JoinColumn vs mappedBy in JPA

@Entity
public class Troop {
    @OneToMany(mappedBy="troop")
    public Set<Soldier> getSoldiers() {
    ...
}

@Entity
public class Soldier {
    @ManyToOne()
    @JoinColumn(name="troop_fk")
    public Troop getTroop() {
    ...
} 

References
https://stackoverflow.com/questions/11938253/jpa-joincolumn-vs-mappedby
https://stackoverflow.com/questions/37047938/jpa-hibernate-onetoone-joincolumn-referencedcolumnname-ignored