Spring Data JPA: EntityManager Explained!

Understanding EntityManager, Entity, Persistence Context

Introduction

• EntityManager is one of the key abstractions that JPA specification defines. EntityManager sits between the database and the application and plays the responsibility of managing the entity in context.
• In this article, we will explore these responsibilities and try to learn about EntityManager.

JPA Architecture

Credits: https://openjpa.apache.org/builds/1.0.2/apache-openjpa-1.0.2/docs/manual/jpa_overview_arch.html

Entity

• An Entity is basically a table representation of a relational database and an instance of that entity corresponds to a row in that table.
• It's a domain object in a persistent context.

Example of an Entity

• @Entity is an annotation that we use to mark a POJO to the persistent context-aware entity.
• @Table allows us to map entities to any custom-named database.
• Other such annotations are @Id @SequenceGenerator @Column etc. that can be used inside the entity.

@Entity
@Table(name="ACCOUNTS")
public class Account {
    @Id @GeneratedValue(strategy= GenerationType.SEQUENCE, generator = "accounts_seq")
    @SequenceGenerator(name = "accounts_seq", sequenceName = "accounts_seq", allocationSize = 1)
    @Column(name = "user_id")
    private int userId;
    private String username;
    private String password;
    private String email;
    private Timestamp createdOn;
    private Timestamp lastLogin;
}

Managing Entity

• Entities live in a persistent context (i.e. data store) and the persistent context defines the scope in which entity lifecycle such as creation, updation, removal, etc happens.
• In order to manage the lifecycle of the entity, the EntityManager instance is used. EntityManager instance provides methods using which we can interact with persistent context.
• Persistent Contexts where the entity & entitymanager lives are of two types, which are container-managed and application managed.

Container Managed

• The container-managed persistent context will make sure that…