Java Persistence API (JPA) with Instance


Overview

Builders of an object oriented language wish to work with objects not the relational knowledge. Once we wish to retailer our objects or knowledge in a database, we have to write database particular queries in our java code utilizing JDBC. Now with this method, there are few issues. First, we have to explicitly convert database queries into java objects and database outcome set into java objects. Second, we have to manually deal with all database transactions. To all these issues, there may be one answer – Java Persistence API.

Java Persistence API aka JPA is a specification that gives functionalities of ORM in Java. ORM stands for Object Relational Mapping. It principally an method to map Java objects to Database tables and vice versa.

JPA permits the java developer to work straight with java objects fairly than writing SQL statements.

With a purpose to map the java objects to database tables, we have to present persistence metadata. It may be written utilizing XML configuration or we will additionally use annotations. If we use xml configuration and annotation each then annotations might be overwritten with xml configuration.

JPA solves not one however many issues, a few of them are talked about beneath:

  1. Java developer want to not know learn how to write SQL statements. They don’t essentially have the information of relational database to retailer their objects in database.
  2. Altering the database in future wouldn’t make a whole lot of code efforts as we write question utilizing question language offered by JPA.

One factor to notice right here is JPA is only a specification and a whole lot of implementations can be found for it. EclipseLink and Hibernate is few of the favored implementations of JPA.

EclipseLink is type of reference implementation of JPA.

To work with JPA, we have to study some key ideas related to it. Let’s perceive every of them adopted by a working instance.

Set up and Configuration for JPA

JPA is only a specification and to be able to make it work, we have to present the implementation for JPA. To avoid wasting time, there are such a lot of implementations for JPA already exists so let’s simply use one in every of them.

Now this tutorial is about Java Persistence API so we’ll use EclipseLink for this tutorial because it’s a pattern implementation for the JPA.

As well as, we’ll use h2 database to make issues less difficult to know.

Let’s create a maven mission and add following dependencies:

<!-- JPA with EclipseLink -->
    <dependency>
        <groupId>org.eclipse.persistence</groupId>
        <artifactId>eclipselink</artifactId>
        <model>3.0.0</model>
    </dependency>

    <dependency>
        <groupId>org.eclipse.persistence</groupId>
        <artifactId>javax.persistence</artifactId>
        <model>2.2.1</model>
    </dependency>

<!-- H2 Database -->

    <dependency>
        <groupId>com.h2database</groupId>
        <artifactId>h2</artifactId>
        <model>1.4.200</model>
    </dependency>

Entity

Entity in JPA is a category whose fields might be continued in a database. Entities in JPA are easy POJO lessons with some annotations. These lessons are annotated with @Entity. @Entity annotation tells JPA to persist this class in database. Every entity will correspond to 1 database desk. Every created object of the Entity class will correspond to 1 row of the desk.

As every entity represents a desk within the database. we will declare the title for the database desk utilizing @Desk annotation.

Now let’s create an entity Pupil to carry and persist Pupil(s) states:

@Entity
@Desk(title="scholar")
public class Pupil {
}

If we don’t present @Desk annotation then it routinely picks the category title because the desk title.

Let’s create some fields that denote the scholars state and create getters and setters methodology for every state:

@Entity
@Desk(title="scholar")
public class Pupil {
	
	non-public int rollNumber;
	non-public String fullName;
	non-public String division;
	
	
	//Getters and Setters	
}

You will need to comply with naming conference when creating getters and setters to let JPA know the corresponding getter and setter for the sector.

Every area/state of the entity/class will correspond to 1 column within the database desk. Now let’s outline column title for every column.

@Entity
	@Desk(title="scholar")
	public class Pupil {
		
		@Column(title="roll_number")
		non-public int rollNumber;
		
		@Column(title="full_name")
		non-public String fullName;
		
		@Column(title="division")
		non-public String division;	
	}

Now we have to outline one column that may act as the first key for the coed desk. The column annotated with @Id will develop into the first key for the desk. JPA offers mechanism to auto generate the worth for the first column. We are able to use @GeneratedValue with GenerationType.AUTO because the parameter. Let’s outline it:

@Entity
	@Desk(title="scholar")
	public class Pupil {
		
		@Id
		@GeneratedValue(technique = GenerationType.AUTO)
		non-public lengthy id;
		
		@Column(title="roll_number")
		non-public int rollNumber;
		
		@Column(title="full_name")
		non-public String fullName;
		
		@Column(title="division")
		non-public String division;
}

There are some guidelines related once we are creating an entity:

  1. The entity class should be denoted utilizing @Entity.
  2. It should have one major key.
  3. It can’t be declared as remaining.
  4. It should have a default constructor.

Suppose, we are not looking for any area to be saved within the database then that area ought to be denoted with @Transient annotation.

Entity Relationships

Within the above part, we created a Pupil entity. OK, that’s straightforward. However it’s possible you’ll ask what if a Pupil has relation with different Entity as properly. How you can handle entity relationship? What if a Pupil entity has an Handle entity.

There are easy annotations offered by JPA to deal with and handle the entity relationship. Let’s take a look at them:

1) One-to-One: When one entity accommodates a reference of one other entity then that is referred to as a one-to-one entity relationship. One entity can solely include single occasion of the referenced entity. The referenced entity is annotated with @OnetoOne annotation.

