Dubbo中@EnableDubbo注解原理

Posted 2022-07-11 敲代码的小小酥

前言

前面解析过很多@EnableXXX注解开启某个功能的原理，其注解内部都是通过@Import注解，来引入一个类，通过这个类，将开启功能的核心类引入spring容器，并进行一系列初始化操作。@EnableDubbo注解也不例外，还是这个套路，下面看其详细实现。

@EnableDubbo原理解析

点进去，看其源码:

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Inherited
@Documented
@EnableDubboConfig
@DubboComponentScan
public @interface EnableDubbo 

    /**
     * Base packages to scan for annotated @Service classes.
     * <p>
     * Use @link #scanBasePackageClasses() for a type-safe alternative to String-based
     * package names.
     *
     * @return the base packages to scan
     * @see DubboComponentScan#basePackages()
     */
    @AliasFor(annotation = DubboComponentScan.class, attribute = "basePackages")
    String[] scanBasePackages() default ;

    /**
     * Type-safe alternative to @link #scanBasePackages() for specifying the packages to
     * scan for annotated @Service classes. The package of each class specified will be
     * scanned.
     *
     * @return classes from the base packages to scan
     * @see DubboComponentScan#basePackageClasses
     */
    @AliasFor(annotation = DubboComponentScan.class, attribute = "basePackageClasses")
    Class<?>[] scanBasePackageClasses() default ;


    /**
     * It indicates whether @link AbstractConfig binding to multiple Spring Beans.
     *
     * @return the default value is <code>false</code>
     * @see EnableDubboConfig#multiple()
     */
    @AliasFor(annotation = EnableDubboConfig.class, attribute = "multiple")
    boolean multipleConfig() default true;

核心是@EnableDubboConfig注解和@DubboComponentScan。先看@EnableDubboConfig注解源码:

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Inherited
@Documented
@Import(DubboConfigConfigurationRegistrar.class)
public @interface EnableDubboConfig 

    /**
     * It indicates whether binding to multiple Spring Beans.
     * <p>
     * Please note that if @link #multiple() is <code>true</code> since 2.6.6, the multiple bean bindings will be
     * enabled, works with single bean bindings, rather than they are mutually exclusive before.
     *
     * @return the default value is <code>true</code> since 2.6.6, the value is inverse earlier.
     * @revised 2.5.9
     */
    boolean multiple() default true;

找到了熟悉的@Import注解，那么就看Import进来的这个类：

public class DubboConfigConfigurationRegistrar implements ImportBeanDefinitionRegistrar 

    @Override
    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) 

        AnnotationAttributes attributes = AnnotationAttributes.fromMap(
                importingClassMetadata.getAnnotationAttributes(EnableDubboConfig.class.getName()));

        boolean multiple = attributes.getBoolean("multiple");

        // Single Config Bindings
        registerBeans(registry, DubboConfigConfiguration.Single.class);

        if (multiple)  // Since 2.6.6 https://github.com/apache/incubator-dubbo/issues/3193
            registerBeans(registry, DubboConfigConfiguration.Multiple.class);
        
    

可以看出，是一个BeanDefinitionRegistrar，是可以将bean注册到spring容器的，那么它将什么注册进了spring容器呢？核心代码如下：

registerBeans(registry, DubboConfigConfiguration.Single.class);

看DubboConfigConfiguration源码:

public class DubboConfigConfiguration 

    /**
     * Single Dubbo @link AbstractConfig Config Bean Binding
     */
    @EnableDubboConfigBindings(
            @EnableDubboConfigBinding(prefix = "dubbo.application", type = ApplicationConfig.class),
            @EnableDubboConfigBinding(prefix = "dubbo.module", type = ModuleConfig.class),
            @EnableDubboConfigBinding(prefix = "dubbo.registry", type = RegistryConfig.class),
            @EnableDubboConfigBinding(prefix = "dubbo.protocol", type = ProtocolConfig.class),
            @EnableDubboConfigBinding(prefix = "dubbo.monitor", type = MonitorConfig.class),
            @EnableDubboConfigBinding(prefix = "dubbo.provider", type = ProviderConfig.class),
            @EnableDubboConfigBinding(prefix = "dubbo.consumer", type = ConsumerConfig.class)
    )
    public static class Single 

    

