Hibernate

• Framework for persisting java objects to a database
• Handles all the low level SQL
• Minimize the amount of JDBC code
• Hibernate uses JDBC for all db communications. Abstract this code from the developer
• Provides Object-to-Relational Mapping (ORM)
• Hibernate is built on top of Java Persistence API (JPA), which is a standard specification for ORM
• Hibernate implements all of JPA annotations

MySQL DB

# MySQL
docker container run \
  -p 3306:3306 \
  --name meusql \
  --env MYSQL_ROOT_PASSWORD=123 \
  --detach \
  mysql:8.0

docker container cp 01-create-user.sql meusql:/tmp
docker container cp 02-student-tracker.sql meusql:/tmp
docker container exec -it meusql bash
mysql -u root -p123 < /tmp/01-create-user.sql
mysql -u root -p123 < /tmp/02-student-tracker.sql

• Url for connection <jdbc:mysql://localhost:3306/hb_student_tracker?useSSL=false&serverTimezone=UTC
• SSL must be set to false

Dependencies

<dependencies>
  <!-- https://mvnrepository.com/artifact/mysql/mysql-connector-java -->
  <dependency>
    <groupId>mysql</groupId>
    <artifactId>mysql-connector-java</artifactId>
    <version>8.0.22</version>
  </dependency>

  <!-- https://mvnrepository.com/artifact/org.hibernate/hibernate-core -->
  <dependency>
    <groupId>org.hibernate</groupId>
    <artifactId>hibernate-core</artifactId>
    <version>5.4.27.Final</version>
  </dependency>
</dependencies>

Hibernate configuration

• hibernate.cfg.xml
• For Maven projects, it should be placed under /src/main/resources

<!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN" "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">

<hibernate-configuration>

    <session-factory>

        <!-- JDBC Database connection settings -->
        <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="connection.url">jdbc:mysql://localhost:3306/hb_student_tracker?useSSL=false&amp;serverTimezone=UTC</property>
        <property name="connection.username">root</property>
        <property name="connection.password">123</property>

        <!-- JDBC connection pool settings ... using built-in test pool -->
        <property name="connection.pool_size">1</property>

        <!-- Select our SQL dialect -->
        <property name="dialect">org.hibernate.dialect.MySQLDialect</property>

        <!-- Echo the SQL to stdout -->
        <property name="show_sql">true</property>

        <!-- Set the current session context -->
        <property name="current_session_context_class">thread</property>

    </session-factory>


</hibernate-configuration>

Entity lifecycle

• The session factory reads the hibernate config file and create session objects
• The session objects are created once in your app

• The session wraps a JDBC connection. Short lived object

• Entity lifecycle

• Detach: entity is not associated with a hibernate session
• Merge: reattach an entity to the session
• Persist: transition new instance to a managed state (next flush/commit will save to db)
• Remove: remove managed entity. (next flush/commit will delete from db)
• Refresh: reload/sync object with data from db. Prevents stale data (data in memory different from data in db)

Entity class

• It's a java class (POJO) that is mapped to a database table
• 2 options for mapping a class
• XML config (legacy)
• Java annotations (modern)

@Entity
@Table(name="student")
public class Student {
    @Id // Primary key
    @Column(name="id")
    private int id;

    @Column(name="first_name")
    private String firstName;

    @Column(name="last_name")
    private String lastName;

    @Column(name="email")
    private String email;
}

Primary keys

@Id // Primary key
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name="id")
private int id;

• ID generation strategies
• Generation.Type.AUTO: Pick strategy for a particular database
• Generation.Type.IDENTITY: Assign primary keys using database identity column. Auto increment
• Generation.Type.SEQUENCE: Assign primary keys using database sequence
• Generation.Type.TABLE: Assign primary keys using underlying database table
• Custom strategy: Override the method public Serializable generate(...) with implementation from org.hibernate.id.IdentifierGenerator