在整个spring框架中,提供的注解非常的多,这些注解简化了我们的很多操作。那么,我们如何自定义注解呢?
第一步:学习已有的注解
@Target({ElementType.METHOD, ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Mapping
public @interface RequestMapping {
String name() default "";
@AliasFor("path")
String[] value() default {};
@AliasFor("value")
String[] path() default {};
RequestMethod[] method() default {};
String[] params() default {};
String[] headers() default {};
String[] consumes() default {};
String[] produces() default {};
}
这是RequestMapping的注解,这个注解里包含了4个其他的注解,我大致介绍一下(其实源码里面说的很清楚):
1.@Target
定义注解修饰的目标,比如@RequestMapping
就是用来修饰方法和类
2.@Retention
定义注解的生命周期,分为以下三种:
/*源码级别*/
// Annotations are to be discarded by the compiler.
SOURCE,
/*编译期级别*/
/*Annotations are to be recorded in the class file by the compiler,but need not be retained by the VM at run time. This is the defaultbehavior.*/
CLASS,
/*运行期级别*/
/*Annotations are to be recorded in the class file by the compiler and retained by the VM at run time, so they may be read reflectively.
RUNTIME
3.@Documented
定义注解会被javadoc或者其他类似工具文档化
4.@Mapping
定义注解是一个web mapping annotation
第二步:创建自定义注解
@Target({ElementType.METHOD, ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface MyFirstAnnotation {
String value() default "";
}
第三步:定义切面类
创建完自定义注解后,很显然的思路是如何让注解起作用。这里以输出日志的注解为例,当用自定义注解来修饰方法时,我们期望在方法执行的前后输出日志记录,那么我们必须采用AOP(面向切面编程)的思想,理所当然地,我们需要定义切面类:
@Aspect
@Component
public class MyFirstAspect {
@Pointcut("@annotation(MyFirstAnnotation)")
public void annotationPointcut() {
}
@Before("annotationPointcut()")
public void beforePointcut(JoinPoint joinPoint) {
MethodSignature methodSignature = (MethodSignature) joinPoint.getSignature();
Method method = methodSignature.getMethod();
MyFirstAnnotation annotation = method.getAnnotation(MyFirstAnnotation.class);
String value = annotation.value();
System.out.println("准备"+value);
}
@After("annotationPointcut()")
public void afterPointcut(JoinPoint joinPoint) {
MethodSignature methodSignature = (MethodSignature) joinPoint.getSignature();
Method method = methodSignature.getMethod();
MyFirstAnnotation annotation = method.getAnnotation(MyFirstAnnotation.class);
String value = annotation.value();
System.out.println("结束"+value);
}
}
重点需要关注的是:切点的定义,切点可以定义成execute(public String sayHello())
的形式,但是这种形式就和咱们的注解不相关了,因此我们采用@annotation(MyFirstAnnotation)
的形式,这样切点就变成了我们自定义注解所修饰的方法
第四步:使用自定义注解
@MyFirstAnnotation("吃饭")
@RequestMapping(value = "/say")
public String sayHello() {
System.out.println("吃饭");
return "hello spring boot";
}
控制台结果输出:
准备吃饭
吃饭
结束吃饭