Spring注解之@Import使用方法讲解

@Import(MapperScannerReGIStrar.class)
public @interface MapperScan {
}

@Import(AspectJAutoProxyRegistrar.class)
public @interface EnableAspectJAutoProxy {
}

@Import(AsyncConfigurationSelector.class)
public @interface EnableAsync {
}

@Configuration
@Import(Person.class)
public class Application {
    public static void main(String[] args) {
        ApplicationContext context = new AnnotationConfigApplicationContext(Application.class);
        Person person = context.getBean(Person.class);
    }
}

public class MyImportSelector implements ImportSelector {
    @Override
    public String[] selectImports(AnnotationMetadata importinGClaSSMetadata) {
        return new String[]{"com.javap.importt.Person"};
    }
}

public class MyImportBeanDefinitionRegistrar implements ImportBeanDefinitionRegistrar {
    @Override
    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
        // 往registry注册BeanDefinition
    }
}

private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,
                            Collection<SourceClass> importCandidates, boolean checkForCircularImports) {
    if (importCandidates.isEmpty()) {
        return;
    }
    if (checkForCircularImports && isChainedImportOnStack(configClass)) {
        this.problemReporter.error(new CircularImportProblem(configClass, this.importStack));
    } else {
        this.importStack.push(configClass);
        try {
            for (SourceClass candidate : importCandidates) {
                if (candidate.isAssignable(ImportSelector.class)) {
                    // 引入的是ImportSelector子类
                    Class<?> candidateClass = candidate.loadClass();
                    ImportSelector selector = BeanUtils.instantiateClass(candidateClass, ImportSelector.class);
                    ParserStrategyUtils.invokeAwareMethods(
                            selector, this.environment, this.resourceLoader, this.registry);
                    if (selector instanceof DeferredImportSelector) {
                        this.deferredImportSelectorHandler.handle(configClass, (DeferredImportSelector) selector);
                    } else {
                        // 调用子类方法，得到要引入的beanNames
                        String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata());
                        Collection<SourceClass> importSourceClasses = asSourceClasses(importClassNames);
                        processImports(configClass, currentSourceClass, importSourceClasses, false);
                    }
                } else if (candidate.isAssignable(ImportBeanDefinitionRegistrar.class)) {
                    // 引入的是ImportBeanDefinitionRegistrar子类
                    Class<?> candidateClass = candidate.loadClass();
                    // 实例化子类对象
                    ImportBeanDefinitionRegistrar registrar =
                            BeanUtils.instantiateClass(candidateClass, ImportBeanDefinitionRegistrar.class);
                    ParserStrategyUtils.invokeAwareMethods(
                            registrar, this.environment, this.resourceLoader, this.registry);
                    // 暂存到Map，稍后触发
                    configClass.addImportBeanDefinitionRegistrar(registrar, currentSourceClass.getMetadata());
                } else {
                    // 引入的是普通类，正常注入到容器
                    this.importStack.registerImport(
                            currentSourceClass.getMetadata(), candidate.getMetadata().getClassName());
                    // 引入的类可能是ConfigurationClass，递归处理
                    processConfigurationClass(candidate.asConfigClass(configClass));
                }
            }
        } catch (BeanDefinitionStoreException ex) {
            throw ex;
        } catch (Throwable ex) {
            throw new BeanDefinitionStoreException(
                    "Failed to process import candidates for configuration class [" +
                            configClass.getMetadata().getClassName() + "]", ex);
        } finally {
            this.importStack.pop();
        }
    }
}

private void loadBeanDefinitionsFromRegistrars(Map<ImportBeanDefinitionRegistrar, AnnotationMetadata> registrars) {
    registrars.forEach((registrar, metadata) ->
            registrar.registerBeanDefinitions(metadata, this.registry));
}