Spring Enable進階應用及原理

Enable*

之前的文章用到了一些Enable*開頭的注解，比如EnableAsync、EnableScheduling、EnableAspectJAutoProxy、EnableCaching等，Enable表示開啟/允許一項功能。

Enable*工作原理

我們隻需要幾個很簡單的注解就能開啟一個複雜的功能，這是多麼簡易的用法，這是怎麼辦到的？

首先來看看計劃任務@EnableScheduling的源代碼

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

}      

主要核心的配置就是導入了一個配置檔案，是以謎底也就接開了。

@Import(SchedulingConfiguration.class)

@Import用法

來看看Import的源碼

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

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

}      

1、Configuration

即上面的用法，直接導入Configuration配置類。

2、ImportSelector

根據條件選擇導入不同的配置類，參考@EnableAsync

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

public class AsyncConfigurationSelector extends AdviceModeImportSelector<EnableAsync> {

    private static final String ASYNC_EXECUTION_ASPECT_CONFIGURATION_CLASS_NAME =
            "org.springframework.scheduling.aspectj.AspectJAsyncConfiguration";

    /**
     * {@inheritDoc}
     * @return {@link ProxyAsyncConfiguration} or {@code AspectJAsyncConfiguration} for
     * {@code PROXY} and {@code ASPECTJ} values of {@link EnableAsync#mode()}, respectively
     */
    @Override
    public String[] selectImports(AdviceMode adviceMode) {
        switch (adviceMode) {
            case PROXY:
                return new String[] { ProxyAsyncConfiguration.class.getName() };
            case ASPECTJ:
                return new String[] { ASYNC_EXECUTION_ASPECT_CONFIGURATION_CLASS_NAME };
            default:
                return null;
        }
    }

}      

3、ImportBeanDefinitionRegistrar

動态注冊Bean，參考@EnableAspectJAutoProxy

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

class AspectJAutoProxyRegistrar implements ImportBeanDefinitionRegistrar {

    /**
     * Register, escalate, and configure the AspectJ auto proxy creator based on the value
     * of the @{@link EnableAspectJAutoProxy#proxyTargetClass()} attribute on the importing
     * {@code @Configuration} class.
     */
    @Override
    public void registerBeanDefinitions(
            AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {

        AopConfigUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(registry);

        AnnotationAttributes enableAspectJAutoProxy =
                AnnotationConfigUtils.attributesFor(importingClassMetadata, EnableAspectJAutoProxy.class);
        if (enableAspectJAutoProxy.getBoolean("proxyTargetClass")) {
            AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);
        }
        if (enableAspectJAutoProxy.getBoolean("exposeProxy")) {
            AopConfigUtils.forceAutoProxyCreatorToExposeProxy(registry);
        }
    }

}