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自定义注解那些事

快乐码农Alan007 03-13 07:40 阅读 3

自定义注解那些事

文章目录

一、自定义注解

  • Operate
package com.example.demo.enumeration;

public enum Operate {
    INSERT, DELETE, UPDATE, SELECT, OTHER;
}

  • LogPoint
package com.example.demo.annotation;

import com.example.demo.enumeration.Operate;

import java.lang.annotation.*;

@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Inherited
@Documented
public @interface LogPoint {
    /**
     * 模块
     *
     * @return 模块
     */
    String module();

    /**
     * 操作类型
     *
     * @return 操作类型
     */
    Operate type() default Operate.OTHER;
}

二、自定义注解可以被继承吗?

注解无Inherited元注解有Inherited元注解
父类注解✔️
父类方法注解-重写父类方法(含抽象类抽象方法实现)
父类方法注解-不重写父类方法✔️✔️
接口注解
接口方法注解
接口方法注解(接口默认方法default)同①情况✔️✔️

:以上能否获取到父类中的注解,指的是通过常规的方式获取,如aClass.getAnnotation(LogPoint.class)method.getAnnotation(LogPoint.class)

三、动态代理会导致自定义注解丢失吗?

注解JDK动态代理CGLIB动态代理
类注解(实现类)不管自定义注解包不包含Inherited元注解,常规方法、AnnotationUtilsAnnotatedElementUtils工具类均无法获取自定义注解。❌若自定义注解包含Inherited元注解,常规方法可获取,若不包含Inherited元注解常规方法无法获取,通过AnnotationUtilsAnnotatedElementUtils工具类可获取。✔️❌
方法注解(实现类)不管自定义注解包不包含Inherited元注解,常规方法、AnnotationUtilsAnnotatedElementUtils工具类均无法获取自定义注解。❌不管自定义注解包不包含Inherited元注解,常规方法均无法获取,通过AnnotationUtilsAnnotatedElementUtils工具类可获取。✔️❌
类注解(接口类)不管自定义注解包不包含Inherited元注解,常规方法无法获取,通过AnnotationUtilsAnnotatedElementUtils工具类可获取。✔️❌不管自定义注解包不包含Inherited元注解,常规方法无法获取,通过AnnotationUtilsAnnotatedElementUtils工具类可获取。✔️❌
方法注解(接口类)不管自定义注解包不包含Inherited元注解,常规方法无法获取,通过AnnotationUtilsAnnotatedElementUtils工具类可获取。✔️❌不管自定义注解包不包含Inherited元注解,常规方法无法获取,通过AnnotationUtilsAnnotatedElementUtils工具类可获取。✔️❌

1.获取自定义注解的可靠方式

  • 类注解
final Class<? extends TestService> aClass = testService.getClass();
final LogPoint annotation = AnnotatedElementUtils.findMergedAnnotation(aClass, LogPoint.class);
  • 方法注解
final Class<? extends TestService> aClass = testService.getClass();
final Method method = aClass.getMethod("test");
final LogPoint annotation = AnnotatedElementUtils.findMergedAnnotation(method, LogPoint.class);

2.AnnotatedElementUtils工具类源码解析

  • 调用链
  • 核心代码
	@Nullable
	private static <C, R> R processClassHierarchy(C context, int[] aggregateIndex, Class<?> source,
			AnnotationsProcessor<C, R> processor, boolean includeInterfaces, boolean includeEnclosing) {

		try {
			R result = processor.doWithAggregate(context, aggregateIndex[0]);
			if (result != null) {
				return result;
			}
			if (hasPlainJavaAnnotationsOnly(source)) {
				return null;
			}
			Annotation[] annotations = getDeclaredAnnotations(source, false);
			result = processor.doWithAnnotations(context, aggregateIndex[0], source, annotations);
			if (result != null) {
				return result;
			}
			aggregateIndex[0]++;
			if (includeInterfaces) {
				for (Class<?> interfaceType : source.getInterfaces()) {
					R interfacesResult = processClassHierarchy(context, aggregateIndex,
						interfaceType, processor, true, includeEnclosing);
					if (interfacesResult != null) {
						return interfacesResult;
					}
				}
			}
			Class<?> superclass = source.getSuperclass();
			if (superclass != Object.class && superclass != null) {
				R superclassResult = processClassHierarchy(context, aggregateIndex,
					superclass, processor, includeInterfaces, includeEnclosing);
				if (superclassResult != null) {
					return superclassResult;
				}
			}
			if (includeEnclosing) {
				// Since merely attempting to load the enclosing class may result in
				// automatic loading of sibling nested classes that in turn results
				// in an exception such as NoClassDefFoundError, we wrap the following
				// in its own dedicated try-catch block in order not to preemptively
				// halt the annotation scanning process.
				try {
					Class<?> enclosingClass = source.getEnclosingClass();
					if (enclosingClass != null) {
						R enclosingResult = processClassHierarchy(context, aggregateIndex,
							enclosingClass, processor, includeInterfaces, true);
						if (enclosingResult != null) {
							return enclosingResult;
						}
					}
				}
				catch (Throwable ex) {
					AnnotationUtils.handleIntrospectionFailure(source, ex);
				}
			}
		}
		catch (Throwable ex) {
			AnnotationUtils.handleIntrospectionFailure(source, ex);
		}
		return null;
	}

3.AnnotationUtils和AnnotatedElementUtils工具类区别

常用方法搜索范围属性合并
AnnotationUtils#getAnnotation(java.lang.reflect.AnnotatedElement, java.lang.Class)SearchStrategy.INHERITED_ANNOTATIONS(基本等同于JDK常规获取方式,但功能更强)
AnnotationUtils#findAnnotation(java.lang.Class<?>, java.lang.Class)SearchStrategy.INHERITED_ANNOTATIONS(基本等同于JDK常规获取方式,但功能更强)
AnnotatedElementUtils#getMergedAnnotationSearchStrategy.TYPE_HIERARCHY(包含父类及接口的注解)
AnnotatedElementUtils#findMergedAnnotationSearchStrategy.TYPE_HIERARCHY(包含父类及接口的注解)

:属性合并=>org.springframework.core.annotation.MergedAnnotations

   @Retention(RetentionPolicy.RUNTIME)
   @RequestMapping(method = RequestMethod.POST)
   public @interface PostMapping {
  
       @AliasFor(attribute = "path")
       String[] value() default {};
  
       @AliasFor(attribute = "value")
       String[] path() default {};
   }

四、扩展

1.Controller可增加事务注解吗?

五、参考

Java元注解与Spring组合注解使用

【小家Spring】Spring贡献的多个注解相关的工具类:AnnotationUtils、AnnotatedElementUtils、AnnotationConfigUtils…

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