Add hibernate-jpa module: 40 runnable Hibernate 7 / JPA interview answers on PostgreSQL 16

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Cfo2K4u4rF18QsPXPFnytf

hibernate-jpa/output/q01-jpa-vs-hibernate.txt

@@ -0,0 +1,4 @@
+Session is an EntityManager      : true
+SessionFactory is an EMF         : true
+em.unwrap(Session.class) == em   : true
+Hibernate-only API reachable     : s.getStatistics().isStatisticsEnabled() = true

hibernate-jpa/output/q02-entity-states.txt

@@ -0,0 +1,5 @@
+new Employee()      : managed=false id=null
+after persist()     : managed=true id assigned=true
+after detach()      : managed=false
+after merge()       : original managed=false, returned copy managed=true, same object=false
+after remove()      : managed=false

hibernate-jpa/output/q03-persist-vs-merge.txt

@@ -0,0 +1,3 @@
+persist(detached) -> jakarta.persistence.EntityExistsException: Detached entity passed to persist: lab.hibernate.model.Employee
+merge(detached)   -> returns a different instance: true
+salary changed on the original after merge is stored: false (db has 100)

hibernate-jpa/output/q04-save-update-removed.txt

@@ -0,0 +1,2 @@
+methods present on org.hibernate.Session : [find, get, getReference, load, merge, persist, refresh, remove]
+methods NOT present                       : [delete, save, saveOrUpdate, update]

hibernate-jpa/output/q05-find-vs-getreference.txt

@@ -0,0 +1,7 @@
+getReference(): SQL statements so far = 0, initialized = false
+ref.name (field) -> null; SQL statements now = 0, initialized = false
+ref.getName()    -> Chen; SQL statements now = 1, initialized = true
+SQL> select e1_0.id,e1_0.city,e1_0.zip,e1_0.department_id,e1_0.level,e1_0.name,e1_0.salary from Employee e1_0 where e1_0.id=?
+find(missing id)       -> null
+getReference(missing)  -> proxy returned without error: true
+first access           -> org.hibernate.ObjectNotFoundException: No row with the given identifier exists for entity [lab.hibernate.model.Employee with id '99999']

hibernate-jpa/output/q06-dirty-checking-flush.txt

@@ -0,0 +1,8 @@
+change a managed entity, no save call:
+SQL> select e1_0.id,e1_0.city,e1_0.zip,e1_0.department_id,e1_0.level,e1_0.name,e1_0.salary from Employee e1_0 where e1_0.id=?
+SQL> update Employee set city=?,zip=?,department_id=?,level=?,name=?,salary=? where id=?
+count visible inside the transaction (AUTO flush before the query): 2
+persist then query:
+SQL> select nextval('Employee_SEQ')
+SQL> insert into Employee (city,zip,department_id,level,name,salary,id) values (?,?,?,?,?,?,?)
+SQL> select count(e1_0.id) from Employee e1_0

hibernate-jpa/output/q07-merge-vs-refresh.txt

@@ -0,0 +1,2 @@
+refresh(): pending in-memory change thrown away, salary = 100
+merge(detached with salary 500) -> database now has 500

hibernate-jpa/output/q08-first-level-cache.txt

@@ -0,0 +1,5 @@
+two find() calls, same session: SQL selects = 1, same instance = true
+after clear(): SQL selects = 2, same instance as before = false
+SQL> select e1_0.id,e1_0.city,e1_0.zip,e1_0.department_id,e1_0.level,e1_0.name,e1_0.salary from Employee e1_0 where e1_0.id=?
+SQL> select e1_0.id,e1_0.city,e1_0.zip,e1_0.department_id,e1_0.level,e1_0.name,e1_0.salary from Employee e1_0 where e1_0.id=?
+a different session: SQL selects = 1 (the first-level cache is per session)

