Record the nth SQL exception

Record No. N Time SQL abnormal

The most important sentence ：Mapper method 'com.handsome.dao.BlogMapper.countBlogViews attempted to return null from a method with a primitive return type (int).

The expected value is int type , The type of return is null, I'm sorry , direct GG. Why did you report this error ？

Mapper How to write the interface

Service Interface writing

ServiceImpl Implementation class writing

Expected return values are int Basic data type , But back null, Result in an error , therefore SQL Methods of should be changed to object type .

After modification Mapper How to write the interface

After modification Service Interface writing

After modification ServiceImpl Implementation class writing

Integer The object type allows the return value to be null, Solve the problem successfully .

In high performance MySQL It is recommended to set a default value for the field as far as possible , Try not to leave the field value blank , Avoid all kinds of things related to null It's abnormal , Unless there is a special need .

Small about entity classes bug

If the attribute of the entity class is defined by the basic data type , Updated in the database , Will not be null The value of is updated into the database . for example ：int The defined value defaults to 0 , When the database is updated 0 Update into the database , Cause data statistics error .

Entity classes should use int The wrapper class Integer,Ingeter yes int The wrapper class ,int The initial value for the 0,Ingeter The initial value for the null, This field will not be updated when updating , Avoided bug Appearance .

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