In this tutorial, we are going to see Hibernate Projection with a simple example. As we already discussed in the previous tutorial Hibernate Criteria, If we want to read a partial entity (selected columns) from the database, hibernate has provided us with a Projection interface.

Hibernate Projection :

• Projection is an interface in Hibernate; it is coming from org.hibernate.criterion package.
• Projection can be applied to the Criteria query.
• Projection is an Object-Oriented Representation of a query result set.
• Hibernate Projection is used to read a partial entity from the database.
• If we want to read more than one property from the database, then we have to add Projection objects to ProjectionList, and then we need to set ProjectionList object to Criteria.

Simple Hibernate Projection Example :

The following code is to read the employee name column from the employee table. It will get all the employeNames from the employee table.

Criteria criteria = session.createCriteria(Employee.class); 
Projection projection = Projections.property("employeeName"); 
criteria.setProjection(projection); 
List list = criteria.list();

The equal SQL query is:

select ename from employee;

Multiple Hibernate Projection Example :

If we want to read more than 1 column (employeeName, salary) with projections, we should have to use ProjectinList class. We should prepare all projections and bind all projections into a single ProjectionList like below.

Criteria criteria = session.createCriteria(Employee.class); 
Projection projection = Projections.property("salary"); 
Projection projection2 = Projections.property("departmentId"); 
Projection projection3 = Projections.property("employeeName"); 
ProjectionList pList = Projections.projectionList(); 
pList.add(projection); 
pList.add(projection2); 
pList.add(projection3); 
criteria.setProjection(pList); 
List list = criteria.list();

On the above example, we will get the salary, departmentId and employeeName from the employee table.

The equal SQL query is:

select sal,deptid,ename from emp;

Since those are multiple projections, first we need to add all projections to ProjectionList object and then add ProjectionList object to criteria object.

Complete Example for Hibernate Projection :

Create Employee Pojo Class: Employee.class

package com.otp.hibernate.pojo;

public class Employee {
    private int employeeId;
    private String employeeName;
    private int departmentId;
    private int salary;

    public int getEmployeeId() {
        return employeeId;
    }

    public void setEmployeeId(int employeeId) {
        this.employeeId = employeeId;
    }

    public String getEmployeeName() {
        return employeeName;
    }

    public void setEmployeeName(String employeeName) {
        this.employeeName = employeeName;
    }

    public int getDepartmentId() {
        return departmentId;
    }

    public void setDepartmentId(int departmentId) {
        this.departmentId = departmentId;
    }

    public int getSalary() {
        return salary;
    }

    public void setSalary(int salary) {
        this.salary = salary;
    }

    @Override
    public String toString() {
        return "Employee [employeeId=" + employeeId + ", employeeName="
                + employeeName + ", departmentId=" + departmentId + ", salary="
                + salary + "]";
    }   

}

Hibernate mapping file :

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
  "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
  "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
  <class name="com.otp.hibernate.pojo.Employee" table="employee"
    schema="onlinetutorialspoint">
    <id name="employeeId" column="id">
      <generator class="increment" />
    </id>
    <property name="employeeName" column="ename" />
    <property name="departmentId" column="deptNo" />
    <property name="salary" column="salary" />
  </class>
</hibernate-mapping>

Run the Application :

import java.util.Iterator;
import java.util.List;

import org.hibernate.Criteria;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
import org.hibernate.cfg.Configuration;
import org.hibernate.criterion.Criterion;
import org.hibernate.criterion.Projection;
import org.hibernate.criterion.ProjectionList;
import org.hibernate.criterion.Projections;
import org.hibernate.criterion.Restrictions;

import com.otp.hibernate.pojo.Employee;

public class Main {

    public static void main(String[] args) {
        Configuration configuration = new Configuration()
                .configure("hibernate.cfg.xml");
        StandardServiceRegistryBuilder builder = new StandardServiceRegistryBuilder()
                .applySettings(configuration.getProperties());
        SessionFactory factory = configuration.buildSessionFactory(builder
                .build());

        Session session = factory.openSession();

        System.out.println("Reading Partial Entity with One Projection object ");

        Criteria criteria = session.createCriteria(Employee.class);
        Projection projection = Projections.property("salary");
        criteria.setProjection(projection);
        List list = criteria.list();
        Iterator it = list.iterator();

        while (it.hasNext()) {
            Integer sal = (Integer) it.next();
            System.out.println("Employee Salary : " + sal);
        }
        
        System.out.println("Reading Partial Entity with multiple Projection objects ");
        
        Criteria crit2 = session.createCriteria(Employee.class);
        Projection projection1 = Projections.property("salary");
        Projection projection2 = Projections.property("departmentId");
        Projection projection3 = Projections.property("employeeName");
        
        ProjectionList pList = Projections.projectionList();
        pList.add(projection1);
        pList.add(projection2);
        pList.add(projection3);
        crit2.setProjection(pList);
        
        List list2 = crit2.list();

        Iterator it2 = list2.iterator();

        while (it2.hasNext()) {
            Object[] obj = (Object[]) it2.next();
            System.out.println("Salary : " + obj[0]+" DeptId : "+obj[1]+" empName : "+obj[2]);
        }
    }
}

Output :

Reading Partial Entity with One Projection object 
Employee Salary : 6000 Employee Salary : 8000 Employee Salary : 4000 Employee Salary : 5000 Employee Salary : 4000 Employee Salary : 3000 
Reading Partial Entity with multiple Projection objects 
Salary : 6000 DeptId : 101 empName : Chandra Salary : 8000 DeptId : 101 empName : Shekhar Salary : 4000 DeptId : 105 empName : Rahul Salary : 5000 DeptId : 103 empName : Mahesh Salary : 4000 DeptId : 101 empName : Vinay Salary : 3000 DeptId : 105 empName : Vijay

The complete example is available for download.

Happy Learning 🙂