●   RetentionPolicy.SOURCE : 在编译阶段丢弃。这些注解在编译结束之后就不再有任何意义，所以它们不会写入字节码。@Override, @SuppressWarnings都属于这类注解。
  
  ●   RetentionPolicy.CLASS : 在类加载的时候丢弃。在字节码文件的处理中有用。注解默认使用这种方式
  
  ●   RetentionPolicy.RUNTIME : 始终不会丢弃，运行期也保留该注解，因此可以使用反射机制读取该注解的信息。我们自定义的注解通常使用这种方式。

● ElementType.CONSTRUCTOR:用于描述构造器
  ● ElementType.FIELD:成员变量、对象、属性（包括enum实例）
  ● ElementType.LOCAL_VARIABLE:用于描述局部变量
  ● ElementType.METHOD:用于描述方法
  ● ElementType.PACKAGE:用于描述包
  ● ElementType.PARAMETER:用于描述参数
  ● ElementType.TYPE:用于描述类、接口(包括注解类型) 或enum声明

class Parent {
    public void run() {
    }
}

class Son extends Parent {
    /**
     * 这个注解是为了检查此方法是否真的是重写父类的方法
     * 这时候就不用我们用肉眼去观察到底是不是重写了
     */
    @Override
    public void run() {
    }
}

/**
 * 此注解代表过时了，但是如果可以调用到，当然也可以正常使用
 * 但是，此方法有可能在以后的版本升级中会被慢慢的淘汰
 * 可以放在类，变量，方法上面都起作用
 */
@Deprecated
public void run() {
}
}

public class JDKAnnotationDemo {
    public static void main(String[] args) {
        Parent parent = new Parent();
        parent.run(); // 在编译器中此方法会显示过时标志
    }
}

// 因为定义的 name 没有使用，那么编译器就会有警告，这时候使用此注解可以屏蔽掉警告
// 即任意不想看到的编译时期的警告都可以用此注解屏蔽掉，但是不推荐，有警告的代码最好还是处理一下
@SuppressWarnings("all")
private String name;
}

for(Annotation annotation : annotations) {
    if(annotation instanceof ApiAuthAnnotation) {
        ApiAuthAnnotation apiAuthAnnotation = (ApiAuthAnnotation) annotation;
        System.out.println("name: " + apiAuthAnnotation.name());
        System.out.println("age: " + apiAuthAnnotation.age());
    }
}
方法注解
Method method = ... //通过反射获取方法对象
Annotation[] annotations = method.getDeclaredAnnotations();

for(Annotation annotation : annotations) {
    if(annotation instanceof ApiAuthAnnotation) {
        ApiAuthAnnotation apiAuthAnnotation = (ApiAuthAnnotation) annotation;
        System.out.println("name: " + apiAuthAnnotation.name());
        System.out.println("age: " + apiAuthAnnotation.age());
    }
}

<aop:config>
    <!--定义切点，切的是我们自定义的注解-->
    <aop:pointcut id="apiAuthAnnotation" expression="@annotation(cn.caijiajia.devops.aspect.ApiAuthAnnotation)"/>
    <!--定义切面，切点是 apiAuthAnnotation，切面类即注解处理器是 apiAuthAspect，主处理逻辑在方法名为 auth 的方法中-->
    <aop:aspect ref="apiAuthAspect">
        <aop:around method="auth" pointcut-ref="apiAuthAnnotation"/>
    </aop:aspect>
</aop:config>

@Component("apiAuthAspect")
public class ApiAuthAspect {

    public Object auth(ProceedingJoinPoint pjp) throws Throwable {
        Method method = ((MethodSignature) pjp.getSignature()).getMethod();
        ApiAuthAnnotation apiAuthAnnotation = method.getAnnotation(ApiAuthAnnotation.class);
        Integer age = apiAuthAnnotation.age();
        if (age > 18) {
            return pjp.proceed();
        } else {
            throw new RuntimeException("你未满18岁，禁止访问");
        }
    }
}

 Class aClass = TheClass.class;
Annotation[] annotations = aClass.getAnnotations();

for(Annotation annotation : annotations){
    if(annotation instanceof MyAnnotation){
        MyAnnotation myAnnotation = (MyAnnotation) annotation;
        System.out.println("name: " + myAnnotation.name());
        System.out.println("value: " + myAnnotation.value());
    }
}

