JPA - Criteria API

The Criteria API is a predefined API used to define queries for entities. It is the alternative way of defining a JPQL query. These queries are type-safe, and portable and easy to modify by changing the syntax. Similar to JPQL it follows abstract schema (easy to edit schema) and embedded objects. The metadata API is mingled with criteria API to model persistent entity for criteria queries. 
The major advantage of the criteria API is that errors can be detected earlier during compile time. String based JPQL queries and JPA criteria based queries are same in performance and efficiency.
The criteria API is included into all versions of JPA therefore each step of criteria API is notified in the specifications of JPA.
In JPA 2.0, the criteria query API, standardization of queries are developed.
In JPA 2.1, Criteria update and delete (bulk update and delete) are included.
The Criteria API and the JPQL are closely related and are allowed to design using similar operators in their queries. It follows javax.persistence.criteria package to design a query. The query structure means the syntax criteria query. 
The following simple criteria query returns all instances of the entity class in the data source.
EntityManager em = ...;
CriteriaBuilder cb = em.getCriteriaBuilder();

CriteriaQuery<Entity class> cq = cb.createQuery(Entity.class);
Root<Entity> from = cq.from(Entity.class);

cq.select(Entity);
TypedQuery<Entity> q = em.createQuery(cq);
List<Entity> allitems = q.getResultList();
The query demonstrates the basic steps to create a criteria.
EntityManager instance is used to create a CriteriaBuilder object.
CriteriaQuery instance is used to create a query object. This query object’s attributes will be modified with the details of the query.
CriteriaQuery.from method is called to set the query root.
CriteriaQuery.select is called to set the result list type.
TypedQuery<T> instance is used to prepare a query for execution and specifying the type of the query result.
getResultList method on the TypedQuery<T> object to execute a query. This query returns a collection of entities, the result is stored in a List.
Let us consider the example of employee database. Let us assume the jpadb.employee table contains following records:
Eid	Ename           Salary	Deg
401	Gopal	        40000	Technical Manager
402	Manisha	        40000	Proof reader
403	Masthanvali     35000	Technical Writer
404     Satish	        30000	Technical writer
405	Krishna	        30000	Technical Writer
406	Kiran	        35000	Proof reader

Create a JPA Project in the eclipse IDE named JPA_Eclipselink_Criteria. All the modules of this project are shown as follows:
Create a package named com.tutorialspoint.eclipselink.entity under ‘src’ package. 
Create a class named Employee.java under given package. The class Employee entity is shown as follows:
package com.tutorialspoint.eclipselink.entity;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

@Entity
public class Employee {
   @Id
   @GeneratedValue(strategy= GenerationType.AUTO) 	
   
   private int eid;
   private String ename;
   private double salary;
   private String deg;
   
   public Employee(int eid, String ename, double salary, String deg) {
      super( );
      this.eid = eid;
      this.ename = ename;
      this.salary = salary;
      this.deg = deg;
   }

   public Employee( ) {
      super();
   }

   public int getEid( ) {
      return eid;
   }
   
   public void setEid(int eid) {
      this.eid = eid;
   }

   public String getEname( ) {
      return ename;
   }
   
   public void setEname(String ename) {
      this.ename = ename;
   }

   public double getSalary( ) {
      return salary;
   }
   
   public void setSalary(double salary) {
      this.salary = salary;
   }

   public String getDeg( ) {
      return deg;
   }
   
   public void setDeg(String deg) {
      this.deg = deg;
   }
   
   @Override
   public String toString() {
   return "Employee [eid = " + eid + ", ename = " + ename + ", salary = " + salary + ", deg = " + deg + "]";
   }
}
Persistence.xml file is required to configure the database and the registration of entity classes.
Persistence.xml will be created by the eclipse IDE while cresting a JPA Project. The configuration details are user specification. The persistence.xml file is shown as follows: 
<?xml version = "1.0" encoding = "UTF-8"?>
<persistence version="2.0" xmlns = "http://java.sun.com/xml/ns/persistence" 
   xmlns:xsi = "http://www.w3.org/2001/XMLSchema-instance" 
   xsi:schemaLocation = "http://java.sun.com/xml/ns/persistence 
   http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd">
   
   <persistence-unit name = "Eclipselink_JPA" transaction-type = "RESOURCE_LOCAL">
      <class>com.tutorialspoint.eclipselink.entity.Employee</class>
      
      <properties>
         <property name = "javax.persistence.jdbc.url" value = "jdbc:mysql://localhost:3306/jpadb"/>
         <property name = "javax.persistence.jdbc.user" value = "root"/>
         <property name = "javax.persistence.jdbc.password" value = "root"/>
         <property name = "javax.persistence.jdbc.driver" 
         value="com.mysql.jdbc.Driver"/>
         <property name = "eclipselink.logging.level" value = "FINE"/>
         <property name = "eclipselink.ddl-generation" 
         value="create-tables"/>
      </properties>
      
   </persistence-unit>
</persistence>
This module contains the service classes, which implements the Criteria query part using the MetaData API initialization. Create a package named ‘com.tutorialspoint.eclipselink.service’. The class named CriteriaAPI.java is created under given package. The DAO class is shown as follows: 

package com.tutorialspoint.eclipselink.service;

import java.util.List;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.TypedQuery;
import javax.persistence.criteria.CriteriaBuilder;
import javax.persistence.criteria.CriteriaQuery;
import javax.persistence.criteria.Root;

import com.tutorialspoint.eclipselink.entity.Employee;

public class CriteriaApi {
   public static void main(String[] args) {
   
   EntityManagerFactory emfactory = Persistence.createEntityManagerFactory( "Eclipselink_JPA" );
   EntityManager entitymanager = emfactory.createEntityManager( );
   CriteriaBuilder criteriaBuilder = entitymanager.getCriteriaBuilder();
   CriteriaQuery<Object> criteriaQuery = criteriaBuilder.createQuery();
   Root<Employee> from = criteriaQuery.from(Employee.class);

   //select all records
   System.out.println(“Select all records”);
   CriteriaQuery<Object> select = c riteriaQuery.select(from);
   TypedQuery<Object> typedQuery = entitymanager.createQuery(select);
   List<Object> resultlist = typedQuery.getResultList();

   for(Object o:resultlist) {
      Employee e = (Employee)o;
      System.out.println("EID : " + e.getEid() + " Ename : " + e.getEname());
   }

   //Ordering the records 
   System.out.println(“Select all records by follow ordering”);
   CriteriaQuery<Object> select1 = criteriaQuery.select(from);
   select1.orderBy(criteriaBuilder.asc(from.get("ename")));
   TypedQuery<Object> typedQuery1 = entitymanager.createQuery(select);
   List<Object> resultlist1 = typedQuery1.getResultList();

   for(Object o:resultlist1){
      Employee e=(Employee)o;
      System.out.println("EID : " + e.getEid() + " Ename : " + e.getEname());
   }

   entitymanager.close( );
   emfactory.close( );
   }
}
After compilation and execution of the above program you will get output in the console panel of Eclipse IDE as follows:
Select All records
EID : 401 Ename : Gopal
EID : 402 Ename : Manisha
EID : 403 Ename : Masthanvali
EID : 404 Ename : Satish
EID : 405 Ename : Krishna
EID : 406 Ename : Kiran
Select All records by follow Ordering
EID : 401 Ename : Gopal
EID : 406 Ename : Kiran
EID : 405 Ename : Krishna
EID : 402 Ename : Manisha
EID : 403 Ename : Masthanvali
EID : 404 Ename : Satish

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