使用JPA查询的java.lang.ClassCastException

我正在尝试创建JavaEEWeb应用程序，但从数据库中获取单个结果时遇到问题。获取结果列表不是问题

    public UserCredentialsDTO findByUsernameAndPassword(String username, String password) {
        EntityManager em = getEntityManager();
        TypedQuery<UserCredentialsDTO> q = em.createNamedQuery("UserCredentialsDTO.findByUsernameAndPassword", UserCredentialsDTO.class);
        //Query q = em.createNamedQuery("UserCredentialsDTO.findByUsernameAndPassword", UserCredentialsDTO.class);
        q.setParameter("un", username);
        q.setParameter("pw", password);
        UserCredentialsDTO r = null;
        try{
            r = q.getSingleResult();                       //This line is a problem
            //r = (UserCredentialsDTO)q.getSingleResult(); 
        } catch(javax.persistence.NoResultException e) {

        }

        return r;
    }

编辑

UserCredentialsDTO：

@NamedQueries(
        {@NamedQuery(name = "UserCredentialsDTO.findByUsernameAndPassword", query = "select uc from UserCredentialsDTO uc where uc.username=:un and uc.password=:pw")}
)
@Entity
@Table(name="USERCREDENTIALS", schema="APP")
public class UserCredentialsDTO implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String username;
    private String password;
    @OneToMany(mappedBy = "userCredentials", cascade = CascadeType.ALL, fetch = FetchType.EAGER, orphanRemoval = true)
    private List<UserGroupDTO> userGroups =  new LinkedList<>();

//getters and setters

    public void add(UserGroupDTO userGroup) {
        userGroup.setUserCredentials(this);
        this.userGroups.add(userGroup);
    }

    public List<UserGroupDTO> getUserGroups() {
        return userGroups;
    }


}

@namedquerys(
{@NamedQuery（name=“UserCredentialsDTO.findByUsernameAndPassword”，query=“从UserCredentialsDTO uc中选择uc，其中uc.username=：un和uc.password=：pw”）}
)
@实体
@表（name=“USERCREDENTIALS”，schema=“APP”）
实现可序列化的公共类UserCredentialsDTO{
@身份证
@GeneratedValue（策略=GenerationType.IDENTITY）
私人长id；
私有字符串用户名；
私有字符串密码；
@OneToMany（mappedBy=“userCredentials”，cascade=CascadeType.ALL，fetch=FetchType.EAGER，orphan=true）
private List userGroups=new LinkedList（）；
//接球手和接球手
public void add（UserGroupDTO userGroup）{
userGroup.setUserCredentials（此）；
this.userGroups.add（userGroup）；
}
公共列表getUserGroups（）{
返回用户组；
}
}

这看起来像是一个环境问题，JPA和应用程序的其余部分使用了不同的类加载器

---

您能将用户证书的代码发布到吗？也许对你有帮助。按你的意愿编辑

@NamedQueries(
        {@NamedQuery(name = "UserCredentialsDTO.findByUsernameAndPassword", query = "select uc from UserCredentialsDTO uc where uc.username=:un and uc.password=:pw")}
)
@Entity
@Table(name="USERCREDENTIALS", schema="APP")
public class UserCredentialsDTO implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String username;
    private String password;
    @OneToMany(mappedBy = "userCredentials", cascade = CascadeType.ALL, fetch = FetchType.EAGER, orphanRemoval = true)
    private List<UserGroupDTO> userGroups =  new LinkedList<>();

//getters and setters

    public void add(UserGroupDTO userGroup) {
        userGroup.setUserCredentials(this);
        this.userGroups.add(userGroup);
    }

    public List<UserGroupDTO> getUserGroups() {
        return userGroups;
    }


}