    /**
     * Multiple Dubbo @link AbstractConfig Config Bean Binding
     */
    @EnableDubboConfigBindings(
            @EnableDubboConfigBinding(prefix = "dubbo.applications", type = ApplicationConfig.class, multiple = true),
            @EnableDubboConfigBinding(prefix = "dubbo.modules", type = ModuleConfig.class, multiple = true),
            @EnableDubboConfigBinding(prefix = "dubbo.registries", type = RegistryConfig.class, multiple = true),
            @EnableDubboConfigBinding(prefix = "dubbo.protocols", type = ProtocolConfig.class, multiple = true),
            @EnableDubboConfigBinding(prefix = "dubbo.monitors", type = MonitorConfig.class, multiple = true),
            @EnableDubboConfigBinding(prefix = "dubbo.providers", type = ProviderConfig.class, multiple = true),
            @EnableDubboConfigBinding(prefix = "dubbo.consumers", type = ConsumerConfig.class, multiple = true)
    )
    public static class Multiple 

    

可以看出，这里是将xml里的一些配置信息，绑定到了类里，加入到了Spring缓存。

下面再看另一个核心注解@DubboComponentScan源码:

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(DubboComponentScanRegistrar.class)
public @interface DubboComponentScan 

    /**
     * Alias for the @link #basePackages() attribute. Allows for more concise annotation
     * declarations e.g.: @code @DubboComponentScan("org.my.pkg") instead of
     * @code @DubboComponentScan(basePackages="org.my.pkg").
     *
     * @return the base packages to scan
     */
    String[] value() default ;

    /**
     * Base packages to scan for annotated @Service classes. @link #value() is an
     * alias for (and mutually exclusive with) this attribute.
     * <p>
     * Use @link #basePackageClasses() for a type-safe alternative to String-based
     * package names.
     *
     * @return the base packages to scan
     */
    String[] basePackages() default ;

    /**
     * Type-safe alternative to @link #basePackages() for specifying the packages to
     * scan for annotated @Service classes. The package of each class specified will be
     * scanned.
     *
     * @return classes from the base packages to scan
     */
    Class<?>[] basePackageClasses() default ;

可以看到@Import进来了DubboComponentScanRegistrar类，看其源码:

public class DubboComponentScanRegistrar implements ImportBeanDefinitionRegistrar 

    @Override
    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) 

        Set<String> packagesToScan = getPackagesToScan(importingClassMetadata);

        registerServiceAnnotationBeanPostProcessor(packagesToScan, registry);

        registerReferenceAnnotationBeanPostProcessor(registry);

    

    /**
     * Registers @link ServiceAnnotationBeanPostProcessor
     *
     * @param packagesToScan packages to scan without resolving placeholders
     * @param registry       @link BeanDefinitionRegistry
     * @since 2.5.8
     */
    private void registerServiceAnnotationBeanPostProcessor(Set<String> packagesToScan, BeanDefinitionRegistry registry) 

        BeanDefinitionBuilder builder = rootBeanDefinition(ServiceAnnotationBeanPostProcessor.class);
        builder.addConstructorArgValue(packagesToScan);
        builder.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
        AbstractBeanDefinition beanDefinition = builder.getBeanDefinition();
        BeanDefinitionReaderUtils.registerWithGeneratedName(beanDefinition, registry);

    

    /**
     * Registers @link ReferenceAnnotationBeanPostProcessor into @link BeanFactory
     *
     * @param registry @link BeanDefinitionRegistry
     */
    private void registerReferenceAnnotationBeanPostProcessor(BeanDefinitionRegistry registry) 

        // Register @Reference Annotation Bean Processor
        BeanRegistrar.registerInfrastructureBean(registry,
                ReferenceAnnotationBeanPostProcessor.BEAN_NAME, ReferenceAnnotationBeanPostProcessor.class);

    

    private Set<String> getPackagesToScan(AnnotationMetadata metadata) 
        AnnotationAttributes attributes = AnnotationAttributes.fromMap(
                metadata.getAnnotationAttributes(DubboComponentScan.class.getName()));
        String[] basePackages = attributes.getStringArray("basePackages");
        Class<?>[] basePackageClasses = attributes.getClassArray("basePackageClasses");
        String[] value = attributes.getStringArray("value");
        // Appends value array attributes
        Set<String> packagesToScan = new LinkedHashSet<String>(Arrays.asList(value));
        packagesToScan.addAll(Arrays.asList(basePackages));
        for (Class<?> basePackageClass : basePackageClasses) 
            packagesToScan.add(ClassUtils.getPackageName(basePackageClass));
        
        if (packagesToScan.isEmpty()) 
            return Collections.singleton(ClassUtils.getPackageName(metadata.getClassName()));
        
        return packagesToScan;
    

可以看到里面有registerServiceAnnotationBeanPostProcessor方法和registerReferenceAnnotationBeanPostProcessor方法。这里注册进来两个BeanPostProcessor，一个用于扫描@Service注解，一个用于扫描@Reference注解。

总结

@EnableDubbo做了两件事，一个是初始化Dubbo核心组件，加载Dubbo配置到内存。另一个是注册BeanPostProcessor，用来扫描@Service和@Reference注解。