JPA data class checklist and example persistence.xml for EclipseLink

Data_class_checklist.md

• Primary Key:

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long oid;

• annotate all data classes with @Entity
• Create attributes
• Annotate relations
• Create getters/Setters, equals/hashCode (required for Sets)
  Eclipse: Shift+Alt+S
  IntelliJ: Alt+Insert

Relations

Inheritance

• annotate superclass with @Inheritance(strategy = InheritanceType.JOINED) (this would create one table per entity, you could also use TABLE_PER_CLASS (one table per superclass and subclasses) or SINGLE_TABLE (one table for the whole hierachy) instead of JOINED)

1:1

bidirectional

• stronger class:

   @OneToOne
   @JoinColumn(name="<name of column>")

• weaker class:

   @OneToOne(mappedBy="<attribute name on other side>")

unidirectional

    @OneToOne
    @JoinColumn(name = "<column name>")

1:n

bidirectional

• Class with 1 in UML diagram (entity that exists only once per relation):

    @OneToMany(mappedBy = "<attribute on other side>")
    List<>

• Class with * in UML diagram (multiple instances of this entity exist per relation

    @ManyToOne
    @JoinColumn(name = "<name of column>")

unidirectional

• Class with * in UML diagram:

   @ManyToOne

OR

• Class with 1 in UML diagram:

    @OneToMany
    List<>

n:m

bidirectional

• stronger class:

    @ManyToMany(cascade = CascadeType.PERSIST)
    @JoinTable(
        name = "<name of table>",
       joinColumns = {@JoinColumn (name = "<column name of attribute>")},
       inverseJoinColumns =  {@JoinColumn(name = "<column name of attribute on other side>")}
    )

• weaker class:

   @ManyToMany(mappedBy = "<name of attribute on other side>")

unidirectional

    @ManyToMany (cascade = CascadeType.PERSIST)
    @JoinTable(
        name = "<name of table>",
        joinColumns = {@JoinColumn (name = "<column name of attribute>")},
        inverseJoinColumns =  {@JoinColumn(name = "<column name of attribute on other side>")}
    )

Hints

• @JoinTable and @JoinColumn are optional. These annotations allow specific configuration how the database should be structured.
• @Table can be added to entities in order to specify table names and unique constrains.

persistence.xml

<!-- example persistence.xml for EclipseLink -->
<?xml version="1.0" encoding="UTF-8" ?>
<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"
	version="2.0" xmlns="http://java.sun.com/xml/ns/persistence">
	<!-- TODO replace DATABASENAME/SCHEMANAME, create that DB, assign permissions to DB, replace USERNAME/PASSWORD with credentials that can access this schema -->
	<persistence-unit name="MSSQL" transaction-type="RESOURCE_LOCAL">
		<exclude-unlisted-classes>false</exclude-unlisted-classes>
		<properties>
			<!-- use jakarta instead of javax if you use jakarta EE -->
			<property name="javax.persistence.jdbc.driver" value="com.microsoft.sqlserver.jdbc.SQLServerDriver" />
			<property name="javax.persistence.jdbc.url"
				value="jdbc:sqlserver://localhost:1433;databaseName=DATABASENAME" />
			<property name="javax.persistence.jdbc.user" value="<USERNAME>" />
			<property name="javax.persistence.jdbc.password" value="<Password>" />

			<!--EclipseLink should create the database schema automatically -->
			<property name="eclipselink.ddl-generation" value="create-or-extend-tables" />
			<property name="eclipselink.ddl-generation.output-mode"
				value="database" />
		</properties>
	</persistence-unit>

	<persistence-unit name="MYSQL" transaction-type="RESOURCE_LOCAL">
		<exclude-unlisted-classes>false</exclude-unlisted-classes>
		<properties>
			<!-- use jskarta instead of javax if you use jakarta EE -->
			<property name="javax.persistence.jdbc.driver" value="com.mysql.cj.jdbc.Driver" />
			<property name="javax.persistence.jdbc.url"
					  value="jdbc:mysql://localhost:3306/SCHEMANAME?serverTimezone=UTC" />
			<property name="javax.persistence.jdbc.user" value="<USERNAME>" />
			<property name="javax.persistence.jdbc.password" value="<PASSWORD>" />

			<!--EclipseLink should create the database schema automatically -->
			<property name="eclipselink.ddl-generation" value="create-or-extend-tables" />
			<property name="eclipselink.ddl-generation.output-mode"
					  value="database" />
		</properties>
		
	</persistence-unit>
</persistence>