spring-data-demo/projections/README.md

projections

Companion code for Spring Data JPA Projections: Interface, Record and Dynamic Projections (With the SQL They Generate) on ankurm.com.

Component	Version
Spring Boot	4.1.2
Spring Data JPA	4.1.2
Hibernate ORM	7.4.13.Final
H2 (in-memory)	2.4.240
JDK	Temurin 25.0.5 LTS

No Docker needed. Every SQL statement is captured by SqlRecorder, a Hibernate StatementInspector, and every test writes the SQL it saw to output/ while asserting the same facts, so the numbers in the post fail the build if they stop being true.

./scripts/run-all.sh      # regenerates every file in output/

Source file	What it shows
EmployeeSummary	closed interface projection
EmployeeFullName	open interface projection (@Value SpEL)
EmployeeFullNameClosed	the fix for the open projection: a default method
EmployeeWithDepartment	nested interface projection
EmployeeDto	record DTO projection
DepartmentPay	target of a JPQL constructor expression
EmployeeRepository	every query-method form, numbered 1 to 14
SqlRecorder	the SQL capture mechanism

Transcript	Shows
output/01-entity-vs-closed-vs-dto.txt	7 columns vs 2 columns, managed entities 4 vs 0
output/02-open-projection-trap.txt	an open projection selects all 7 columns
output/11-default-method-fix.txt	a default method keeps the projection closed (2 columns)
output/03-nested-projection.txt	nested projection pulls the whole department row
output/04-dynamic-projection.txt	one method, three shapes
output/05-query-forms.txt	@Query aliases, constructor expression, native query
output/06-pitfall-entity-query.txt	select e declared as an interface projection loads everything
output/07-pitfall-missing-aliases.txt	tuple select without aliases returns nulls
output/08-pitfall-unknown-property.txt	unknown projection property
output/09-paging-and-proxy.txt	paging runs a count query; projections are JDK proxies
output/10-entity-query-as-dto-rewrite.txt	select e declared as a record is rewritten to 2 columns

There is no docs/ folder: the explanations live in the post.