Java Persistence - Java

• Types
  • JDBC-ODBC Driver
  • Native API Driver
  • Thin Driver, Pure Driver
  • Network Protocol Driver

• java.sql & javax.sql

    public class DBUtility {
        public static String url = “jdbc:mysql://localhost:3306/jdbcdemos";
        public static String username = "root";
        public static String password = "root";
        private static Connection connection = null;
        public static Connection getDBConnection() throws ClassNotFoundException, SQLException {
            Class.forName("com.mysql.jdbc.Driver"); // No longer required from JDBC 4.0
            Connection connection = DriverManager.getConnection (url, username, password);
            return connection;
        }
    }

• connection.close();

• executeQuery(“”)
• execute(“”)
• executeUpdate(“”)

• PreparedStatement pstatement = connection.prepareStatement (retrievedata);
• ResultSet resultSet = pstatement.executeQuery();
• while(resultset.next()){}

• Use ? in SQL query to pass parameter
• Connection connection = DBUtility.getDBConnection();
• PreparedStatement pstatement = connection.prepareStatement(String);
• pstatement.setInt(N, var);
• int rowsInsertCount = pstatement.executeUpdate();

• Logical collection of data that can be modified from a database
• Annotated with javax.persistence.Entity
• ID property annotated with javax.persistence.Id
• By default Entity class is mapped to the table have same name
• Mandatory Annotations => @Entity & @Id
• Optional Annotations => @Table(name=””) & @Column(name=””)
• Temporal(TemporalType.DATE) => Used to convert to java.sql Date type

• Helps in managing entity and performing CRUD operations

• EntityManagerFactory entityManagerFactory = Persistence.createEntityManagerFactory("unit-name");
• EntityManager entityManager = entityManagerFactory.createEntityManager();
• entityManager.getTransaction().commit();
• EmployeeEntity employee = new EmployeeEntity();
• employee.setEmployeeld(1);
• employee.setEmployeeName("ABCDE");
• employee.setInsertTime(new Date());
• entityManager.getTransaction().begin();
• entityManager.persist(employee);
• Find => Returns null if record not found
  • Employee employee = em.find(Employee.class, idValue)
• Remove
  • entityManage.remove(employeeEntity)

• @GeneratedValue(strategy=GenerationType.AUTO)
• AUTO, INDENTITY, SEQUENCE, TABLE

• If entity already presents then acts as update, if not present then acts as save operation
• Creates the copy of the entity and manages it instead of original

• OneToOne
  • Code
    • @OneToOne
    • JoinColumn(name=”colName”, unique=true)
    • private ClassName classEntity
  • Find & Execute functionality will also be executed for target entity
  • Cascade Types => Persist parent entity before child entity
    • Defined in source entity
    • @OneToOne(cascade=CascadeType.ALL)
    • Types
      • ALL
      • MERGE
      • PERSIST, REFRESH, REMOVE
• ManyToOne
  • Removal operation may fail
  • Code
    • @ManyToOne(cascade=CascadeType.ALL)
    • JoinColumn(name=”colName”, unique)
• OneToMany
  • Find & Remove will cascade all child entities also
  • Code
    • @OneToMany (cascade = CascadeType.ALL)
    • @JoinTable(name = "tableName", joinColumns = {@JoinColumn(name = "colName1 ") }, inverseJoinColumns = {@JoinColumn(name = “colName2”, unique = true) })
• ManyToMany
  • Find & Remove may lead to Faliure
  • Code
    • @ManyToMany (cascade = CascadeType.ALL)
    • @JoinTable(name = "tableName", joinColumns = {@JoinColumn(name = "colName1 ") }, inverseJoinColumns = {@JoinColumn(name = “colName2”) })
    • private List<NameEntity> names;

• Get the reference to EntityManager
• Write Query => Use package javax.persistence.Query
• Set the Parameters
• Execute the Query (Select, Update, Delete)
• Get the Result

• Query query = entityManager.createQuery("SELECT k FROM EmployeeEntity k");
• List<EmployeeEntity> list = query.getResultList();
  • Entity reference k has reference to all the variables/properties of the entity
• HQL (Hibernate Query Language) => Using hibernate provider
  • FROM EmployeeEntity

• Contains reference to the single attribute of the entity only
• Returns list of objects for the selected attribute type
• Query query = entityManager.createQuery("SELECT e.name FROM EmployeeEntity e");
• Query query = entityManager.createQuery("SELECT e.name, e.id FROM EmployeeEntity e");
• List<String> list = query.getResultList();
• In case no data is found, empty list will be returned
• Parameter
  • Positional
    • Query query = entityManager.createQuery("SELECT e FROM EmployeeEntity e WHERE e.salary>=?1 and e.salary<=?2");
    • query.setParameter(1, lowerbound);
    • query.setParameter(2, upperbound);
  • Named
    • Query query = entityManager.createQuery("SELECT e FROM EmployeeEntity e WHERE e.salary>= :lb and e.salary<= :ub");
    • query.setParameter(lb, lowerbound);
    • query.setParameter(ub, upperbound);

• UPPER, LOWER, LENGTH, SUBSTRING
• CURRENT_DATE, CURRENT_TIME, CURRENT_TIMESTAMP => Returns as java.sql
• AVG, COUNT, MAX, MIN, SUM

• Update

      Query query = entityManager.createQuery("UPDATE EmployeeEntity e SET e.salary=?2 WHERE e.name= :ub");
      query.setParameter(22, salary);
      query.setParameter("ub", name);
      entityManager.getTransaction().begin();
      Integer res = query = executeUpdate();
      entityManager.getTransaction().commit();
  

• Delete

• Configured with a predefined unchangable query string
• Code

      // Entity Class                
      @Entity
      @Table(name = "Employee")
      @NamedQuery(name="findAllEmployeeSalaryRangePositionalParam",
          query="SELECT e FROM Employee Entity e where e.salary>=?1 and e.salary<=?2")
      public class Employee Entity {}
      
      @Entity
      @Table(name = "Employee")
      @NamedQueries({
          @NamedQuery(name="findAllEmployeeSalaryRangePositionalParam",
          query="SELECT e FROM Employee Entity e where e.salary>=?1 and e.salary<=?2"),
          @NamedQuery(name="findAllEmployeeSalaryRangePositionalParam",
          query="SELECT e FROM Employee Entity e where e.salary>=?1 and e.salary<=?2")
      })
      public class Employee Entity {}
  
      // Employee DAOIMPL
      Query query = entityManager.createNamedQuery("findAllEmployeeSalaryRangePositionalParam");
      query.setParameter(1, lowerBound);
      query.setParameter(2, upperBound);
      List<EmployeeEntity> employeeList = query.getResultList();
  

• Named queries can be placed in orm.xml file

      <named-query name="">
          <query></query>
      </named-query>
  

• Query query = entityManager.createQuery("select emp, ast from Employee Entity emp, AssetEntity ast where emp.assetId = ast.assetId");

• Applicaple for OneToOne & ManyToOne, when there is single target entity involved
• Target is found automatically due to cascade type