hibernate-lazy-initialization-exception's Introduction

hibernate-lazy-initialization-exception

The main goal of this project is to show typical problems with lazy fetching - LazyInitializationException.

Reference: http://docs.jboss.org/hibernate/orm/5.3/userguide/html_single/Hibernate_User_Guide.html
Reference: http://download.oracle.com/otn-pub/jcp/persistence-2_2-mrel-spec/JavaPersistence.pdf?AuthParam=1538677370_fbd3bf5088a447d9721552ad131ae9c7

preface

Fetching, essentially, is the process of grabbing data from the database and making it available to the application.

We have two approaches (provided by JPA 2.2):

public enum FetchType { LAZY, EAGER };
- EAGER - requirement on the persistence provider runtime that the value must be eagerly fetched,
- LAZY - hint to the persistence provider runtime.

annotations

Annotation	Default
`@OneToOne`	`EAGER`
`@OneToMany`	`LAZY`
`@ManyToOne`	`EAGER`
`@ManyToMany`	`LAZY`
`@Basic`	`EAGER`

LazyInitializationException

LazyInitializationException indicates an attempt to access not-yet-fetched data outside of a session context.
For example, when an uninitialized proxy or collection is accessed after the session was closed.

tests

noSession_onlyGetter - there is no exception because we don't fire loading:
```
repository.findById(1).map(Employee::getIssues);
```
doesn't consume the collection so there is no need to load it.
noSession_consumingCollection
```
repository.findById(1).map(Employee::getIssues).map(Collection::size);
```
consumes the collection so query is fired and throws the exception (there is no open session).
transactional()
```
repository.findById(1).map(Employee::getIssues).map(Collection::size);
```
we consume the collection but we also provided the open session by @Transactional annotation.