JPA/JPQL 自动方法查询生成

Question

I have a co-worker that just blew my mind. We have a Java 11/Spring Boot/Hibernate/JPA app talking to a MySQL DB. Apparently JPA JPQL (or something similar to that) is capable of -- but only if you write the repository methods correctly -- building out queries based on your method name.

So for instance if we have a JPA entity:

@Entity
@Table(name = "accounts")
@Data
public class Account {
    @Column(name = "account_email")
    private String email;

    // ... many more fields down here
}

And then a repository for it:

@Repository
public interface AccountRepository extends JpaRepository<Account,Long> {
    @Query("FROM Account WHERE email = :email")
    Account findByEmail(@Param(value = "email") String email);
}

Apparently (and this might be a bad example) I could just simplify that to:

@Repository
public interface AccountRepository extends JpaRepository<Account,Long> {
    Account findByEmail(String email);
}

And JPA/JPQL will figure out that since I want to " findByEmail " and Account#email exists, it just wants me to do a SELECT * FROM accounts where email = ? . Amazing!

The only problem is: I don't see this documented anywhere well , and I don't see it documented anywhere officially . There's a few old blogs that I was able to find that insinuate the same things, but nowhere official (JPA docs, JPQL docs, etc.) that go into detail as to how it works and what its limitations are.

Can anyone point me in the right direction? What is this mysterious feature/technology called and what are its limitations/capabilities? Can it only work on SELECTs or can it handle inserts/updates/deletes as well?

Answer 1

This is part of the Spring Data support for JPA. You can find more info at the documentation Query Methods and all the supported query-keywords in the appendix section. Here is an excerpt from the documentation:

Query subject keywords

Keyword	Description
find…By, read…By, get…By, query…By, search…By, stream…By	General query method returning typically the repository type, a Collection or Streamable subtype or a result wrapper such as Page, GeoResults or any other store-specific result wrapper. Can be used as findBy…, findMyDomainTypeBy… or in combination with additional keywords.
exists…By	Exists projection, returning typically a boolean result.
count…By	Count projection returning a numeric result.
delete…By, remove…By	Delete query method returning either no result (void) or the delete count.
…First…, …Top…	Limit the query results to the first of results. This keyword can occur in any place of the subject between find (and the other keywords) and by.
…Distinct…	Use a distinct query to return only unique results. Consult the store-specific documentation whether that feature is supported. This keyword can occur in any place of the subject between find (and the other keywords) and by.

Query predicate keywords

Logical keyword	Keyword expressions
AND	And
OR	Or
AFTER	After, IsAfter
BEFORE	Before, IsBefore
CONTAINING	Containing, IsContaining, Contains
BETWEEN	Between, IsBetween
ENDING_WITH	EndingWith, IsEndingWith, EndsWith
EXISTS	Exists
FALSE	False, IsFalse
GREATER_THAN	GreaterThan, IsGreaterThan
GREATER_THAN_EQUALS	GreaterThanEqual, IsGreaterThanEqual
IN	In, IsIn
IS	Is, Equals, (or no keyword)
IS_EMPTY	IsEmpty, Empty
IS_NOT_EMPTY	IsNotEmpty, NotEmpty
IS_NOT_NULL	NotNull, IsNotNull
IS_NULL	Null, IsNull
LESS_THAN	LessThan, IsLessThan
LESS_THAN_EQUAL	LessThanEqual, IsLessThanEqual
LIKE	Like, IsLike
NEAR	Near, IsNear
NOT	Not, IsNot
NOT_IN	NotIn, IsNotIn
NOT_LIKE	NotLike, IsNotLike
REGEX	Regex, MatchesRegex, Matches
STARTING_WITH	StartingWith, IsStartingWith, StartsWith
TRUE	True, IsTrue
WITHIN	Within, IsWithin

Answer 2

Not much of a mystery and quite well documented to be perfectly honest. What you mention is called a derived query which basically infers the JPQL query from the method name. A simple Google search can turn up quite some documentation of this feature. Also the Spring Data JPA documentation clearly documents this and well as its usage. Check here for more.