leejunkim

QueryDSL

2 min read

Overview

A framework that allows you to construct type-safe queries in Java

  • It generates metamodel classes (Q-classes) from your entities, enabling compile-time checking of queries instead of runtime errors with string-based queries.

The advantages

  • Type safety
    • At compile time, it tracks the classes annotated with @Entity and makes a QClass for each (contains their meta data)
    • The types for the fields
      • Integer → NumberPath
      • String → StringPath
      • The *Path extends the *Expression abstract class
        • The *Expression abstract class implements Expression interface
          • This abstract class contains specific methods you can use to calculate, like NumberExpression abstract class contains max(), min(), random(), etc
        • Ex) NumberPath extends the NumberExpression abstract class, which implements Expression interface
  • Can reuse repeated conditionals
    • Entity conditionals are expressed by the Expression interface, and the JPQL creation and execution are by the JPAQueryFactory
      • They are separated, unlike traditional methods
    • So you can make different conditional methods A, B, C, and use them however u want to make a query!!
  • Clean static queries (깔끔한 동적 쿼리)
where (
	BooleanExpression1,
	BooleanExpression2,
	BooleanExpression3
)
  • If any BooleanExpression is null, it is just ignored like it didn’t exist!!

Quick Example

BooleanExpression isSameMemberId(Long memberId) {
	if (memberId == null) return null;
	return reservation.member.id.eq(memberId); // checking if it's equal with eq
}
 
BooleanExpression isSameThemeId(Long themeId) {
	if (themeId == null) return null;
	return reservation.reservationSlot.theme.eq(themeId);
}
 
// ============= query result!!
return jpaQueryFactory.selectFrom(reservation)
	.join(reservation.member, member)
	.fetchJoin()
	.where(
		isSameMemberId(memberId), // 에약자
		isSameThemeId(themeId), // 테마
		greaterThanEqualDate(startDate), // 시작날짜
		lessThanEqualDate(endDate) // 종료날짜
	)
	.fetch();

Using with JPARepository

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