SpringMVC: spring mvc No converter found for return value of type: class

关于json转换器缺失的问题，报错内容：No converter found for return value of type

aop.AopInvocationException: Null return value from advice does not match primitive return type for:

Return value (true) was not iterable.

attempted to return null from a method with a primitive return type (int).

Java_procedure with return value(oracle)

【Mybatis报错】attempted to return null from a method with a primitive return type (int).

Android之提示Method return type must not include a type variable or wildcard:

message [Failed to convert property value of type

springboot 异常 - Failed to convert value of type