JPA Lifecycle and Annotations

Objective

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

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Instructions

1. Create a new Java project using Maven.
2. Define a simple entity class called “Student” with attributes like id, firstName, lastName, email and age. Remember to include a no-arg constructor.
3. Use JPA annotations to map the entity class to a database table named students.
4. Add a constraint to the email attribute to ensure that the email address is unique.
5. Include appropriate annotations such as @Entity, @Table, @Id, @GeneratedValue, and @Column to define the primary key and attributes mapping.
6. Add a Main.class including a main method
7. Create the following methods in the Main class:
   • public static void createStudent(Student student) - This method should create a new student and persist it to the database.
   • public static Student readStudent(int id) - This method should read a student from the database using the student’s id.
   • public static Student updateStudent(Student updStd) - This method should update an existing student in the database.
   • public static void deleteStudent(int id) - This method should delete a student from the database using the student’s id.
   • public static List<Student> readAllStudents() - This method should retrieve all students from the database and return them as a list. Use a TypedQuery to retrieve all students.

In all the methods above, remember to open and close the EntityManager and EntityManagerFactory objects. You can use either the try-with-resources or the finally block to close the objects.

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1. In all the methods above, write small comments that explains when an object is transient, detached, removed or managed. (See example below)

    public static void main(String[] args) {
    // entity is in transient state
    Student student = new Student("Michelle", "Schmidt", "schmidt@mail.com", 30);

    }
    
    public static void createStudent(Student student) {
        try(EntityManager em = emf.createEntityManager()) {
            em.getTransaction().begin();
            // entity is in managed state (after persist)
            em.persist(student);
            // entity is in detached state after the transaction is committed
            em.getTransaction().commit();
        }
    }

1. Add a @PrePersist method to the Student class that verifies that the email address is valid. If the email address is not valid, throw an exception.
2. Use the same logic as above but this time in the @PreUpdate method.