar { @Override public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) { RootBeanDefinition rootBeanDefinition = new RootBeanDefinition(TestBean4.class); registry.registerBeanDefinition('TestBean4', rootBeanDefinition); }}package com.example.demo;import com.example.demo.bean.TestBean1;import com.example.demo.bean.TestConfig;import com.example.demo.bean.TestImportBeanDefinitionRegistrar;import com.example.demo.bean.TestImportSelector;import org.springframework.context.annotation.Configuration;import org.springframework.context.annotation.Import;@Import({TestBean1.class,TestConfig.class,TestImportSelector.class,TestImportBeanDefinitionRegistrar.class})@Configurationpublic class AppConfig {}

最后，我们来看下导入结果：

package com.example.demo;import org.junit.Test;import org.junit.runner.RunWith;import org.springframework.boot.test.context.SpringBootTest;import org.springframework.context.annotation.AnnotationConfigApplicationContext;import org.springframework.test.context.junit4.SpringRunner;import java.util.Arrays;@RunWith(SpringRunner.class)@SpringBootTestpublic class DemoApplicationTests { @Test public void test() { AnnotationConfigApplicationContext annotationConfigApplicationContext = new AnnotationConfigApplicationContext(AppConfig.class); String[] beanDefinitionNames = annotationConfigApplicationContext.getBeanDefinitionNames(); System.out.println('--------------------------------------------------------'); for (String beanDefinitionName: beanDefinitionNames) { System.out.println(beanDefinitionName); } System.out.println('--------------------------------------------------------'); }}

打印结果如下：

--------------------------------------------------------org.springframework.context.annotation.internalConfigurationAnnotationProcessororg.springframework.context.annotation.internalAutowiredAnnotationProcessororg.springframework.context.annotation.internalRequiredAnnotationProcessororg.springframework.context.annotation.internalCommonAnnotationProcessororg.springframework.context.event.internalEventListenerProcessororg.springframework.context.event.internalEventListenerFactoryappConfigcom.example.demo.bean.TestBean1com.example.demo.bean.TestConfiggetTestBean2com.example.demo.bean.TestBean3TestBean4--------------------------------------------------------

可以看出TestBean1,TestBean2,TestBean3,TestBean4通过不同的4种导入方法被导入SpringIOC容器中。

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