Spring Data JPA All in One

when we call save(entity) behind the seen it call persist method of entity manager

and when we update the object it calls merge method on the entity manager.

So same save method is called for persisting and updating the object.

Here when we call save method it will call behind the seen entity manager persist method and it issues the insert query (because y persist ke saath saath sql query ko flush bhi kar deta hain so alag se entity manager dot getTransaction dot comit karne ki jarurt nahi hoti hain y automatically ho jaata hain.) in sql at runtime.

Now suppose we want to make change in returnedStudent object then we will call the save method but entity manager behind the seen will call to merge method.

You might be asking this point that why we are calling save method for updating the student ?

So let's go to the update process again and see how the merge method of the entity manager is actually doing update when save method is called. so lets say line 20 has been executed and we have persisted student object into student table. so here both the student and returnedStudent variable the both would be refering to same student object in java memory which would be in a detched state. After that we realised that we have misspelled name alisa we correct that in next time 21 by updating it using the returnedStudent variable but the object being referenced by the returned by the student variable is in the detched state so whatever changes we make to it we will not save to database automatically and will updated only in java memory its like client or user working with detched object rendered on his screen or a web page an example whatever update he makes to detached object unless he saves the detched object

When we delete a record then why an extra select sql statement get executed when we call delete method

Because before deleting student hibernate force check weather this student is exist in our database weather even we have that student in our database or not the way it does that under the hood it calls entity manager. find the student whose id is one.

Then it checks weather find method is null or not. Only if we have not null means we do have student with id one then only it deletes. So here due to this find method it fire the select query.

Spring Data Commons

If we want to delete in batch then we suffix as deleteAllInBatch (InBatch as suffix)

If our database is not a relational db then still we have Interface like JpaRepository like show in below image.

Question:

Here you might be thinking why C option is correct because sometimes we don't want to expose all the methods so we can create only the methods that we need it so we can only extend the Repository interface like shown below.

So we can create our own customer repository like this with only the methods we need it.

Derived Queries, Paging and Sorting

Question :

If all query available for spring data jpa then it will execute as follow

First Priority for @Query()

If @Query is not available then Second Priority for @NamedQuery() and if NamedQuery also not available then it will move to the drived query by method name.

Note: Never Pass String literal into your JPQL query like in @Query() always used the name parameter in the query