Now we will take a look at how to fetch objects using a Session. Here, we will also see how to get only one record using the primary key column. We override a toString() method in the Employee and Department classes so that it's easy to display the data within an object, as shown in the following code:
@Override
public String toString() {
return "
Employee"
+ "
id: " + this.getId()
+ "
first name: " + this.getFirstName()
+ "
salary: " + this.getSalary()
+ "
department: " + this.getDepartment().getDeptName();
}Here, we are trying to get an employee having id equals 1.
The equivalent SQL query is as follows:
SELECT * FROM employee WHERE id=1;
Now, let's look at how to do the same using hibernate:
- Enter the following code to fetch an object of the
employeetype, where theidis1:SessionFactory sessionFactory = HibernateUtil.getSessionFactory(); Session session = sessionFactory.openSession(); Employee employee = (Employee) session.get(Employee.class, 1l); if(employee != null){ System.out.println(employee.toString()); } session.close(); HibernateUtil.shutdown();
The output of the preceding code will be as follows:
Hibernate: select employee0_.id as id0_1_, employee0_.department as department0_1_, employee0_.firstName as firstName0_1_, employee0_.salary as salary0_1_, department1_.id as id1_0_, department1_.deptName as deptName1_0_ from employee employee0_ left outer join department department1_ on employee0_.department=department1_.id where employee0_.id=? Employee id: 1 first name: yogesh salary: 50000.0 department: developement
Now, let's take a look at how the preceding code works. The first thing I want to highlight here is the query shown on the console/output window preceded by hibernate. Here, the hibernate engine internally creates a core SQL query to perform the operation that we can find in the console/output window. Hibernate internally uses JDBC to execute this query.
Another thing that needs to be highlighted is that we used the Session.get(...) method to fetch the data from the database. When you use the Session.get(...) method to fetch a record, it will perform the following actions:
Let's consider another method named load(...) to fetch the data.
Now we will take a look at how to fetch data using the load(...) method and the difference between the load() and get() methods.
The load() method works in the following manner:
- It returns the
proxy(hibernate term) object. This means that it returns the dummy object without hitting the database if the same object is found in a persistent state for the given identifier. - If the object is not found in the session cache, it will hit the database.
- If no row is found in the session cache as well as in the database, then it will throw an
ObjectNotFoundExceptionerror.
Let's take a look at some real-time scenarios.
The record is in the session cache when load() is invoked.
SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
Session session = sessionFactory.openSession();
System.out.println("Employee get...");
Employee employeeGet = (Employee) session.get(Employee.class, Long.valueOf(2));
System.out.println(employeeGet.toString());
System.out.println("Employee load...");
Employee employeeLoad = (Employee) session.load(Employee.class,Long.valueOf(2));
System.out.println(employeeLoad.toString());
session.close();
HibernateUtil.shutdown();Employee get... Hibernate: select employee0_.id as id0_1_, employee0_.department as department0_1_, employee0_.firstName as firstName0_1_, employee0_.salary as salary0_1_, department1_.id as id1_0_, department1_.deptName as deptName1_0_ from employee employee0_ left outer join department department1_ on employee0_.department=department1_.id where employee0_.id=? Employee id: 2 first name: aarush salary: 35000.0 department: developement Employee load... Employee id: 2 first name: aarush salary: 35000.0 department: developement
From the output, it's clear that when the first get() method is invoked, the persisted object, Employee#2, is stored in the session cache at that time. When load() is invoked, it is directly loaded from the session; there is no need to hit the database. Here, we can show that the Select query is executed only once.
The record is not in the session cache when load() is invoked.
SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
Session session = sessionFactory.openSession();
System.out.println("Employee get...");
Employee employeeGet = (Employee) session.get(Employee.class, new Long(1));
System.out.println(employeeGet .toString());
System.out.println("Employee load...");
Employee employeeLoad = (Employee) session.load(Employee.class, new Long(2));
System.out.println(employeeLoad .toString());
session.close();
HibernateUtil.shutdown();Employee get... Hibernate: select employee0_.id as id0_1_, employee0_.department as department0_1_, employee0_.firstName as firstName0_1_, employee0_.salary as salary0_1_, department1_.id as id1_0_, department1_.deptName as deptName1_0_ from employee employee0_ left outer join department department1_ on employee0_.department=department1_.id where employee0_.id=? Employee id: 1 first name: yogesh salary: 50000.0 department: developement Employee load... Hibernate: select employee0_.id as id0_1_, employee0_.department as department0_1_, employee0_.firstName as firstName0_1_, employee0_.salary as salary0_1_, department1_.id as id1_0_, department1_.deptName as deptName1_0_ from employee employee0_ left outer join department department1_ on employee0_.department=department1_.id where employee0_.id=? Employee id: 2 first name: aarush salary: 35000.0 department: developement
The record is neither in the session cache nor in the database when load() is invoked.
SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
Session session = sessionFactory.openSession();
System.out.println("
Employee get...");
Employee employeeGet = (Employee) session.get(Employee.class, new Long(1));
System.out.println(employeeGet.toString());
System.out.println("
Employee load...");
Employee employeeLoad = (Employee) session.load(Employee.class, new Long(3));
System.out.println(employeeLoad .toString());
session.close();
HibernateUtil.shutdown();Employee get...
Hibernate: select employee0_.id as id0_1_, employee0_.department as department0_1_, employee0_.firstName as firstName0_1_, employee0_.salary as salary0_1_, department1_.id as id1_0_, department1_.deptName as deptName1_0_ from employee employee0_ left outer join department department1_ on employee0_.department=department1_.id where employee0_.id=?
Employee
id: 1
first name: yogesh
salary: 50000.0
department: developement
Employee load...
Hibernate: select employee0_.id as id0_1_, employee0_.department as department0_1_, employee0_.firstName as firstName0_1_, employee0_.salary as salary0_1_, department1_.id as id1_0_, department1_.deptName as deptName1_0_ from employee employee0_ left outer join department department1_ on employee0_.department=department1_.id where employee0_.id=?
Exception in thread "main" org.hibernate.ObjectNotFoundException: No row with the given identifier exists: [vo.Employee#3]
at org.hibernate.impl.SessionFactoryImpl$2.handleEntityNotFound(SessionFactoryImpl.java:435)
at org.hibernate.proxy.AbstractLazyInitializer.checkTargetState(AbstractLazyInitializer.java:189)
at org.hibernate.proxy.AbstractLazyInitializer.initialize(AbstractLazyInitializer.java:178)
at org.hibernate.proxy.AbstractLazyInitializer.getImplementation(AbstractLazyInitializer.java:215)
at org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.invoke(JavassistLazyInitializer.java:190)
at vo.Employee_$$_javassist_1.getId(Employee_$$_javassist_1.java)
at ch2.Load6.main(Load6.java:21)