我在 Wildfly 10.1.0-Final 上使用 Java EE 7 与 Java 8 和 Hibernate (5.0.X),并且需要使用投影将 JPQL 查询结果加载到 DTO 中,但我找不到任何有关如何执行此操作的文档同时加载子集合 DTO。
例如,如果我有以下用户、角色和权限实体:
@Entity
public class User
{
@Id
private long id;
private String userName;
private String firstName;
private String lastName;
private JobTitle jobTitle;
private Email email;
private boolean isRemote;
@ManyToMany
private Set<Tag> tags;
@ManyToMany
// @JoinColumn definitions...
private Set<Role> roles;
// getters/setters...
}
@Entity
public class Role
{
@Id
private long id;
private String name;
private String description;
@ManyToMany
// @JoinColumn definitions...
private Set<Privilege> privileges;
// getters/setters...
}
@Entity
public class Privilege
{
@Id
private long id;
private String key;
private String name;
private String description;
// getters/setters...
}
我想使用投影将一些查询结果加载到以下不可变的 DTO 中(假设所有 DTO 都有基于 id 实现的 hashCode 和 equals):
public class UserDTO
{
private final long id;
private final String userName;
private final Set<RoleDTO> roles = new HashSet<>();
public UserDTO(long id, String userName, Collection<RoleDTO> roles) // not sure if this is correct for projection..
{
this.id = id;
this.userName = userName;
this.roles.addAll(roles);
}
public Set<Role> getRoles()
{
return Collections.unmodifiableSet(roles);
}
// getters
}
public class RoleDTO
{
private final long id;
private final String name;
private final Set<PrivilegeDTO> privileges = new HashSet<>();
public RoleDTO(long id, String name, Set<PrivilegeDTO> privileges)
{
this.id = id;
this.name = name;
this.privileges.addAll(privileges);
}
public Set<Privilege> getPrivileges()
{
return Collections.unmodifiableSet(privileges);
}
// other getters
}
public class PrivilegeDTO
{
private final long id;
private final String key;
public PrivilegeDTO(long id, String key)
{
this.id = id;
this.key = key;
}
// getters
}
为了实现这一目标,JPQL 查询的结构是什么样的?我很确定我可以通过执行连接然后将结果处理到 DTO 对象中来完成工作,如下所示(按 ID 加载 50 个最新用户):
List<Object[]> results = em.createQuery("SELECT u.id, u.userName, r.id, "
+ "r.name, p.id, p.key FROM User u "
+ "LEFT JOIN u.roles r "
+ "LEFT JOIN r.privileges p "
+ "ORDER BY u.id DESC")
.setMaxResults(50).getResultList();
Map<Long, UserDTO> users = new HashMap<>();
Map<Long, RoleDTO> roles = new HashMap<>();
Map<Long, PrivilegeDTO> privileges = new HashMap<>();
for(Object[] objArray : results)
{
// process these into the DTO objects,
}
重建必须从 PrivilegeDTO 对象开始,然后是 RoleDTO,最后是 UserDTO。这将允许不变性,因为在构建 RoleDTO 对象时需要 PrivilegeDTO 对象,否则必须稍后添加它们,这意味着 RoleDTO 不是不变的。
在 Streams 中这将是一个有趣的练习,但我更希望能够从查询中构建它,看起来它必须更快。这可能吗?
多谢!