@Enable 注解原理解析

这几天项目中用到Apollo，顺便看一下源码，然后看到Apollo再Spring中注册配置的类ApolloConfigRegistrar，里面大致就是注册了一堆processor

然后这个ApolloConfigRegistrar是通过@EnableApolloConfig注解去Import的

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

  String[] value() default {ConfigConsts.NAMESPACE_APPLICATION};

  int order() default Ordered.LOWEST_PRECEDENCE;
}

那这个@EnableApolloConfig是通过什么加载到Spring中的呢，然后我就在IDEA中搜了一下，发现并没有任何地方用到了这个注解，这就让我很奇怪了

然后我去翻了下@EnableWebMvc注解的代码，然后就在@Configuration注解中找到了如下的注释

<h2>Enabling built-in Spring features using {@code @Enable} annotations</h2>

Spring features such as asynchronous method execution, scheduled task execution,
annotation driven transaction management, and even Spring MVC can be enabled and
configured from {@code @Configuration}
classes using their respective "{@code @Enable}" annotations. See
{@link org.springframework.scheduling.annotation.EnableAsync @EnableAsync},
{@link org.springframework.scheduling.annotation.EnableScheduling @EnableScheduling},
{@link org.springframework.transaction.annotation.EnableTransactionManagement @EnableTransactionManagement},
{@link org.springframework.context.annotation.EnableAspectJAutoProxy @EnableAspectJAutoProxy},
and {@link org.springframework.web.servlet.config.annotation.EnableWebMvc @EnableWebMvc}
for details.

总结下来就一句话，就是Spring会自动加载@Enable*的注解起到初始化配置的作用

那Spring是如何自动加载@Enable注解的呢

我自己写了一个@Enable注解dubug了一下，看了调用栈发现是ConfigurationClassParser在做处理，仔细一看，其实是递归调用，找Import注解然后进行后续的配置处理

/**
 * Recursively collect all declared {@code @Import} values. Unlike most
 * meta-annotations it is valid to have several {@code @Import}s declared with
 * different values; the usual process of returning values from the first
 * meta-annotation on a class is not sufficient.
 * <p>For example, it is common for a {@code @Configuration} class to declare direct
 * {@code @Import}s in addition to meta-imports originating from an {@code @Enable}
 * annotation.
 * @param sourceClass the class to search
 * @param imports the imports collected so far
 * @param visited used to track visited classes to prevent infinite recursion
 * @throws IOException if there is any problem reading metadata from the named class
 */
private void collectImports(SourceClass sourceClass, Set<SourceClass> imports, Set<SourceClass> visited)
        throws IOException {

    if (visited.add(sourceClass)) {
        for (SourceClass annotation : sourceClass.getAnnotations()) {
            String annName = annotation.getMetadata().getClassName();
            if (!annName.startsWith("java") && !annName.equals(Import.class.getName())) {
                collectImports(annotation, imports, visited);
            }
        }
        imports.addAll(sourceClass.getAnnotationAttributes(Import.class.getName(), "value"));
    }
}

其实是Import注解在发生作用，而不是@Enable注解

那不用@Enable注解也可以自动配置吗

答案是可以的

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(XXX.class)
public @interface EnableXXX {
}

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(XXX.class)
public @interface DisableXXX {
}

上面2个注解都可以生效

PS：有Spring Boot的AutoConfig机制，就不需要搞这么麻烦的配置了，这个只是Spring Framework的一部分罢了

最后编辑于：2019.04.12 17:02:07