Practice topics

Configuration

• Add dependencies: Hibernate and MySQL connector
• Create database [create-db.sql]
• Configure Hibernate by XML [hibernate.cfg.xml]

Concepts

• CRUD features
• SessionFactory, Session and Transaction
• Uni-directional and bi-directional relationship
• Cascade types: CascadeType.ALL, CascadeType.PERSIST (save), CascadeType.REMOVE, CascadeType.DETACH, CascadeType.MERGE, CascadeType.REFRESH
  • 📌 Use persist() with CascadeType.ALL/PERSIST to save all associated objects.
• Eager loading and Lazy loading; Fetch types: Fetch.LAZY (prefer), Fetch.EAGER
  • To retrieve lazy data, a Hibernate session need to be open (connect to the database).
  • 📌 Resolve Lazy loading issue (because of loading after closing session)
    • Option 1: Call getter method while session is still open [EagerLazyDemo]
    • Option 2: Use Hibernate query with HQL [hibernate.query.Query] [FetchJoinDemo]

Entity Class

• Add no-args constructor (required by Hibernate)
• 📌 Add convenience methods for bi-directional one/many-to-many relationship [Instructor]
• @Entity [javax.persistence]
• @Id and @GeneratedValue (prefer GenerationType.IDENTITY)
• @Table: map to table in the database [javax.persistence]
• @Column: map to column of the table
• @OneToOne [Instructor /InstructorDetail]
  • Uni-directional (use @JoinColumn) and bi-directional (add mappedBy to @OneToOne)
  • @JoinColum: used in the entity class containing the foreign key.
  • mappedBy attribute: used in the class entity which does not contain the foreign key.
• @OneToMany [Course /Review]
  • Uni-directional: @JoinColumn used in the class entity which does not contain the foreign key.
• @ManyToMany [Student /Course]
  • fetch should be FetchType.LAZY
  • cascade should omit CascadeType.REMOVE
  • @JoinTable with joinColumns and inverseJoinColumns for each foreign key

App Flow

[CreateDemo]

1. Create a Session Factory including all the entities
2. Create a Session
3. Start a Transaction
4. Perfrom CRUD (save/permit if create or update entities)
5. Commit the Transaction
6. Handle connection leak issue: catch error and close Session.
7. Close the Factory