一、注解在类上

package annotation5;
import annotation4.*;
public class ReflectAnnotationTest {
    public static void main(String[] args) throws Exception {
        Class c =Class.forName("annotation4.AnnotationTest01");
        //判断类上面是否有@annotation4.MyAnnotation
        //System.out.println(c.isAnnotationPresent(MyAnnotation.class));
        if (c.isAnnotationPresent(MyAnnotation.class)){
            MyAnnotation myAnnotation = (MyAnnotation) c.getAnnotation(MyAnnotation.class);
            //获取注解对象的属性
            String value = myAnnotation.value();
            System.out.println(value);
        }
        //判断String类上面是否存在注解
        Class StringClass = Class.forName("java.lang.String");
        System.out.println(StringClass.isAnnotationPresent(MyAnnotation.class));
    }
}

二、注解在类方法中

package annotation6;
import java.lang.reflect.Method;
public class MyAnnotationTest {
    @MyAnnotation(username = "admin",password = "123")
    public void doSome(){
    }
    public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException {
        Class c = Class.forName("annotation6.MyAnnotationTest");
        //获取doSome方法
        Method doSomeMethord =c.getDeclaredMethod("doSome");
        if (doSomeMethord.isAnnotationPresent(MyAnnotation.class)){
            MyAnnotation myAnnotation = doSomeMethord.getAnnotation(MyAnnotation.class);
            System.out.println(myAnnotation.username());
            System.out.println(myAnnotation.password());
        }
    }
}

三、注解的应用

package annotation7;
import java.lang.reflect.Field;
public class Test {
    public static void main(String[] args) throws Exception{
        //获取类
        Class userClass = Class.forName("annotation7.User");
        if (userClass.isAnnotationPresent(Id.class)){
            Field[] fields = userClass.getDeclaredFields();
            boolean isOk = false;
            for (Field field : fields){
                if ("id".equals(field.getName()) && "int".equals(field.getType().getSimpleName())){
                    isOk = true;
                    break;
                }
            }
            //判断是否合法
            if (!isOk){
                throw new HasNotIdPropertyException("被Id注解标注的类中必须要有一个int类型的id属性");
            }
        }
    }
}

package annotation7;
@Id
public class User {
    String name;
}

package annotation7;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface Id {
}

package annotation7;
public class HasNotIdPropertyException extends RuntimeException {
    public HasNotIdPropertyException() {
    }
    public HasNotIdPropertyException(String message) {
        super(message);
    }
}