從源碼分析@EnableDubbo的作用

前言：
最近研究Zookeeper以及Dubbo，在使用Dubbo時有配置文件以及註解的兩種方式，爲了圖方便使用了註解的方式，但是爆出了空指針的問題，分析才發現自己沒有加@EnableDubbo註解，於是想研究下@EnableDubbo是如何實現配置文件功能的。

從源碼分析@EnableDubbo的作用

1. 查看@EnableDubbo

/**
 * Enables Dubbo components as Spring Beans, equals
 * {@link DubboComponentScan} and {@link EnableDubboConfig} combination.
 * <p>
 * Note : {@link EnableDubbo} must base on Spring Framework 4.2 and above
 *
 * @see DubboComponentScan
 * @see EnableDubboConfig
 * @since 2.5.8
 */
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Inherited
@Documented
@EnableDubboConfig
@DubboComponentScan
public @interface EnableDubbo {
		......
}

在源碼中我們可以看到@EnableDubbo是@EnableDubboConfig以及@DubboComponentScan合併的一個註解。

• EnableDubboConfig：實現Dubbo的配置，功能類似與xml配置文件中的各種配置，在該註解中可以實現類似的功能，通過綁定的方式來實現。
• DubboComponentScan：掃描Dubbo的組件類，將Dubbo的組件類自動注入到Spring容器中。

2. 查看@EnableDubboConfig註解

2.1、首先查看註釋，可以通過註釋看到，各種配置類綁定了對應的配置文件中的配置功能。

• ApplicationConfig 綁定 dubbo.application
• ModuleConfig 綁定 dubbo.module
• …
  通過這些註釋我們大概知道各種配置類要實現的Dubbo的功能，通過讀取Dubbo服務的相關類，將這些相關類設置相應的Dubbo配置。

/**
 * As  a convenient and multiple {@link EnableDubboConfigBinding}
 * in default behavior , is equal to single bean bindings with below convention prefixes of properties:
 * <ul>
 * <li>{@link ApplicationConfig} binding to property : "dubbo.application"</li>
 * <li>{@link ModuleConfig} binding to property :  "dubbo.module"</li>
 * <li>{@link RegistryConfig} binding to property :  "dubbo.registry"</li>
 * <li>{@link ProtocolConfig} binding to property :  "dubbo.protocol"</li>
 * <li>{@link MonitorConfig} binding to property :  "dubbo.monitor"</li>
 * <li>{@link ProviderConfig} binding to property :  "dubbo.provider"</li>
 * <li>{@link ConsumerConfig} binding to property :  "dubbo.consumer"</li>
 * </ul>
 * <p>
 * In contrast, on multiple bean bindings that requires to set {@link #multiple()} to be <code>true</code> :
 * <ul>
 * <li>{@link ApplicationConfig} binding to property : "dubbo.applications"</li>
 * <li>{@link ModuleConfig} binding to property :  "dubbo.modules"</li>
 * <li>{@link RegistryConfig} binding to property :  "dubbo.registries"</li>
 * <li>{@link ProtocolConfig} binding to property :  "dubbo.protocols"</li>
 * <li>{@link MonitorConfig} binding to property :  "dubbo.monitors"</li>
 * <li>{@link ProviderConfig} binding to property :  "dubbo.providers"</li>
 * <li>{@link ConsumerConfig} binding to property :  "dubbo.consumers"</li>
 * </ul>
 *
 * @see EnableDubboConfigBinding
 * @see DubboConfigConfiguration
 * @see DubboConfigConfigurationRegistrar
 * @since 2.5.8
 */

2.2、查看具體配置

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

    /**
     * It indicates whether binding to multiple Spring Beans.
     *
     * @return the default value is <code>false</code>
     * @revised 2.5.9
     */
    boolean multiple() default true;

}

在該註解中可以看到通過Import導入了一個DubboConfigConfigurationRegistrar.class的類

3、查看DubboConfigConfigurationRegistrar類

/**
 * Dubbo {@link AbstractConfig Config} {@link ImportBeanDefinitionRegistrar register}, which order can be configured
 *
 * @see EnableDubboConfig
 * @see DubboConfigConfiguration
 * @see Ordered
 * @since 2.5.8
 */
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/dubbo/issues/3193
            registerBeans(registry, DubboConfigConfiguration.Multiple.class);
        }