2) One-to-Many: One entity can include a set of references of one other entity. This relationship is named one-to-many entity relationship. On this case, the entity accommodates a number of cases of the referenced entity. For this @OnetoMany annotation is used.

3) Many-to-One: When a number of cases of an entity are associated to 1 occasion of one other entity, Many-to-One entity relationship is established. @ManyToOne annotation is used on this context.

4) Many-to-Many: When a number of cases of an entity accommodates references of a number of cases of one other entity then Many-to-Many relationship is established. That is annotated with @ManyToMany.

Entity Supervisor

Entity Supervisor is the utility that’s accountable for all of the operations from and to the database. It handles and manages a number of operations like discovering the objects to persist, persisting the objects and eradicating the objects.

All entities are managed by the entity supervisor of JPA.

The entity supervisor is routinely out there in JavaEE utility. For different forms of utility, we have to handle it manually. It’s out there in javax.persistence package deal.

EntityManagerFactory and Persistence Unit

With a purpose to persist the entity, we’ll outline our persistence unit for the appliance. We are able to outline it within the persistence.xml file below the META-INF folder.

The entities are managed (created, up to date, and deleted) through EntityManager which is created by the EntityManagerFactory which is configured utilizing our persistence unit aka persistence.xml file.

The persistence.xml file accommodates the database configuration particulars equivalent to driver, consumer particulars and so forth.

That is how our persistence.xml file is in search of the Pupil utility instance that we’re engaged on. This MUST BE saved within the META-INF folder inside assets folder of your mission.

<persistence xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://java.solar.com/xml/ns/persistence 
                      http://java.solar.com/xml/ns/persistence/persistence_2_0.xsd"
  model="2.0" xmlns="http://java.solar.com/xml/ns/persistence">
  
  <persistence-unit title="student-persistence" transaction-type="RESOURCE_LOCAL">
    <class>com.app.Studentt</class>
    <properties>
      <property title="javax.persistence.jdbc.driver" worth="org.h2.Driver" />
      <property title="javax.persistence.jdbc.url"    worth="jdbc:h2:mem:take a look at" />
      <property title="javax.persistence.jdbc.consumer" worth="admin" />
      <property title="javax.persistence.jdbc.password" worth="admin" />
    </properties>
  </persistence-unit>
</persistence>

Discover that we’re utilizing h2 database for simplicity. We’ve additionally offered a reputation to this persistence.xml file contained in the title attribute of persistence-unit tag. It should later be used once we are creating occasion for the EntityManagerFactory.

Persistence of Entity

Tell us some steps that includes in persisting an entity in JPA.

1. Get the EntityManager

EntityManager is offered by the EntityManagerFactory. First, we’ll create the item of EntityManagerFactory after which we’ll create our entity supervisor.

EntityManagerFactory managerFactory =Persistence.createEntityManagerFactory("student-persistence");  

EntityManagerFactory is configured utilizing the persistence unit that’s why we now have offered the title of our persistence unit within the createEntityManagerFactory methodology.

EntityManager supervisor =managerFactory.createEntityManager();  

Now we’re utilizing the occasion of EntityManagerFactory to create the EntityManager occasion.

2. Initializing the EntityManager / Beginning the transaction

supervisor.getTransaction().start();

It will get the transaction from the EntityManager occasion and begins it by calling the start() methodology.

3. Persisting the entity

Now we’ll persist our Pupil entity within the database. Let’s create and occasion of Pupil and persist it utilizing JPA EntityManager.

Pupil scholar = new Pupil();
scholar.setDepartment("CS");
scholar.setFullName("CHANDLER BING");
scholar.setRollNumber(123);
		
supervisor.persist(scholar);

4. Closing the transaction

Final step is to shut the began transaction.

supervisor.getTransaction().commit();

That is how our remaining code appears to be like like in our App.java:

package deal com.app;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

public class App {
	public static void predominant(String[] args) {
		EntityManagerFactory managerFactory = Persistence.createEntityManagerFactory("student-persistence");

		EntityManager supervisor = managerFactory.createEntityManager();

		attempt {

			supervisor.getTransaction().start();

			Pupil scholar = new Pupil();
			scholar.setDepartment("CS");
			scholar.setFullName("CHANDLER BING");
			scholar.setRollNumber(123);

			supervisor.persist(scholar);

			supervisor.getTransaction().commit();
		} catch (Exception e) {
			e.printStackTrace();
		} lastly {
			managerFactory.shut();
			supervisor.shut();
		}
	}

}

JPQL (Java Persistence Question Language)

JPQL is a question language particularly designed for Java Persistence API. It’s an object oriented question language. We use this language to carry out database operations on our entities. It doesn’t matter what database we’re utilizing, JPQL stays similar as it really works on entities not database tables.

JPA transforms the JPQL to database particular question language to truly carry out the database operations.

Conclusion

This tutorial helped you in understanding the ORM and JPA as its core. This API helps java builders to work on objects fairly than relational knowledge. Builders want to not know database particular syntax to retailer their objects into database. It additionally avoids the handbook operation of changing objects into relational knowledge and vice versa.

I hope you discover this tutorial useful. Thanks.



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