Java 如何在数据库中存储递归对象？

如何存储引用了同一类的其他对象的对象

我尝试了很多使用注释和XML的方法，但都失败了

在最好的情况下，我会得到以下错误：

警告：插入对象“pmo.domain.entities”。Unit@1129829c“使用语句”插入单元（“名称”，父单元ID，ID）值（？，？）失败：未设置参数“#2”；SQL语句： 在单元中插入（“名称”，父单元ID，ID）值（？，？）[90012-196]

ExampleTest.java：

package pmo.domain.entities;

public class ExampleTest {
    public static void Main(String[] args) {
        Unit unit1 = new Unit(0, "Unit 1");
        Unit unit2 = new Unit(1, "Unit 2", unit1);
        Unit unit3 = new Unit(2, "Unit 3", unit2);

        PersistenceManagerFactory pmf = JDOHelper.getPersistenceManagerFactory("Test");
        PersistanceManager pm = pmf.getPersistenceManager();

        Transaction transaction = pm.currentTransaction();

        try {
            transaction.begin();
            pm.makePersistent(unit1);
            pm.makePersistent(unit2);
            pm.makePersistent(unit3);
            transaction.commit();
        } finally {
            if (transaction.isActive()) {
                transaction.rollback();
            }

            pm.close();
        }
    }
}

package pmo.domain.entities;

public class ExampleTest {
    public static void Main(String[] args) {
        Unit unit1 = new Unit(0, "Unit 1");
        Unit unit2 = new Unit(1, "Unit 2");
        Unit unit3 = new Unit(2, "Unit 3");

        unit2.addUnit(unit3);
        unit1.addUnit(unit2);

        PersistenceManagerFactory pmf = JDOHelper.getPersistenceManagerFactory("Test");
        PersistanceManager pm = pmf.getPersistenceManager();

        Transaction transaction = pm.currentTransaction();

        try {
            transaction.begin();
            pm.makePersistent(unit1);
            transaction.commit();
        } finally {
            if (transaction.isActive()) {
                transaction.rollback();
            }

            pm.close();
        }
    }
}

Unit.java：

package pmo.domain.entities;

public class Unit {
    private long id;
    private String name;
    private Unit parentUnit;

    public Unit(long id, String name, Unit parentUnit) {
        this.id = id;
        this.name = name;
        this.parentUnit = parentUnit;
    }

    public Unit(long id, String name) {
        this(id, name, null);
    }
}

package pmo.domain.entities;

public class Unit {
    private long id;
    private String name;
    private Collection<Unit> childUnits;

    public Unit(long id, String name, Collection<Unit> childUnits) {
        this.id = id;
        this.name = name;
        this.childUnits = childUnits;
    }

    public Unit(long id, String name) {
        this(id, name, new ArrayList<>());
    }

    public void addUnit(Unit unit) {
        this.childUnits.add(unit);
    }

    public void removeUnit(long id) {
        childUnits.removeIf(unit -> unit.id == id);
    }
}

package.jdo：

<?xml version="1.0" encoding="UTF-8" ?>
<jdo xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xmlns="http://xmlns.jcp.org/xml/ns/jdo/jdo"
     xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/jdo/jdo http://xmlns.jcp.org/xml/ns/jdo/jdo_3_1.xsd" version="3.1">
    <package name="pmo.domain.entities">
        <class name="Unit" table="unit">
            <field name="id" primary-key="true">
                <column name="id"/>
            </field>
            <field name="parentUnit">
                <column name="parent_unit_id" target-field="id"/>
                <foreign-key name="unit_id_fk" />
            </field>
        </class>
    </package>
</jdo>

<?xml version="1.0" encoding="UTF-8" ?>
<jdo xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xmlns="http://xmlns.jcp.org/xml/ns/jdo/jdo"
     xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/jdo/jdo http://xmlns.jcp.org/xml/ns/jdo/jdo_3_1.xsd" version="3.1">
    <package name="es.indra.pmo.pmomanagement.domain.entities">
        <class name="Employee" table="employee">
            <field name="id" primary-key="true"/>
        </class>
        <class name="Unit" table="unit">
            <field name="id" primary-key="true">
                <column name="id"/>
            </field>
            <field name="childUnits">
                <order column="parent_unit_id"/>
            </field>
        </class>
    </package>
</jdo>

---

我通过颠倒关系解决了这个问题。 从孩子到父母。现在，它是使用集合的父到子

ExampleTest.java：

package pmo.domain.entities;

public class ExampleTest {
    public static void Main(String[] args) {
        Unit unit1 = new Unit(0, "Unit 1");
        Unit unit2 = new Unit(1, "Unit 2", unit1);
        Unit unit3 = new Unit(2, "Unit 3", unit2);