hibernate-jpa/output/q09-no-arg-constructor.txt

@@ -0,0 +1,4 @@
+SessionFactory started with an entity that has no no-arg constructor: true
+persist(): ok
+find() -> org.hibernate.InstantiationException: No default constructor for entity 'lab.hibernate.model.BadEntity'
+root cause -> org.hibernate.InstantiationException: No default constructor for entity 'lab.hibernate.model.BadEntity'

hibernate-jpa/output/q10-identity-vs-sequence-batching.txt

@@ -0,0 +1,2 @@
+SEQUENCE: 25 entities -> 1 prepared insert statement(s) with jdbc.batch_size=10, 2 sequence selects
+IDENTITY: 25 entities -> 25 prepared insert statements (one per row)

hibernate-jpa/output/q11-embeddable-columns.txt

@@ -0,0 +1 @@
+columns of table employee: [city, department_id, id, level, name, salary, zip]

hibernate-jpa/output/q12-enum-ordinal-vs-string.txt

@@ -0,0 +1,2 @@
+@Enumerated(STRING)  stores LEAD as: LEAD
+@Enumerated (default) stores LEAD as: 2  (its position in the enum: [JUNIOR, SENIOR, LEAD])

hibernate-jpa/output/q13-element-collection-table.txt

@@ -0,0 +1 @@
+tables: [comment, post, post_tags]

hibernate-jpa/output/q14-inheritance-strategies.txt

@@ -0,0 +1,5 @@
+tables: [bankpayment, cardpayment, payment, shape]
+JOINED     'from Payment' returned 2 rows
+SINGLE_TABLE 'from Shape' returned 2 rows
+SQL> select p1_0.id,case when p1_2.id is not null then 1 when p1_1.id is not null then 2 end,p1_0.amount,p1_1.iban,p1_2.last4 from Payment p1_0 left join BankPayment p1_1 on p1_0.id=p1_1.id left join CardPayment p1_2 on p1_0.id=p1_2.id
+SQL> select s1_0.id,s1_0.kind,s1_0.label,s1_0.radius,s1_0.side from Shape s1_0

hibernate-jpa/output/q15-mapped-superclass.txt

@@ -0,0 +1,3 @@
+tables: [note]
+select from the entity Note: 1 row(s), created_by column inherited = system
+select from the @MappedSuperclass -> org.hibernate.query.sqm.UnknownEntityException: Could not resolve root entity 'Auditable'

hibernate-jpa/output/q16-equals-hashcode-generated-id.txt

@@ -0,0 +1,2 @@
+before persist: id=null, set.contains(item)=true
+after persist : id assigned=true, hashCode changed, set.contains(item)=false

hibernate-jpa/output/q17-default-fetch-types.txt

@@ -0,0 +1,7 @@
+default of fetch() on each JPA annotation, read from the API jar:
+  @Basic             -> EAGER
+  @ManyToOne         -> EAGER
+  @OneToOne          -> EAGER
+  @OneToMany         -> LAZY
+  @ManyToMany        -> LAZY
+  @ElementCollection -> LAZY

hibernate-jpa/output/q18-owning-side.txt

@@ -0,0 +1,2 @@
+employee.department_id after d.employees.add(e) only: null
+after setting employee.department too            : 1

hibernate-jpa/output/q19-n-plus-one.txt

@@ -0,0 +1,7 @@
+naive loop over 3 departments: 4 SQL statements (1 + 3)
+SQL> select d1_0.id,d1_0.name from Department d1_0
+SQL> select e1_0.department_id,e1_0.id,e1_0.city,e1_0.zip,e1_0.level,e1_0.name,e1_0.salary from Employee e1_0 where e1_0.department_id=?
+SQL> select e1_0.department_id,e1_0.id,e1_0.city,e1_0.zip,e1_0.level,e1_0.name,e1_0.salary from Employee e1_0 where e1_0.department_id=?
+SQL> select e1_0.department_id,e1_0.id,e1_0.city,e1_0.zip,e1_0.level,e1_0.name,e1_0.salary from Employee e1_0 where e1_0.department_id=?
+with join fetch: 1 SQL statement
+SQL> select distinct d1_0.id,e1_0.department_id,e1_0.id,e1_0.city,e1_0.zip,e1_0.level,e1_0.name,e1_0.salary,d1_0.name from Department d1_0 join Employee e1_0 on d1_0.id=e1_0.department_id

