@Entity
@Table(name = "B")
public class B {
    @ManyToOne( fetch = FetchType.LAZY, cascade = { CascadeType.ALL } )
    @JoinColumn( name = "A_ID" )
    private A a;
    //bi-directional one-to-many association to C
    @OneToMany( mappedBy = "b", cascade = { CascadeType.ALL }, fetch = FetchType.LAZY )
    private List<C> c = new ArrayList<>();
    public void add_C(C c) {
        this.c.add(c);
        c.setB(this);
    }
}

---------a的Map器

------实体b

-------实体c

@Entity
@Table(name = "A")
public class A {
    //bi-directional one-to-many association to B
    @OneToMany( mappedBy = "a", cascade = { CascadeType.ALL }, fetch = FetchType.LAZY )
    private List<B> b = new ArrayList<>();
    public void add_B(B b) {
        this.b.add(b);
        b.setA(this);
    }
}

@Entity
@Table(name = "C")
public class C {
    @ManyToOne( fetch = FetchType.LAZY, cascade = { CascadeType.ALL } )
    @JoinColumn( name = "B_ID" )
    private B b;
}

--实体a

@Mapper(
        componentModel = "spring",
        unmappedTargetPolicy = ReportingPolicy.WARN
)
public abstract class A_Mapper {
    public abstract A toEntity(DTO_A dto_a);
    @AfterMapping
    public void mapToEntity(@MappingTarget A a)
    {
        List<B> bs = new ArrayList<>(a.getB());
        for (B b: bs) { 
            a.add_B(b);
            for (C c:
                    b.getC() {
                b.add_C(c);
            }
        }
}

保存实体a后，a的id应设置为b项，b的id应设置为c项。但是抛出了一个“nullpointer异常”。有人能告诉我怎么修吗。如何修改代码，使a和b的id都设置为c？提前谢谢