        PersistenceManagerFactory pmf = JDOHelper.getPersistenceManagerFactory("Test");
        PersistanceManager pm = pmf.getPersistenceManager();

        Transaction transaction = pm.currentTransaction();

        try {
            transaction.begin();
            pm.makePersistent(unit1);
            pm.makePersistent(unit2);
            pm.makePersistent(unit3);
            transaction.commit();
        } finally {
            if (transaction.isActive()) {
                transaction.rollback();
            }

            pm.close();
        }
    }
}

package pmo.domain.entities;

public class ExampleTest {
    public static void Main(String[] args) {
        Unit unit1 = new Unit(0, "Unit 1");
        Unit unit2 = new Unit(1, "Unit 2");
        Unit unit3 = new Unit(2, "Unit 3");

        unit2.addUnit(unit3);
        unit1.addUnit(unit2);

        PersistenceManagerFactory pmf = JDOHelper.getPersistenceManagerFactory("Test");
        PersistanceManager pm = pmf.getPersistenceManager();

        Transaction transaction = pm.currentTransaction();

        try {
            transaction.begin();
            pm.makePersistent(unit1);
            transaction.commit();
        } finally {
            if (transaction.isActive()) {
                transaction.rollback();
            }

            pm.close();
        }
    }
}

Unit.java：

package pmo.domain.entities;

public class Unit {
    private long id;
    private String name;
    private Unit parentUnit;

    public Unit(long id, String name, Unit parentUnit) {
        this.id = id;
        this.name = name;
        this.parentUnit = parentUnit;
    }

    public Unit(long id, String name) {
        this(id, name, null);
    }
}

package pmo.domain.entities;

public class Unit {
    private long id;
    private String name;
    private Collection<Unit> childUnits;

    public Unit(long id, String name, Collection<Unit> childUnits) {
        this.id = id;
        this.name = name;
        this.childUnits = childUnits;
    }

    public Unit(long id, String name) {
        this(id, name, new ArrayList<>());
    }

    public void addUnit(Unit unit) {
        this.childUnits.add(unit);
    }

    public void removeUnit(long id) {
        childUnits.removeIf(unit -> unit.id == id);
    }
}

包pmo.domain.entities；
公营课组{
私人长id；
私有字符串名称；
私人收藏单位；
公共单位（长id、字符串名称、集合子单位）{
this.id=id；
this.name=名称；
this.childUnits=childUnits；
}
公共单位（长id、字符串名称）{
此（id、名称、新ArrayList（））；
}
公共单位（单位）{
this.childUnits.add（单位）；
}
公共无效删除单元（长id）{
childUnits.removeIf（unit->unit.id==id）；
}
}

package.jdo：

<?xml version="1.0" encoding="UTF-8" ?>
<jdo xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xmlns="http://xmlns.jcp.org/xml/ns/jdo/jdo"
     xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/jdo/jdo http://xmlns.jcp.org/xml/ns/jdo/jdo_3_1.xsd" version="3.1">
    <package name="pmo.domain.entities">
        <class name="Unit" table="unit">
            <field name="id" primary-key="true">
                <column name="id"/>
            </field>
            <field name="parentUnit">
                <column name="parent_unit_id" target-field="id"/>
                <foreign-key name="unit_id_fk" />
            </field>
        </class>
    </package>
</jdo>

<?xml version="1.0" encoding="UTF-8" ?>
<jdo xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xmlns="http://xmlns.jcp.org/xml/ns/jdo/jdo"
     xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/jdo/jdo http://xmlns.jcp.org/xml/ns/jdo/jdo_3_1.xsd" version="3.1">
    <package name="es.indra.pmo.pmomanagement.domain.entities">
        <class name="Employee" table="employee">
            <field name="id" primary-key="true"/>
        </class>
        <class name="Unit" table="unit">
            <field name="id" primary-key="true">
                <column name="id"/>
            </field>
            <field name="childUnits">
                <order column="parent_unit_id"/>
            </field>
        </class>
    </package>
</jdo>

无论是否属于同一类，都不能存储一条记录，该记录的键指向另一条不存在的记录。在您的情况下，第一次保存将失败，因为父级是null@jr593在其他的测试中，我在没有父母的情况下做的测试很好。只有在证明它失败的情况下才可以。我正在寻找以另一种方式实现这种关系的方法。