Class aClass = TheClass.class;
Annotation annotation = aClass.getAnnotation(MyAnnotation.class);

if(annotation instanceof MyAnnotation){
    MyAnnotation myAnnotation = (MyAnnotation) annotation;
    System.out.println("name: " + myAnnotation.name());
    System.out.println("value: " + myAnnotation.value());
}

public class TheClass {
  @MyAnnotation(name="someName",  value = "Hello World")
  public void doSomething(){}
}

Method method = ... //获取方法对象
Annotation[] annotations = method.getDeclaredAnnotations();

for(Annotation annotation : annotations){
    if(annotation instanceof MyAnnotation){
        MyAnnotation myAnnotation = (MyAnnotation) annotation;
        System.out.println("name: " + myAnnotation.name());
        System.out.println("value: " + myAnnotation.value());
    }
}

Method method = ... // 获取方法对象
Annotation annotation = method.getAnnotation(MyAnnotation.class);

if(annotation instanceof MyAnnotation){
    MyAnnotation myAnnotation = (MyAnnotation) annotation;
    System.out.println("name: " + myAnnotation.name());
    System.out.println("value: " + myAnnotation.value());
}

public class TheClass {
  public static void doSomethingElse(
        @MyAnnotation(name="aName", value="aValue") String parameter){
  }
}

Method method = ... //获取方法对象
Annotation[][] parameterAnnotations = method.getParameterAnnotations();
Class[] parameterTypes = method.getParameterTypes();

int i=0;
for(Annotation[] annotations : parameterAnnotations){
  Class parameterType = parameterTypes[i++];

  for(Annotation annotation : annotations){
    if(annotation instanceof MyAnnotation){
        MyAnnotation myAnnotation = (MyAnnotation) annotation;
        System.out.println("param: " + parameterType.getName());
        System.out.println("name : " + myAnnotation.name());
        System.out.println("value: " + myAnnotation.value());
    }
  }
}

public class TheClass {

  @MyAnnotation(name="someName",  value = "Hello World")
  public String myField = null;
}

Field field = ... //获取方法对象</pre>
<pre>Annotation[] annotations = field.getDeclaredAnnotations();

for(Annotation annotation : annotations){
 if(annotation instanceof MyAnnotation){
 MyAnnotation myAnnotation = (MyAnnotation) annotation;
 System.out.println("name: " + myAnnotation.name());
 System.out.println("value: " + myAnnotation.value());
 }
}

Field field = ...//获取方法对象</pre>
<pre>
Annotation annotation = field.getAnnotation(MyAnnotation.class);

if(annotation instanceof MyAnnotation){
 MyAnnotation myAnnotation = (MyAnnotation) annotation;
 System.out.println("name: " + myAnnotation.name());
 System.out.println("value: " + myAnnotation.value());
}

enum Complexity {
    LOW, HIGH
}

public @interface ComplexAnnotation {
    Class<? extends Object> value();

    int[] types();

    Complexity complexity();
}

public @interface FailingAnnotation {
    Object complexity();
}

@SimpleAnnotation
public class Apply {
    @SimpleAnnotation
    private String aField;

    @SimpleAnnotation
    public Apply() {
        // ...
    }
}

@SimpleAnnotation
public void aMethod(@SimpleAnnotation String param) {
    // ...
}

@SimpleAnnotation
int i = 10;

for (@SimpleAnnotation int j = 0; j < i; j++) {
    // ...
}

try (@SimpleAnnotation FileWriter writer = getWriter()) {
    // ...
} catch (Exception ex) {
    // ...
}

@SimpleAnnotation
public @interface ComplexAnnotation {
    // ...
}

@PackageAnnotation
package com.baeldung.interview.annotations;

@Target(ElementType.TYPE_USE)
public @interface SimpleAnnotation {
    // ...
}

new @SimpleAnnotation Apply();

aString = (@SimpleAnnotation String) something;

public class SimpleList<T>
  implements @SimpleAnnotation List<@SimpleAnnotation T> {
    // ...
}

void aMethod() throws @SimpleAnnotation Exception {
    // ...
}

@Target(ElementType.FIELD)
public @interface SimpleAnnotation {
    // ...
}

@Target({ ElementType.FIELD, ElementType.METHOD, ElementType.PACKAGE })

@Target({})
public @interface NoTargetAnnotation {
    // ...
}

@Target(ElementType.ANNOTATION_TYPE)
public @interface SimpleAnnotation {
    // ...
}

@Target({ ElementType.FIELD, ElementType.TYPE, ElementType.FIELD })
public @interface TestAnnotation {
    int[] value() default {};
}

@Target({ ElementType.FIELD, ElementType.TYPE})