hibernate-jpa/output/q20-join-fetch-vs-entity-graph.txt

@@ -0,0 +1,4 @@
+join fetch   : department initialized = true
+SQL> select e1_0.id,e1_0.city,e1_0.zip,d1_0.id,d1_0.name,e1_0.level,e1_0.name,e1_0.salary from Employee e1_0 join Department d1_0 on d1_0.id=e1_0.department_id
+entity graph : department initialized = true
+SQL> select e1_0.id,e1_0.city,e1_0.zip,d1_0.id,d1_0.name,e1_0.level,e1_0.name,e1_0.salary from Employee e1_0 left join Department d1_0 on d1_0.id=e1_0.department_id

hibernate-jpa/output/q21-batch-fetch-size.txt

@@ -0,0 +1,4 @@
+default_batch_fetch_size=2, 3 departments: 3 SQL statements (1 + ceil(3/2))
+SQL> select d1_0.id,d1_0.name from Department d1_0
+SQL> select e1_0.department_id,e1_0.id,e1_0.city,e1_0.zip,e1_0.level,e1_0.name,e1_0.salary from Employee e1_0 where e1_0.department_id = any (?)
+SQL> select e1_0.department_id,e1_0.id,e1_0.city,e1_0.zip,e1_0.level,e1_0.name,e1_0.salary from Employee e1_0 where e1_0.department_id=?

hibernate-jpa/output/q22-lazy-initialization-exception.txt

@@ -0,0 +1,3 @@
+read a lazy collection after the session closed -> org.hibernate.LazyInitializationException: Cannot lazily initialize collection of role 'lab.hibernate.model.Department.employees' with key '1' (no session)
+cure 1: fetch it inside the query -> 3 employees, 1 SQL statement
+cure 2: ask for exactly the columns you need -> [E11, E12, E13], 1 SQL statement

hibernate-jpa/output/q23-multiple-bag-fetch.txt

@@ -0,0 +1,2 @@
+two List collections in one join fetch -> java.lang.IllegalArgumentException: org.hibernate.loader.MultipleBagFetchException: cannot simultaneously fetch multiple bags: [lab.hibernate.model.Post.comments, lab.hibernate.model.Post.tags]
+fix: one collection per query -> comments=2 tags=2, 2 SQL statements

hibernate-jpa/output/q24-pagination-join-fetch.txt

@@ -0,0 +1,6 @@
+join fetch of a collection + setMaxResults(2) -> 2 departments
+SQL contains a row limit: true
+SQL> select d1_0.id,e1_0.department_id,e1_0.id,e1_0.city,e1_0.zip,e1_0.level,e1_0.name,e1_0.salary,d1_0.name from (select d1_0.id,d1_0.name from Department d1_0 where exists(select 1 from Employee e1_0 where d1_0.id=e1_0.department_id) order by d1_0.id offset ? rows fetch first ? rows only) d1_0(id,name) join Employee e1_0 on d1_0.id=e1_0.department_id order by d1_0.id
+two-step fix -> 2 departments
+SQL> select d1_0.id from Department d1_0 order by d1_0.id fetch first ? rows only
+SQL> select distinct d1_0.id,e1_0.department_id,e1_0.id,e1_0.city,e1_0.zip,e1_0.level,e1_0.name,e1_0.salary,d1_0.name from Department d1_0 join Employee e1_0 on d1_0.id=e1_0.department_id where d1_0.id in (?,?) order by d1_0.id

