Added the single-table, joined and table-per-entity inheritance

strategy examples.

Added a mapped superclass example.

jpa10/pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <groupId>example</groupId>
+    <artifactId>jpa10</artifactId>
+    <version>1.0-SNAPSHOT</version>
+    <packaging>jar</packaging>
+    <properties>
+        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+        <maven.compiler.source>17</maven.compiler.source>
+        <maven.compiler.target>17</maven.compiler.target>
+    </properties>
+    <dependencies>
+        <dependency>
+            <groupId>org.hibernate</groupId>
+            <artifactId>hibernate-core-jakarta</artifactId>
+            <version>5.5.7.Final</version>
+        </dependency>
+        <dependency>
+            <groupId>org.glassfish.jaxb</groupId>
+            <artifactId>jaxb-runtime</artifactId>
+            <version>3.0.0</version>
+        </dependency>
+        <dependency>
+            <groupId>org.postgresql</groupId>
+            <artifactId>postgresql</artifactId>
+            <version>42.2.23.jre7</version>
+        </dependency>
+        <dependency>
+            <groupId>example</groupId>
+            <artifactId>jpaunit</artifactId>
+            <version>1.0-SNAPSHOT</version>
+        </dependency>
+    </dependencies>
+</project>

jpa10/src/main/java/example/jpa/inheritance/TestJoined.java

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+package example.jpa.inheritance;
+
+import example.jpa.inheritance.entities.Chocolate;
+import example.jpa.inheritance.entities.Product;
+import example.jpa.inheritance.entities.Wine;
+import example.jpaunit.JPAUnit;
+
+public class TestJoined {
+    public static void main(String[] args) {
+        var product = new Product();
+        product.setName("Sugar");
+
+        var choco = new Chocolate();
+        choco.setName("Ritter Sport");
+        choco.setCalories(575.0);
+
+        var wine = new Wine();
+        wine.setName("Tsinandali");
+        wine.setYear(2018);
+
+        try (var unit = new JPAUnit("postgres-unit")) {
+            unit.runTransaction(entityManager -> {
+                entityManager.persist(product);
+                entityManager.persist(choco);
+                entityManager.persist(wine);
+            });
+        }
+    }
+}

jpa10/src/main/java/example/jpa/inheritance/TestMappedSuperclass.java

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+package example.jpa.inheritance;
+
+import example.jpa.inheritance.entities.Bicycle;
+import example.jpa.inheritance.entities.Car;
+import example.jpaunit.JPAUnit;
+
+public class TestMappedSuperclass {
+    public static void main(String[] args) {
+        var car = new Car();
+        car.setColor("Black");
+        car.setHorsePower(140);
+
+        var bike = new Bicycle();
+        bike.setColor("Silver");
+        bike.setModel("Juggernaut");
+
+        try (var unit = new JPAUnit("postgres-unit")) {
+            unit.runTransaction(entityManager -> {
+                entityManager.persist(car);
+                entityManager.persist(bike);
+            });
+        }
+    }
+}

jpa10/src/main/java/example/jpa/inheritance/TestSingleTable.java

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+package example.jpa.inheritance;
+
+import example.jpa.inheritance.entities.Bug;
+import example.jpa.inheritance.entities.Feature;
+import example.jpa.inheritance.entities.Issue;
+import example.jpaunit.JPAUnit;
+
+public class TestSingleTable {
+    public static void main(String[] args) {
+        var issue = new Issue();
+        issue.setTitle("Make our software great again");
+        issue.setDescription("Our software is not great anymore. " +
+                "This has to be sorted out!!!");
+
+        var bug = new Bug();
+        bug.setTitle("It doesn't work");
+        bug.setDescription("Whatever I do, nothing happens.");
+        bug.setSeverity(1);
+
+        var feature = new Feature();
+        feature.setTitle("Integrate the spreadsheet into our software");
+        feature.setDescription("I feel lazy switching between windows " +
+                "and it greatly reduces my productivity as a user.");
+        feature.setTargetVersion("15RC");
+
+        try (var unit = new JPAUnit("postgres-unit")) {
+            unit.runTransaction(entityManager -> {
+                entityManager.persist(issue);
+                entityManager.persist(bug);
+                entityManager.persist(feature);
+            });
+        }
+    }
+}

jpa10/src/main/java/example/jpa/inheritance/TestTablePerEntity.java

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+package example.jpa.inheritance;
+
+import example.jpa.inheritance.entities.Employee;
+import example.jpa.inheritance.entities.Manager;
+import example.jpaunit.JPAUnit;
+
+public class TestTablePerEntity {
+    public static void main(String[] args) {
+        var employee = new Employee();
+        employee.setName("Вася Пупкин");
+        var manager = new Manager();
+        manager.setName("Кощей Бессмертный");
+        manager.setResponsibility("Сеять плохое, злое и вечное.");
+
+        try (var unit = new JPAUnit("postgres-unit")) {
+            unit.runTransaction(entityManager -> {
+                entityManager.persist(employee);
+                entityManager.persist(manager);
+            });
+        }
+    }
+}

jpa10/src/main/java/example/jpa/inheritance/entities/Bicycle.java

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+package example.jpa.inheritance.entities;
+
+import jakarta.persistence.Entity;
+import jakarta.persistence.Table;
+
+@Entity
+@Table(name = "bicycles")
+public class Bicycle extends Vehicle {
+    private String model;
+
+    public String getModel() {
+        return model;
+    }
+
+    public void setModel(String model) {
+        this.model = model;
+    }
+
+}

