REST CRUD API with JPA

Application Architecture

What is JPA?

• Java Presistence API(JPA)

  • Standard API for Object-to-Relation-Mapping (ORM)

• Only a specification

  • Defines a set of interfaces
  • Requires an implementation to be usable

JPA - Vendor Implementations

What are Benefits of JPA

• By having a standard API, we are not locked to vendor's implementation
• Maintain portable, flexible code by coding to JPA spec(interfaces)
• Can theoretically switch vendor implementations
  • For example, if Vendor ABC stops supporting their product
  • We could switch to Vendor XYZ without vendor lock in

Auto Data Source Configuration

• In Spring Boot, Hibernate is default implementation of JPA
• EntityManager is similar to Hibernate SessionFactory
• EntityManager can serve as a wrapper for Hibernate Session object
• We can inject the EntityManager into our DAO

Various DAO Techniques

• Version 1: Use EntityManager but leverage native Hibernate API
• Version 2: Use EntityManager and standard JPA API
• Version 3: Spring Data JPA

Standard JSPA API

• The JPA API methods are similar to Native Hibernate API
• JPA also support a query language: JPQL (JPQ Query Language)
  • For more details on JPQL https://docs.oracle.com/javaee/7/tutorial/persistence-querylanguage.htm#BNBTG

Comparing JPA to Native Hibernate Methods

Action	native Hibernate method	JPA method
Create/save new entity	session.save(...)	entityManager.persist(...)
Retrieve entity by id	session.get(...) / load(...)	entitymanager.find(...)
Retrieve list of entities	session.createQuery(...)	entityManager.createQuery(...)
Save or update entity	session.saveOrUpdate(...)	entityManager.merge(...)
Delete entity	session.delete(...)	entityManager.remove(...)

• High-level comparison, Other options depending on context ...

Development Process

1. Set up Database Dev Environment
2. Create Spring Boot project using Spring Initializer
3. Get list of employees
4. Get single employee by ID
5. Add a new employee
6. Update an existing employee
7. Delete an existing employee

DAO Interface

public interface EmployeeDAO
  
  public List<Employee> findAll();
}

DAO Implementation

@Repository
public class EmployeeDAOJpaImpl implements EmployeeDAO {
  
  private EntityManager entityManager;
  
  @Autowired
  public EmployeeDAOJpaImpl (EntityManager theEntityManager) {
    entityManager = theEntityManager;
  }
  
  @Override
  public List<Employee> findAll() {
  
    // create a query
    TypedQuery<Employee> theQuery = 
      entityManager.createQuery("from Employee", Employee.class);
    
    // execute query and get result list
    List<Employee> employees = theQuery.getResultList();
    
    // return the result
    return employees;
}

Add or Update employee

@Override
public void save(Employee theEmployee) {
  
  // save or update the employee
  Employee dbEmployee = entityManager.merge(theEmployee);
  
  // Update with id from db ... so we can get generated id for save/insert
  theEmployee.setId(dbEmployee.getId());
}

• entityManager.merge(theEmployee) if id == 0 then save/insert else update
• theEmployee.setId(dbEmployee.getId()); useful in our REST API to return generated id