spring 方法拦截器从service层的方法上获取事务注解为什么是null

网上很多文档说：

Transactional tx = invocation.getMethod().getAnnotation(Transactional.class);

获取的注解tx=null;

解决办法是

Method realMethod = invocation.getThis().getClass().getDeclaredMethod(invocation.getMethod().getName(),invocation.getMethod().getParameterTypes());//        Transactional tx1 = realMethod.getAnnotation(Transactional.class);//tx1 = null

但是我实践的结果是相反的，请问为什么呢？

下面是我的代码

package com.yh.consumerfinance.pmc.omc.interceptor;

import org.aopalliance.intercept.MethodInterceptor;
import org.aopalliance.intercept.MethodInvocation;
import org.springframework.transaction.annotation.Transactional;

/**
 * @Description: Description
 * @Author: 张颖辉(yh)
 * @CreateDate: 2020/12/25 13:51
 * @UpdateUser: 张颖辉(yh)
 * @UpdateDate: 2020/12/25 13:51
 * @UpdateRemark: The modified content
 * @Version: 1.0
 */
public class ReadWriteInterceptor implements MethodInterceptor{
    

    public ReadWriteInterceptor() {
    }

    public Object invoke(MethodInvocation invocation) throws Throwable {
        String methodName = invocation.getMethod().getName();
        // 下面两种方法  获取方法上的注解都是null
//        Method method = invocation.getThis().getClass().getMethod(methodName, invocation.getMethod().getParameterTypes());
//        Method realMethod = invocation.getThis().getClass().getDeclaredMethod(invocation.getMethod().getName(),invocation.getMethod().getParameterTypes());
//        Transactional tx0 = method.getAnnotation(Transactional.class); //tx0 = null
//        Transactional tx1 = realMethod.getAnnotation(Transactional.class);//tx1 = null
 // 下面 方法  获取方法上的注解不是null
        Transactional tx = invocation.getMethod().getAnnotation(Transactional.class);
        if (tx == null) {
            tx = invocation.getThis().getClass().getAnnotation(Transactional.class);
        }

        if (tx != null && tx.readOnly()) {
            //
        } else {
           //
        }

        Object var5;
        try {
            var5 = invocation.proceed();
        } catch (Throwable var9) {
            throw var9;
        } finally {
            //
        }

        return var5;
    }

}