jpa10/src/main/java/example/jpa/inheritance/entities/Bug.java

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+package example.jpa.inheritance.entities;
+
+import jakarta.persistence.DiscriminatorValue;
+import jakarta.persistence.Entity;
+
+@Entity
+@DiscriminatorValue("B")
+public class Bug extends Issue {
+    private int severity;
+
+    public int getSeverity() {
+        return severity;
+    }
+
+    public void setSeverity(int severity) {
+        this.severity = severity;
+    }
+
+}

jpa10/src/main/java/example/jpa/inheritance/entities/Car.java

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+package example.jpa.inheritance.entities;
+
+import jakarta.persistence.Column;
+import jakarta.persistence.Entity;
+import jakarta.persistence.Table;
+
+@Entity
+@Table(name = "cars")
+public class Car extends Vehicle{
+    @Column(name = "horse_power")
+    private int horsePower;
+
+    public int getHorsePower() {
+        return horsePower;
+    }
+
+    public void setHorsePower(int horsePower) {
+        this.horsePower = horsePower;
+    }
+
+}

jpa10/src/main/java/example/jpa/inheritance/entities/Chocolate.java

@@ -0,0 +1,19 @@
+package example.jpa.inheritance.entities;
+
+import jakarta.persistence.Entity;
+import jakarta.persistence.Table;
+
+@Entity
+@Table(name = "chocolates")
+public class Chocolate extends Product {
+    double calories;
+
+    public double getCalories() {
+        return calories;
+    }
+
+    public void setCalories(double calories) {
+        this.calories = calories;
+    }
+
+}

jpa10/src/main/java/example/jpa/inheritance/entities/Employee.java

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+package example.jpa.inheritance.entities;
+
+import jakarta.persistence.Entity;
+import jakarta.persistence.GeneratedValue;
+import jakarta.persistence.GenerationType;
+import jakarta.persistence.Id;
+import jakarta.persistence.Inheritance;
+import jakarta.persistence.InheritanceType;
+import jakarta.persistence.SequenceGenerator;
+import jakarta.persistence.Table;
+
+/**
+ * Employee entity.
+ * 
+ * This entity demonstrates table-per-entity inheritance strategy.
+ * It can't be used with the IDENTITY ID generation, so a SEQUENCE generation
+ * is used instead.
+ */
+@Entity
+@Table(name = "employees")
+@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
+@SequenceGenerator(name = "employee_id_generator",
+                   sequenceName = "employee_id_sequence",
+                   allocationSize = 5)
+public class Employee {
+    @Id
+    @GeneratedValue(strategy = GenerationType.SEQUENCE,
+                    generator = "employee_id_generator")
+    private int id;
+
+    private String name;
+
+    public int getId() {
+        return id;
+    }
+
+    public void setId(int id) {
+        this.id = id;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+}

jpa10/src/main/java/example/jpa/inheritance/entities/Feature.java

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+package example.jpa.inheritance.entities;
+
+import jakarta.persistence.Column;
+import jakarta.persistence.DiscriminatorValue;
+import jakarta.persistence.Entity;
+
+@Entity
+@DiscriminatorValue("F")
+public class Feature extends Issue {
+    @Column(name = "target_version")
+    private String targetVersion;
+
+    public String getTargetVersion() {
+        return targetVersion;
+    }
+
+    public void setTargetVersion(String targetVersion) {
+        this.targetVersion = targetVersion;
+    }
+
+}

jpa10/src/main/java/example/jpa/inheritance/entities/Issue.java

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+package example.jpa.inheritance.entities;
+
+import jakarta.persistence.DiscriminatorColumn;
+import jakarta.persistence.DiscriminatorValue;
+import jakarta.persistence.Entity;
+import jakarta.persistence.GeneratedValue;
+import jakarta.persistence.GenerationType;
+import jakarta.persistence.Id;
+import jakarta.persistence.Inheritance;
+import jakarta.persistence.InheritanceType;
+import jakarta.persistence.Table;
+
+/**
+ * Issue entity.
+ * 
+ * This entity demonstrates the single table inheritance strategy.
+ * The discriminator column is defined via {@code @DiscriminatorColumn},
+ * its value is defined via {@code @DiscriminatorValue}.
+ */
+@Entity
+@Table(name = "issues")
+@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
+@DiscriminatorColumn(name = "type")
+@DiscriminatorValue("I")
+public class Issue {
+
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    private int id;
+
+    private String title;
+
+    private String description;
+
+    public int getId() {
+        return id;
+    }
+
+    public void setId(int id) {
+        this.id = id;
+    }
+
+    public String getTitle() {
+        return title;
+    }
+
+    public void setTitle(String title) {
+        this.title = title;
+    }
+
+    public String getDescription() {
+        return description;
+    }
+
+    public void setDescription(String description) {
+        this.description = description;
+    }
+
+}

jpa10/src/main/java/example/jpa/inheritance/entities/Manager.java

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+package example.jpa.inheritance.entities;
+
+import jakarta.persistence.Entity;
+import jakarta.persistence.Table;
+
+@Entity
+@Table(name = "managers")
+public class Manager extends Employee {
+    private String responsibility;
+
+    public String getResponsibility() {
+        return responsibility;
+    }
+
+    public void setResponsibility(String responsibility) {
+        this.responsibility = responsibility;
+    }
+
+}