hibernate-mapping package="model">
   <class name="BuchungLeistung" table="BuchungLeistung">
      <!--  <id name="id" column="BuchungLeistung_ID">
         <generator class="native" />
      </id>-->
      <composite-id name="pk">
          <key-many-to-one name="leistung" column="Leistung_ID" class="Leistung" />
          <key-many-to-one name="buchung" column="Buchung_ID" class="Buchung" />
          <!--
         <key-property name="leistung" column="Leistung_ID">
         </key-property>
         <key-property name="buchung" column="Buchung_ID">
         </key-property> -->
      </composite-id>
      <property name="anzahl" column="Anzahl" />
      
      <many-to-one name="leistung" column="Leistung_ID" not-null="true" lazy="false"/>
      <many-to-one name="buchung" column="Buchung_ID" not-null="true" />
   </class>
</hibernate-mapping>

public class BuchungLeistung {

   //private long id;
   private int anzahl;
   
   private Buchung buchung;
   private Leistung leistung;
   
   public BuchungLeistung() {
      
   }
...

14:14:12,203 DEBUG Configuration:1634 - null<-org.domException in thread "main" java.lang.ExceptionInInitializerError
   at hibUtils.SessionFactoryUtil.<clinit>(SessionFactoryUtil.java:18)
   at model.DatabaseService.getPersonen(DatabaseService.java:109)
   at tableModels.PersonenTableModel.<init>(PersonenTableModel.java:28)
   at delegate.MainWindow.createTable(MainWindow.java:650)
   at delegate.MainWindow.<init>(MainWindow.java:639)
   at Hotelverwaltung.main(Hotelverwaltung.java:15)
Caused by: org.hibernate.InvalidMappingException: Could not parse mapping document from resource cfg/BuchungLeistung.hbm.xml4j.tree.DefaultAttribute@1b4fad5

@Entity
public class Inventory {

    private Long id;

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "INVENTORY_ID")
    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    private List<InventoryUser> inventoryUser = new ArrayList<InventoryUser>();

    @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER, mappedBy = "inventory")
    public List<InventoryUser> getInventoryUser() {
        return inventoryUser;
    }

    public void setInventoryUser(List<InventoryUser> productUser) {
        this.inventoryUser = productUser;
    }
    // more attributes

@Entity
@Table(name = "User_Table")
public class User {
    private String id;

    @Id
    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    private Set<InventoryUser> inventoryUser = new LinkedHashSet<InventoryUser>();

    @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER, mappedBy = "user")
    public Set<InventoryUser> getInventoryUser() {
        return inventoryUser;
    }

    public void setInventoryUser(Set<InventoryUser> inventoryUser) {
        this.inventoryUser = inventoryUser;
    }

    // more attributes

@Entity
@Table(name = "Inventory_User")
@IdClass(InventoryUser.ProductUserPK.class)
public class InventoryUser {

    private Long inventoryId;

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    public Long getInventoryId() {
        return inventoryId;
    }

    public void setInventoryId(Long inventoryId) {
        this.inventoryId = inventoryId;
    }

    private String userId;

    @Id
    public String getUserId() {
        return userId;
    }

    public void setUserId(String userId) {
        this.userId = userId;
    }

    private User user;

    @ManyToOne
    @JoinColumn(name = "userId", insertable = false)
    public User getUser() {
        return user;
    }

    public void setUser(User user) {
        this.user = user;
    }

    private Inventory inventory;

    @ManyToOne
    @JoinColumn(name = "INVENTORY_ID", insertable = false)
    public Inventory getInventory() {
        return inventory;
    }

    public void setInventory(Inventory inventory) {
        this.inventory = inventory;
    }

    private Date obtainDate = new Date();

    @Basic
    public Date getObtainDate() {
        return obtainDate;
    }

    public void setObtainDate(Date obtainDate) {
        this.obtainDate = obtainDate;
    }

    public static class ProductUserPK implements Serializable {
        private Long inventoryId;
        private String userId;

        public Long getInventoryId() {
            return inventoryId;
        }

        public void setInventoryId(Long inventoryId) {
            this.inventoryId = inventoryId;
        }

        public String getUserId() {
            return userId;
        }

        public void setUserId(String userId) {
            this.userId = userId;
        }

        @Override
        public boolean equals(Object o) ...

        @Override
        public int hashCode() ...
    }
}

        Inventory inventory = new Inventory();
        User user = new User();
        InventoryUser inventoryUser = new InventoryUser();
       
        inventoryUser.setInventory(inventory);
        inventoryUser.setUser(user);
     
        inventory.getInventoryUser().add(inventoryUser);
        user.getInventoryUser().add(inventoryUser);

Could not execute JDBC batch update [insert into Inventory_User (obtainDate, INVENTORY_ID, userId) values (?, ?, ?)]
java.sql.BatchUpdateException: Column 'INVENTORY_ID' cannot be null