在Spring Data JPA规范中的三个表联接中允许空集

Question

我将在Spring Data JPA Specification加入三个表，例如：

public static Specification<Contact> findByStructureCode(final String code) {
    return new Specification<Contact>() {
        @Override
        public Predicate toPredicate(Root<Contact> root, CriteriaQuery<?> criteriaQuery, CriteriaBuilder criteriaBuilder) {
            return criteriaBuilder.equal(root.join(Contact_.contactToStructures)
                    .join(ContactToStructure_.structureCodes)
                    .get(StructureCode_.code), code);
        }
    };
}

但是，这给了我一个很好的NullPointerException 。

java.lang.NullPointerException: null
    at org.hibernate.jpa.criteria.path.AbstractFromImpl.constructJoin(AbstractFromImpl.java:345) ~[hibernate-entitymanager-4.3.8.Final.jar:4.3.8.Final]
    at org.hibernate.jpa.criteria.path.AbstractFromImpl.join(AbstractFromImpl.java:333) ~[hibernate-entitymanager-4.3.8.Final.jar:4.3.8.Final]
    at org.hibernate.jpa.criteria.path.AbstractFromImpl.join(AbstractFromImpl.java:324) ~[hibernate-entitymanager-4.3.8.Final.jar:4.3.8.Final]
    at my.package.ContactSpecification$3.toPredicate(ContactSpecification.java:83) ~[MyProject-1.0.0.jar:na]

请注意，我已用伪造信息替换了最后一行中的package和jar信息。 我的代码中堆栈跟踪引用的行是.join(ContactToStructure_.structureCodes) 。

我相信这是由于并非所有的Contact行在我的数据库中都有对应的ContactToStructure行而引起的。

因此，如何进行第一次contactToStructures以允许具有空contactToStructures集的Contact实体，而只返回具有非空contactToStructures集的联系人？

---

这是实体及其元模型的相关部分。

Contact.java

@Entity
@Table(name = "CONTACT", schema = "myschema")
public class Contact {    
    @OneToMany(mappedBy = "contact", fetch = FetchType.LAZY, cascade = CascadeType.REMOVE)
    private Set<ContactToStructure> contactToStructures = new HashSet<>(0);    
}

Contact_.java

@StaticMetamodel(Contact.class)
public class Contact_ {
    public static volatile SetAttribute<Contact, ContactToStructure> contactToStructures;
}

ContactToStructure.java

@Entity
@Table(name = "CONTACT_TO_STRUCTURE", schema = "myschema")
public class ContactToStructure {
    @OneToMany(fetch=FetchType.LAZY, cascade = {CascadeType.ALL}, mappedBy="contactToStructure")
    private Collection<StructureCode> structureCodes = new ArrayList<>();
}

ContactToStructure_.java

@StaticMetamodel(ContactToStructure.class)
public class ContactToStructure_ {
    public static volatile SetAttribute<ContactToStructure, StructureCode> structureCodes;
}

StructureCode.java

@Entity
@Table(name = "STRUCTURE_CODE", schema = "myschema")
public class StructureCode {
    @Column(name = "CODE")
    private String code;
}

StructureCode_.java

@StaticMetamodel(StructureCode.class)
public class StructureCode_ {
    public static volatile SingularAttribute<StructureCode, String> code;
}

Answer 1

如果仍然相关，请尝试进行以下更改ContactToStructure_.java ：

public static volatile SetAttribute<Version, UseCase> useCases;

至：

public static volatile CollectionAttribute<Version, UseCase> useCases;