注解编程 之 注解合并
组合注解
Spring4.2之后，就提供了组合注解的实现方式，就是将多个注解作用于一个注解，用一个注解就和依赖实现多个注解的功能。是作用的注解元素看上去更简洁美观，更强大之处是属性覆盖功能。
例如：Spring的@RestController，它将@ResponseBody和@Controller两个注解组合为一个，那么在Controller类上只需加上@RestController即可实现加两个注解才能实现的功能。
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@Target(ElementType.TYPE)
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@Retention(RetentionPolicy.RUNTIME)
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@Documented
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@Controller //组合Controller使其实现Bean注册
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@ResponseBody //组合ResponseBody使其支持将结果转化为json
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public @interface RestController {
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/**
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* The value may indicate a suggestion for a logical component name,
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* to be turned into a Spring bean in case of an autodetected component.
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* @return the suggested component name, if any (or empty String otherwise)
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* @since 4.0.1
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*/
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@AliasFor(annotation = Controller.class)
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String value() default "";
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}
自定义注解
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public class SelfAnnotationTest {
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@Target({ ElementType.ANNOTATION_TYPE, ElementType.FIELD, ElementType.TYPE })
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@Retention(RetentionPolicy.RUNTIME)
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@interface TestOne {
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String testOne() default "testOne";
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}
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@Target({ ElementType.ANNOTATION_TYPE, ElementType.FIELD, ElementType.TYPE })
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@Retention(RetentionPolicy.RUNTIME)
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@interface TestTwo {
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String testTwo() default "testTwo";
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}
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@TestTwo
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@Target({ ElementType.ANNOTATION_TYPE, ElementType.FIELD, ElementType.TYPE })
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@Retention(RetentionPolicy.RUNTIME)
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@interface TestThree {
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String testThree() default "testThree";
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}
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@TestThree
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static class Element {}
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public static void main(String[] args) {
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TestTwo testTwo = AnnotatedElementUtils.getMergedAnnotation(Element.class, TestTwo.class);
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TestThree testThree = AnnotatedElementUtils.getMergedAnnotation(Element.class, TestThree.class);
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System.out.println(testTwo);
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System.out.println(testThree);
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}
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}
输出：
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@com.example.helloworld.annotation.SelfAnnotationTest$TestTwo(testTwo=testTwo)
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@com.example.helloworld.annotation.SelfAnnotationTest$TestThree(testThree=testThree)
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Process finished with exit code 0
可以看出，AnnotatedElemnetUtils.getMergedAnnotation()方法可以返回组合注解本身，及此注解上的元注解。
自定义注解合并
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@TestOne
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@TestTwo
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@Target({ ElementType.ANNOTATION_TYPE, ElementType.FIELD, ElementType.TYPE })
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@Retention(RetentionPolicy.RUNTIME)
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@interface TestThree { // 合并 @TestOne 及 @TestTwo 注解
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String testThree() default "testThree";
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String testTwo(); // 自动对应 @TestTwo 中的属性
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String testOne(); // 自动对应 @TestOne 中的属性
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}
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@TestThree(testTwo = "test2", testOne = "test1")
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static class Element {}
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public static void main(String[] args) {
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TestTwo testTwo = AnnotatedElementUtils.getMergedAnnotation(Element.class, TestTwo.class);
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System.out.println(testTwo.testTwo());
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TestThree testThree = AnnotatedElementUtils.getMergedAnnotation(Element.class, TestThree.class);
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System.out.println(testThree.testThree());
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TestOne testOne = AnnotatedElementUtils.getMergedAnnotation(Element.class, TestOne.class);
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System.out.println(testOne.testOne());
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}
输出：
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test2
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testThree
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test1
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Process finished with exit code 0
组合注解实现属性值覆盖
Spring组合注解中的属性覆盖功能，即更底层的注解属性方法覆盖高层次注解的属性方法。实现该功能需要Spring提供的另外一个注解@AliasFor配合完成。
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public class SelfAnnotationTest {
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@Target({ ElementType.ANNOTATION_TYPE, ElementType.FIELD, ElementType.TYPE })
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@Retention(RetentionPolicy.RUNTIME)
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@interface TestOne {
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String testOne1() default "testOne";
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}
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@Target({ ElementType.ANNOTATION_TYPE, ElementType.FIELD, ElementType.TYPE })
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@Retention(RetentionPolicy.RUNTIME)
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@TestOne
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@interface TestTwo {
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String testTwo2() default "testTwo";
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}
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@Target({ ElementType.ANNOTATION_TYPE, ElementType.FIELD, ElementType.TYPE })
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@Retention(RetentionPolicy.RUNTIME)
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@TestTwo
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@interface TestThree {
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@AliasFor(annotation = TestTwo.class, attribute = "testTwo2")
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String testThree() default "testThree";
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@AliasFor(annotation = TestOne.class, attribute = "testOne1")
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String testThree2() default "testThree2";
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}
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@TestThree(testThree = "testThree 覆盖了 testTwo2", testThree2 = "testThree2 覆盖了 testOne1")
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static class Element {}
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public static void main(String[] args) {
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TestTwo testTwo = AnnotatedElementUtils.getMergedAnnotation(Element.class, TestTwo.class);
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System.out.println(testTwo.testTwo2());
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TestOne testOne = AnnotatedElementUtils.getMergedAnnotation(Element.class, TestOne.class);
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System.out.println(testOne.testOne1());
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}
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}
输出：
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testThree 覆盖了 testTwo2
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testThree2 覆盖了 testOne1
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Process finished with exit code 0
以上为三层属性覆盖，支持无限层覆盖。