JPA EntityManager 查找方法不起作用[关闭]

Question

我想编写一个方法，通过cardId 找回customerId，但这不起作用，Eclipse 说customerId 无法解析为变量

    public CustomerMapping getCustomerMapping(String cardId) {

            this.getEntityManager();
            return em.find(CustomerMapping.class, customerId);
    }




@Stateless
public class CustomerMappingEJB {

    private EntityManager em;


    private void getEntityManager() {
        em = MultiTenantEntityManagerFactory.getEntityManager();
    }

    private void beginTransaction() {
        em.getTransaction().begin();
    }

    private void commitTransaction() {
        em.getTransaction().commit();
    }


    private void insert(CustomerMapping customerMapping) {
        em.persist(customerMapping);
    }

    public CustomerMapping getCustomerMapping(long id) {

        this.getEntityManager();
        return em.find(CustomerMapping.class, id);
    }

    public List<CustomerMapping> getCustomerMappings() {

        TypedQuery<CustomerMapping> query = null;

            this.getEntityManager();
            query = em.createNamedQuery("AllCustomerIds", CustomerMapping.class);       

        return query.getResultList();
    }

    public CustomerMappingHelper getCustomerMappingHelper(String cardId) {

        this.getEntityManager();
        return em.find(CustomerMappingHelper.class, cardId);
    }

    public CustomerMapping addNew(CustomerMapping customerMapping) {
        this.getEntityManager();
        this.beginTransaction();
        this.insert(customerMapping);
        this.commitTransaction();
        return customerMapping;
    }
}

我没有把 MultiTenantEntityManagerFactory 类放在这里，因为我猜你不需要那个，然后我有 CustomerMapping 类和你所说的“一堆列表”，只有一些设置和获取 id、cardId 和 customerId 的方法我也没有复制它，因为如果你也想要，这将非常大，我会这样做

public class CustomerMapping {

    @Id
    @GeneratedValue
    public Long id;

    @Basic
    private String customerId;

    @Basic
    private String cardId;

    @Basic
    private String hashKey;

}

Answer 1

您可以通过cardId 使用JPQL 获得CustomerMapping。更优化的解决方案是使用投影。

public String getCustomerMappingCustomerId(String cardId) {
    getEntityManager();        
    CustomerMapping result = first(em.createQuery(
        "select c from CustomerMapping c where c.cardId = :cardId")
        .setParameter("cardId", cardId).getResultList());
    return result ==  null ? null : result.getCustomerId(); 
}

private static <T> T first(List<T> items) {
    return items == null || items.isEmpty() ? null : items.get(0);
}