@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