        // Since 2.7.6
        registerCommonBeans(registry);
    }
}

在該類中主要實現的是註冊Dubbo的相關類，通過registerBeans來實現。
同時我們在這段源碼中可以看到通過 multiple判斷是否爲單例，如果爲單例則調用DubboConfigConfiguration.Single.class，如果爲多對象則調用DubboConfigConfiguration.Multiple.class

4、進入DubboConfigConfiguration.Single.class類

在該類中我們可以看到通過各種配置類ApplicationConfig.class、ModuleConfig.class、.....來實現了xml配置文件中的各種配置

public class DubboConfigConfiguration {

    /**
     * Single Dubbo {@link AbstractConfig Config} Bean Binding
     */
    @EnableConfigurationBeanBindings({
            @EnableConfigurationBeanBinding(prefix = "dubbo.application", type = ApplicationConfig.class),
            @EnableConfigurationBeanBinding(prefix = "dubbo.module", type = ModuleConfig.class),
            @EnableConfigurationBeanBinding(prefix = "dubbo.registry", type = RegistryConfig.class),
            @EnableConfigurationBeanBinding(prefix = "dubbo.protocol", type = ProtocolConfig.class),
            @EnableConfigurationBeanBinding(prefix = "dubbo.monitor", type = MonitorConfig.class),
            @EnableConfigurationBeanBinding(prefix = "dubbo.provider", type = ProviderConfig.class),
            @EnableConfigurationBeanBinding(prefix = "dubbo.consumer", type = ConsumerConfig.class),
            @EnableConfigurationBeanBinding(prefix = "dubbo.config-center", type = ConfigCenterBean.class),
            @EnableConfigurationBeanBinding(prefix = "dubbo.metadata-report", type = MetadataReportConfig.class),
            @EnableConfigurationBeanBinding(prefix = "dubbo.metrics", type = MetricsConfig.class),
            @EnableConfigurationBeanBinding(prefix = "dubbo.ssl", type = SslConfig.class)
    })
    public static class Single {

    }

    /**
     * Multiple Dubbo {@link AbstractConfig Config} Bean Binding
     */
    @EnableConfigurationBeanBindings({
            @EnableConfigurationBeanBinding(prefix = "dubbo.applications", type = ApplicationConfig.class, multiple = true),
            @EnableConfigurationBeanBinding(prefix = "dubbo.modules", type = ModuleConfig.class, multiple = true),
            @EnableConfigurationBeanBinding(prefix = "dubbo.registries", type = RegistryConfig.class, multiple = true),
            @EnableConfigurationBeanBinding(prefix = "dubbo.protocols", type = ProtocolConfig.class, multiple = true),
            @EnableConfigurationBeanBinding(prefix = "dubbo.monitors", type = MonitorConfig.class, multiple = true),
            @EnableConfigurationBeanBinding(prefix = "dubbo.providers", type = ProviderConfig.class, multiple = true),
            @EnableConfigurationBeanBinding(prefix = "dubbo.consumers", type = ConsumerConfig.class, multiple = true),
            @EnableConfigurationBeanBinding(prefix = "dubbo.config-centers", type = ConfigCenterBean.class, multiple = true),
            @EnableConfigurationBeanBinding(prefix = "dubbo.metadata-reports", type = MetadataReportConfig.class, multiple = true),
            @EnableConfigurationBeanBinding(prefix = "dubbo.metricses", type = MetricsConfig.class, multiple = true)
    })
    public static class Multiple {

    }
}

5、@DubboComponentScan的源碼分析

5.1、DubboComponentScan 解析

/**
 * Dubbo Component Scan {@link Annotation},scans the classpath for annotated components that will be auto-registered as
 * Spring beans. Dubbo-provided {@link Service} and {@link Reference}.
 *
 * @see Service
 * @see Reference
 * @since 2.5.7
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(DubboComponentScanRegistrar.class)
public @interface DubboComponentScan {
		........
}

