JPQL, Criteria, and Querydsl

Start with the smallest query form

Use findAs for an ID, typed JPQL for a fixed query, and Criteria or Querydsl when conditions must be composed.

val active = entityManager
    .createQueryAs<Account>("select a from Account a where a.active = true")
    .setPaging(0, 50)
    .resultList

findOneOrNull converts only NoResultException to null. Non-unique and database failures still propagate. Natural-ID helpers support one @NaturalId or a non-empty map of composite attributes.

Criteria helpers

val cb = entityManager.criteriaBuilder
val query = cb.createQueryAs<Account>()
val root = query.from<Account>()
query.select(root).where(cb.eq(root.attribute(Account::active), true))

attribute uses the Kotlin property name as the JPA path. It reduces strings but is not a generated metamodel check.

Querydsl and projections

Querydsl fits joins, subqueries, projections, and reusable predicates. Q types still require annotation processing. Select constructor, bean, or @QueryProjection mapping based on compile-time checks and coupling.

Query shape	Start with
ID or natural ID	reified lookup helper
Short fixed query	typed JPQL
JPA-standard dynamic query	Criteria
Kotlin joins and projections	Querydsl
Entity Views and keyset pages	Blaze-Persistence demo

Avoid deprecated Querydsl fetchCount() and fetchResults() shortcuts for complex queries; use an explicit count or Blaze-Persistence page metadata.

Executable examples

./gradlew :bluetape4k-hibernate:test --tests '*SimpleQuerydslExamples'
./gradlew :bluetape4k-examples-jpa-querydsl-demo:test
./gradlew :bluetape4k-examples-jpa-blazepersistence-demo:test

Sources and tests

• CriteriaSupport.kt
• TypedQuerySupport.kt
• SimpleQuerydslExamples.kt
• NaturalIdTest.kt