hibernate-jpa/output/q25-cascade-transient.txt

@@ -0,0 +1,2 @@
+persist Employee that points at an unsaved Department -> java.lang.IllegalStateException: org.hibernate.TransientPropertyValueException: Persistent instance of 'lab.hibernate.model.Employee' references an unsaved transient instance of 'lab.hibernate.model.Department' (persist the transient instance before flushing) [lab.hibernate.model.Employee.department -> lab.hibernate.model.Department]
+Post -> comments has CascadeType.ALL: comments stored = 1

hibernate-jpa/output/q26-orphan-removal.txt

@@ -0,0 +1,9 @@
+removed a line from the collection only:
+SQL> select po1_0.id,po1_0.customer from PurchaseOrder po1_0 where po1_0.id=?
+SQL> select l1_0.order_id,l1_0.id,l1_0.sku from OrderLine l1_0 where l1_0.order_id=?
+SQL> delete from OrderLine where id=?
+removed the order (cascade ALL):
+SQL> select po1_0.id,po1_0.customer from PurchaseOrder po1_0 where po1_0.id=?
+SQL> select l1_0.order_id,l1_0.id,l1_0.sku from OrderLine l1_0 where l1_0.order_id=?
+SQL> delete from OrderLine where id=?
+SQL> delete from PurchaseOrder where id=?

hibernate-jpa/output/q27-eager-many-to-one.txt

@@ -0,0 +1,6 @@
+'from EagerEmployee' with the default @ManyToOne: 4 SQL statements for 9 employees in 3 departments
+SQL> select ee1_0.id,ee1_0.department_id,ee1_0.name from EagerEmployee ee1_0
+SQL> select d1_0.id,d1_0.name from Department d1_0 where d1_0.id=?
+SQL> select d1_0.id,d1_0.name from Department d1_0 where d1_0.id=?
+SQL> select d1_0.id,d1_0.name from Department d1_0 where d1_0.id=?
+'from Employee' with @ManyToOne(fetch = LAZY): 1 SQL statement

hibernate-jpa/output/q28-second-level-cache.txt

@@ -0,0 +1,6 @@
+right after persist() the row is already in L2 (READ_WRITE caches on insert): SQL selects = 0, hits = 1
+after evictAllRegions(), three find() calls, three different sessions:
+  SQL selects         : 1
+  L2 cache hits       : 2
+  L2 cache misses     : 1
+  L2 cache puts       : 1

hibernate-jpa/output/q29-query-cache.txt

@@ -0,0 +1,5 @@
+run 1: query cache hits=0 misses=1 puts=1
+run 2: query cache hits=1 misses=1 puts=1
+SQL selects for Product after 2 runs: 1
+run 3 after inserting a Product: query cache hits=1 misses=2
+SQL selects for Product after 3 runs: 3

hibernate-jpa/output/q30-bulk-update-vs-l2.txt

@@ -0,0 +1,3 @@
+after an HQL bulk update to 11   : find() sees 11 (L2 hits=0 misses=1)
+after a native bulk update to 12 : find() sees 12 (L2 hits=0 misses=1)
+after raw JDBC update to 13      : find() sees 12 (L2 hits=1 misses=0)  <- stale: Hibernate was not told

hibernate-jpa/output/q31-optimistic-locking.txt

@@ -0,0 +1,6 @@
+both sessions read balance=100 version=0
+session A commits +50 -> version is now 1
+session B commits +25 -> jakarta.persistence.OptimisticLockException: Unexpected row count (expected row count 1 but was 0) [update Account set balance=?,owner=?,version=? where id=? and version=?] for entity [lab.hibernate.model.Account with id '1']
+  cause: org.hibernate.StaleObjectStateException: Unexpected row count (expected row count 1 but was 0) [update Account set balance=?,owner=?,version=? where id=? and version=?] for entity [lab.hibernate.model.Account with id '1']
+SQL> update Account set balance=?,owner=?,version=? where id=? and version=?
+final balance: 150 (B's stale write was rejected, A's +50 kept)