在該源碼中可以通過註釋看到，通過路徑掃描到的Dubbo組件，將會自動注入到Spring中。通過@Service以及@Reference實現注入。
5.2、DubboComponentScanRegistrar解析

public class DubboComponentScanRegistrar implements ImportBeanDefinitionRegistrar {
    @Override
    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {

        Set<String> packagesToScan = getPackagesToScan(importingClassMetadata);

        registerServiceAnnotationBeanPostProcessor(packagesToScan, registry);

        // @since 2.7.6 Register the common beans
        registerCommonBeans(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);

    }

    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;
    }

}

5.3、查看registerCommonBeans類

public interface DubboBeanUtils {

    /**
     * Register the common beans
     *
     * @param registry {@link BeanDefinitionRegistry}
     * @see ReferenceAnnotationBeanPostProcessor
     * @see DubboConfigDefaultPropertyValueBeanPostProcessor
     * @see DubboConfigAliasPostProcessor
     * @see DubboLifecycleComponentApplicationListener
     * @see DubboBootstrapApplicationListener
     */
    static void registerCommonBeans(BeanDefinitionRegistry registry) {

        // Since 2.5.7 Register @Reference Annotation Bean Processor as an infrastructure Bean
        registerInfrastructureBean(registry, ReferenceAnnotationBeanPostProcessor.BEAN_NAME,
                ReferenceAnnotationBeanPostProcessor.class);

        // Since 2.7.4 [Feature] https://github.com/apache/dubbo/issues/5093
        registerInfrastructureBean(registry, DubboConfigAliasPostProcessor.BEAN_NAME,
                DubboConfigAliasPostProcessor.class);

        // Since 2.7.5 Register DubboLifecycleComponentApplicationListener as an infrastructure Bean
        registerInfrastructureBean(registry, DubboLifecycleComponentApplicationListener.BEAN_NAME,
                DubboLifecycleComponentApplicationListener.class);

        // Since 2.7.4 Register DubboBootstrapApplicationListener as an infrastructure Bean
        registerInfrastructureBean(registry, DubboBootstrapApplicationListener.BEAN_NAME,
                DubboBootstrapApplicationListener.class);

        // Since 2.7.6 Register DubboConfigDefaultPropertyValueBeanPostProcessor as an infrastructure Bean
        registerInfrastructureBean(registry, DubboConfigDefaultPropertyValueBeanPostProcessor.BEAN_NAME,
                DubboConfigDefaultPropertyValueBeanPostProcessor.class);
    }
}

在該類中看第一個方法，通過註釋大概可以知道註解@Reference實現注入類，進入ReferenceAnnotationBeanPostProcessor.class類，

/**
 * {@link org.springframework.beans.factory.config.BeanPostProcessor} implementation
 * that Consumer service {@link Reference} annotated fields
 *
 * @since 2.5.7
 */
public class ReferenceAnnotationBeanPostProcessor extends AbstractAnnotationBeanPostProcessor implements
        ApplicationContextAware, ApplicationListener<ServiceBeanExportedEvent> {

    /**
     * The bean name of {@link ReferenceAnnotationBeanPostProcessor}
     */
    public static final String BEAN_NAME = "referenceAnnotationBeanPostProcessor";

    /**
     * Cache size
     */
    private static final int CACHE_SIZE = Integer.getInteger(BEAN_NAME + ".cache.size", 32);

    private final ConcurrentMap<String, ReferenceBean<?>> referenceBeanCache =
            new ConcurrentHashMap<>(CACHE_SIZE);

    private final ConcurrentMap<InjectionMetadata.InjectedElement, ReferenceBean<?>> injectedFieldReferenceBeanCache =
            new ConcurrentHashMap<>(CACHE_SIZE);

    private final ConcurrentMap<InjectionMetadata.InjectedElement, ReferenceBean<?>> injectedMethodReferenceBeanCache =
            new ConcurrentHashMap<>(CACHE_SIZE);

    private final ConcurrentMap<String, ReferencedBeanInvocationHandler> referencedBeanInvocationHandlersCache =
            new ConcurrentHashMap<>();

    private ApplicationContext applicationContext;
    .
    .
    .
    }

在該類中具體實現了向Spring容器中注入的方法，在該方法中有創建了Map容器，以及Spring中的ApplicationContext…通過以上來實現Dubbo類的注入。