Spring Boot, Hibernate, bidirectional One-To-Many. Strange behaviour. Why is there two selects insdead of an error? - spring-boot

Question

Spring Boot, Hibernate, bidirectional One-To-Many. Strange behavior. Why is there two selects instead of an error?
I have a basic Spring boot application.
It simulates throwing dices.
I have two entity classes Dice and DiceBatch.
DiceBatch has List<Dice> dices;
Dice has DiceBatch diceBatch; as two sides of bidirectional ManyToOne, or OneToMany.
I use JpaRepository<DiceBatch, UUID> to get one instance of DiceBatch by callig a method of JpaRepository findById(UUID id)
I call this method inside DiceBatchService's method findDiceBatchById(UUID diceBatchId).
Method is marked as #Transactional.
When i do that Hibernate logs one SQL select:
/* select
d
from
DiceBatch d
where
d.id = ?1 */ select
dicebatch0_.dice_batch_id as dice_bat1_1_,
dicebatch0_.batch_creation_time as batch_cr2_1_,
dicebatch0_.batch_name as batch_na3_1_
from
dice_batch dicebatch0_
where
dicebatch0_.dice_batch_id=?
At this point everything is ok.
Method returns DiceBatch entity with lazily initialized List<Dice> dices.
This is important. Method is #Transactional when method returns I should leave transactionla context.
Lazy fields should stay lazy and should cause LazyInitializationException if I try to access them.
Now control goes back to the controller method of DiceBatchController findDiceBatchById(UUID diceBatchId)
And here something strange happens.
Hibernate logs another select
select
dices0_.dice_batch_id as dice_bat5_0_0_,
dices0_.dice_id as dice_id1_0_0_,
dices0_.dice_id as dice_id1_0_1_,
dices0_.dice_batch_id as dice_bat5_0_1_,
dices0_.sequential_number as sequenti2_0_1_,
dices0_.throw_result as throw_re3_0_1_,
dices0_.throw_time as throw_ti4_0_1_
from
dice dices0_
where
dices0_.dice_batch_id=?
...and response JSON contains DiceBatch with all Dice entities related to it.
So I have several question.
Why didn't I get LazyInitializationException?
How come the List<Dice> inside DiceBatch got initialized outside of Transactional context?
How Spring managed to build a complete entity of DiceBatch, including the content of the List<Dice> without any exceptions?
How to modify my code to avoid this strange implicit bahavior?
Here is all the relevant code.
package org.dice.model;
#Entity
#Builder
#Getter
#Setter
#NoArgsConstructor
#AllArgsConstructor
public class Dice {
#Id
#GenericGenerator(name = "UUID",
strategy = "org.hibernate.id.UUIDGenerator")
#GeneratedValue(strategy = javax.persistence.GenerationType.AUTO,
generator = "UUID")
#Column(name = "dice_id",
nullable = false)
private UUID id;
#Column(name = "throw_result",
nullable = false)
private Integer throwResult;
#Column(name = "throw_time",
nullable = false)
private LocalDateTime throwTime;
#ManyToOne(fetch = FetchType.LAZY)
#JoinColumn(name = "dice_batch_id",
nullable = false,
foreignKey = #ForeignKey(name = "fk_dice_dice_batch_id_dice_batch_dice_batch_id")
)
#JsonBackReference
private DiceBatch diceBatch;
#Embedded
private SequentialNumber sequentialNumber;
}
package org.dice.model;
#Entity
#Builder
#Getter
#Setter
#NoArgsConstructor
#AllArgsConstructor
public class DiceBatch {
#Id
#GenericGenerator(name = "UUID",
strategy = "org.hibernate.id.UUIDGenerator")
#GeneratedValue(strategy = GenerationType.AUTO,
generator = "UUID")
#Column(name = "dice_batch_id",
nullable = false)
private UUID id;
#Column(name = "batch_name",
nullable = false)
private String batchName;
#Column(name = "batch_creation_time",
nullable = false)
private LocalDateTime batchCreationTime;
#OneToMany(
mappedBy = "diceBatch",
cascade = CascadeType.ALL,
orphanRemoval = true,
fetch = FetchType.LAZY)
#JsonManagedReference
private List<Dice> dices = new ArrayList<>();
public void addDice(Dice dice) {
dices.add(dice);
dice.setDiceBatch(this);
}
public void removeDice(Dice dice) {
dices.remove(dice);
dice.setDiceBatch(null);
}
}
package org.dice.repo;
#Repository
public interface DiceBatchRepo extends JpaRepository<DiceBatch, UUID> {}
package org.dice.service;
#Service
#RequiredArgsConstructor
public class DiceBatchService {
#Transactional
public DiceBatch findDiceBatchById(UUID diceBatchId) {
DiceBatch diceBatch = diceBatchRepo
.findById_my(diceBatchId)
.orElseThrow();
return diceBatch;
}
}
package org.dice.controller;
public class DiceBatchController {
#GetMapping(path = "/get/{diceBatchId}")
public ResponseEntity<DiceBatch> findDiceBatchById(
#PathVariable(name = "diceBatchId") UUID diceBatchId) {
log.info("<C>[/batch/get] endpoint reached.\n" +
"Dice Batch Id: {}\n",
diceBatchId);
return ResponseEntity.ok(diceBatchService.findDiceBatchById(diceBatchId));
}
}