Hibernate

Concepts

JDBC -> JPA -> Hibernate -> Spring Data JPA / Spring Data Mongodb / Spring Data Elastic Search
Session Factory grabs the configFile: username + password, then creates sessions
Hibernate use reflection API to auto generate SQL statements
Hibernate allows us to write:
1. HQL(Hibernate Query Language), just in case the APIs are not flexible enough to catering some situations.
2. Native SQL. cons: with dialect language(mysql, postgres, oracle)
Cache strategies in Hibernate:
- First Level(default): in session level. The session(a user create a session) is private, it means it cannot access any content belong to other sessions.
- Second Level: in session factory level. Add extra configurations to the ConfigFile.
  - For example, there are many users sending the same queries and they are supposed to return the same result. with second level caching strategy, the cache will be pubic to all sessions.
github

Setup

pom.xml

        <dependency>
            <groupId>org.hibernate</groupId>
            <artifactId>hibernate-core</artifactId>
            <version>5.4.20.Final</version>
        </dependency>
        <dependency>
            <groupId>com.h2database</groupId>
            <artifactId>h2</artifactId>
            <version>1.4.200</version>
        </dependency>

Old style cfg

src/main/resources/hibernate.cfg-one-to-one.xml

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
        <property name="hibernate.connection.driver_class">org.h2.Driver</property>
        <property name="hibernate.connection.url">jdbc:h2:mem:test</property>
        <property name="hibernate.connection.username">sa</property>
        <property name="hibernate.connection.password"></property>
        <property name="hibernate.dialect">org.hibernate.dialect.H2Dialect</property>
        <property name="show_sql">true</property>
        <property name="hbm2ddl.auto">create-drop</property>
        <mapping class="com.yixianwang.hibernate.onetoone.dto.foreignKeyAsso.EmployeeEntity"/>
        <mapping class="com.yixianwang.hibernate.onetoone.dto.foreignKeyAsso.AccountEntity"/>
        <mapping class="com.yixianwang.hibernate.onetoone.dto.sharedPrimaryKey.EmployeeEntity"/>
        <mapping class="com.yixianwang.hibernate.onetoone.dto.sharedPrimaryKey.AccountEntity"/>
        <mapping class="com.yixianwang.hibernate.onetoone.dto.joinTable.EmployeeEntity"/>
        <mapping class="com.yixianwang.hibernate.onetoone.dto.joinTable.AccountEntity"/>
        <mapping class="com.yixianwang.hibernate.onetoone.dto.mapsById.EmployeeEntity"/>
        <mapping class="com.yixianwang.hibernate.onetoone.dto.mapsById.AccountEntity"/>
    </session-factory>
</hibernate-configuration>

src/main/resources/hibernate.cfg-one-to-many.xml

src/main/resources/hibernate.cfg-many-to-many.xml

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
        <property name="hibernate.connection.driver_class">org.h2.Driver</property>
        <property name="hibernate.connection.url">jdbc:h2:mem:test</property>
        <property name="hibernate.connection.username">sa</property>
        <property name="hibernate.connection.password"></property>
        <property name="hibernate.dialect">org.hibernate.dialect.H2Dialect</property>
        <property name="show_sql">true</property>
        <property name="hbm2ddl.auto">create-drop</property>
        <mapping class="com.yixianwang.hibernate.manyToMany.joinTable.ReaderEntity"/>
        <mapping class="com.yixianwang.hibernate.manyToMany.joinTable.SubscriptionEntity"/>
    </session-factory>
</hibernate-configuration>

New style cfg

only use annotation

One to One

two way one to one

EmployeeEntity.java

    @OneToOne(cascade = CascadeType.REMOVE)
    @PrimaryKeyJoinColumn
    private AccountEntity account;

AccountEntity.java

    @OneToOne(mappedBy="account", cascade = CascadeType.REMOVE)
    private EmployeeEntity employee;

One to Many

one employee has multiple accounts

EmployeeEntity.java

    @OneToMany(cascade=CascadeType.ALL)
    @JoinTable(name="EMPLOYEE_ACCOUNT", joinColumn={@JoinColumn(name="EMPLOYEE_ID", referencedColumnName="ID")}
                                  , inverseJoinColumns={@JoinColumn(name="ACCOUNT_ID", referencedColumnName="ID")})
    private Set<AccountEntity> accounts;

use Hibernate

Session session = HibernateUtil.getSessionFactory().oepnSession();
session.beginTransaction();

// ...

// save(insert)
session.save(firstEmployee);
session.save(secondEmployee);

session.getTransaction().commit();
HibernateUtil.shutdown();

CascadeType

In practice, we should always use the minimum scope.

CascadeType.class

package javax.persistence;

public enum CascadeType {
    All,
    PERSIST,
    MERGE,
    REMOVE,
    REFRESH,
    DETACH;

    private CascadeType() {
    }
}

@OneToMany(cascade=CascadeType.ALL), when delete(or update) a employee, it also deletes all corresponding accounts.