JPA Book Exercise

Exercise 1: Mapping Entities and Annotations

Objective: Apply JPA annotations to map Java classes to database tables and understand entity lifecycle.

1. Set Up a Java Project:
   • Create a new Maven project. Use JPA starter if needed.
2. Define the Book Entity:
   • Create a simple entity class called Book with the following attributes:
     • id (primary key)
     • title
     • author
     • isbn (must be unique)
     • publishedYear
   • Use JPA annotations to map the entity class to a database table named books.
   • Ensure the isbn field is unique.
   • Include a no-arg constructor (@NoArgsConstructor).
   • Add Lombok annotations like @Getter, @Setter, @ToString, and @Builder.
3. Add JPA Annotations:
   • Use @Entity, @Table, @Id, @GeneratedValue, and @Column to define the primary key and attribute mappings.
4. Set Up the Main Class:
   • Add a static property: EntityManagerFactory.

   • Create methods for CRUD operations in a BookDAO class:

   • public static void createBook(Book book)
     • This method persists a new book to the database.
   • public static Book readBook(int id)
     • This method retrieves a book by ID.
   • public static Book updateBook(Book updatedBook)
     • This method updates an existing book.
   • public static void deleteBook(int id)
     • This method deletes a book by ID.
   • public static List<Book> readAllBooks()
     • This method retrieves all books from the database using a TypedQuery.

5. Use try-with-resources or finally blocks to properly close EntityManager objects.

6. Add JPA Entity Lifecycle Comments
   • Add comments explaining when an entity is in:
     • Transient State (before persist)
     • Managed State (after persist)
     • Detached State (after transaction commit)
     • Removed State (before deletion)

   Example:

   public static void main(String[] args) {
       // Entity is in transient state
       Book book = new Book("Effective Java", "Joshua Bloch", "9780134685991", 2018);
   }
   
   public static void createBook(Book book) {
       try (EntityManager em = emf.createEntityManager()) {
           em.getTransaction().begin();
           // Entity is in managed state after persist()
           em.persist(book);
           // Entity becomes detached after transaction is committed
           em.getTransaction().commit();
       }
   }
   

7. Validate ISBN Before Persisting a Book
   • Add a method in the Book class to verify that the ISBN is valid.
   • Use @PrePersist to validate ISBN before saving a book.
   • If ISBN is invalid, throw an exception.
   • Add a @PreUpdate method to validate ISBN before updating.
   • Hint: Use the ISBNValidator class from the ISBN validation exercise
8. Create a Test Class
   • Add a test class that contains a test method for each CRUD operation. It’s not an integration test, but just a sort of main method that calls each method in the BookDAO class.
9. Populate the Database with Books
   • Add 10 books to the database
   • At least 3 books should be authored by “J.K. Rowling”
   • At least 4 books should be authored by “George Orwell”
   • The oldest book should be from 1945
   • The most recent book should be from 2024
10. Aggregate Data with Queries
    • Calculate the average publication year of all books
    • Find the average publication year of books authored by “George Orwell”
    • Count how many books are authored by “J.K. Rowling”
    • Find the title of the oldest book** in the database.
    • Retrieve the most recently published book
    • Sum the publication years of all books

Exercise 2: Q & A

1. Why do we need a no-arg constructor in an entity class?
2. Investigate where in the code we can:
   • Specify the database dialect?
   • Define JDBC connection properties?
   • Add annotated classes?
3. What is the purpose of @GeneratedValue?
   • What are the different primary key generation strategies in JPA?
4. What is try-with-resources, and how can we use it in JPA?
5. What is the difference between persist() and merge() in JPA?
6. What is the difference between TypedQuery and Query?
7. What is the difference between getSingleResult() and getResultList()?
8. What are the different states of an entity in JPA?

Remember: Always add your entity classes to the HibernateConfig