hibernate-jpa/output/q32-pessimistic-locking.txt

@@ -0,0 +1,3 @@
+session 1: find(..., PESSIMISTIC_WRITE) ran: select a1_0.id,a1_0.balance,a1_0.owner,a1_0.version from Account a1_0 where a1_0.id=? for no key update of a1_0
+session 2 with lock.timeout=0 -> jakarta.persistence.PessimisticLockException: JDBC exception executing SQL [ERROR: could not obtain lock on row in relation "account"] [select a1_0.id,a1_0.balance,a1_0.owner,a1_0.version from Account a1_0 where a1_0.id=? for no key update of a1_0 nowait]
+  cause: org.hibernate.exception.LockTimeoutException: JDBC exception executing SQL [ERROR: could not obtain lock on row in relation "account"] [select a1_0.id,a1_0.balance,a1_0.owner,a1_0.version from Account a1_0 where a1_0.id=? for no key update of a1_0 nowait]

hibernate-jpa/output/q33-lost-update.txt

@@ -0,0 +1,2 @@
+SQL> update PlainAccount set balance=?,owner=? where id=?
+two deposits (+50, +25) on 100 -> final balance 125, expected 175: A's deposit silently lost

hibernate-jpa/output/q34-isolation-levels.txt

@@ -0,0 +1,3 @@
+PostgreSQL default isolation level: read committed
+read committed  : first read 100, another transaction commits 150, second read 150 -> non-repeatable read
+repeatable read : first read 100, another transaction commits 150, second read 100 -> repeatable

hibernate-jpa/output/q35-version-bump-collections.txt

@@ -0,0 +1,2 @@
+Wallet (owns a unidirectional @OneToMany): add a Coin -> version 0 -> 1
+Folder (inverse, mappedBy @OneToMany)   : add a Doc  -> version 0 -> 0

hibernate-jpa/output/q36-dynamic-update.txt

@@ -0,0 +1,6 @@
+default entity, change one column:
+SQL> select p1_0.id,p1_0.a,p1_0.b,p1_0.c from Plain p1_0 where p1_0.id=?
+SQL> update Plain set a=?,b=?,c=? where id=?
+@DynamicUpdate entity, change one column:
+SQL> select w1_0.id,w1_0.a,w1_0.b,w1_0.c from Wide w1_0 where w1_0.id=?
+SQL> update Wide set a=? where id=?

hibernate-jpa/output/q37-hql-criteria-native.txt

@@ -0,0 +1,6 @@
+HQL      -> [Cy, Ana]
+Criteria -> [Cy, Ana]
+Native   -> [Cy, Ana]
+SQL> select e1_0.id,e1_0.city,e1_0.zip,e1_0.department_id,e1_0.level,e1_0.name,e1_0.salary from Employee e1_0 where e1_0.salary>? order by e1_0.salary
+SQL> select e1_0.id,e1_0.city,e1_0.zip,e1_0.department_id,e1_0.level,e1_0.name,e1_0.salary from Employee e1_0 where e1_0.salary>? order by e1_0.salary
+SQL> select * from employee where salary > 150 order by salary

hibernate-jpa/output/q38-dto-projection.txt

@@ -0,0 +1,4 @@
+record projection -> [EmployeeRow[name=Bo, salary=100], EmployeeRow[name=Cy, salary=200], EmployeeRow[name=Ana, salary=300]]
+entities loaded: 0
+SQL> select e1_0.name,e1_0.salary from Employee e1_0 order by e1_0.salary
+full entity query loads 3 entities

hibernate-jpa/output/q39-bulk-update-stale-entity.txt

@@ -0,0 +1,3 @@
+HQL bulk update touched 3 rows
+the entity already loaded still says salary = 100
+after refresh() it says salary = 999

hibernate-jpa/output/q40-named-query-validation.txt

@@ -0,0 +1,3 @@
+buildSessionFactory() failed with org.hibernate.query.NamedQueryValidationException:
+  Errors in named queries:
+    [1] Error in query named 'BrokenQuery.byTitle': Could not resolve attribute 'titel' of 'lab.hibernate.model.BrokenQuery' [select b from BrokenQuery b where b.titel = :t]