Entity
@Table(name = "widgets")
public class Widget implements Persistable<Long> {

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Long id;

    @Column(name = "name", unique = false, nullable = false, length = 255)
    private String name;

    @Column(name = "widget_type", unique = false, nullable = false, length = 255)
    private String widgetType;

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "pk.widget", cascade={CascadeType.PERSIST, CascadeType.MERGE})
    Set<Location> WidgetLocations = new HashSet<Location>();
    ...
}


@Entity
@Table(name = "workspaces")
public class Workspace implements Persistable<Long> {

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Long id;

    @Column(name = "name", unique = false, nullable = false, length = 255)
    private String name;

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "pk.workspace", cascade={CascadeType.PERSIST, CascadeType.MERGE})
    Set<Location> WidgetLocations = new HashSet<Location>();
    ...
}


@Entity
@Table(name = "widget_location")
@AssociationOverrides({ @AssociationOverride(name = "pk.workspace", joinColumns = @JoinColumn(name = "workspace_id") ),
@AssociationOverride(name = "pk.widget", joinColumns = @JoinColumn(name = "widget_id") ) })
public class Location {

    private LocationID pk = new LocationID();

    @Column(name = "row_number", nullable = false)
    private int rowNumber;

    @Column(name = "column_number", nullable = false)
    private int columnNumber;

    @EmbeddedId
    public LocationID getPk() {
        return pk;
    }

    public void setPk(LocationID pk) {
        this.pk = pk;
    }

    @Transient
    public Workspace getWorkspace() {
        return getPk().getWorkspace();
    }

    public void setWorkspace(Workspace workspace) {
        getPk().setWorkspace(workspace);
    }

    @Transient
    public Widget getWidget() {
        return getPk().getWidget();
    }

    public void setWidget(Widget widget) {
        getPk().setWidget(widget);
    }
    ...
}


@Embeddable
public class LocationID implements Persistable<Long> {

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Long id;

    private Workspace workspace;
    private Widget widget;

    @ManyToOne
    public Workspace getWorkspace() {
        return workspace;
    }

    @ManyToOne
    public Widget getWidget() {
        return widget;
    }
}

Location newLocation = new Location();
newLocation.setWorkspace(workspace);
newLocation.setWidget(widget);
newLocation.setColumnNumber(columnNumber);
newLocation.setRowNumber(rowNumber);

workspace.getWidgetLocations().add(newLocation);
workspaceRepository.save(workspace);