JPA: 如何从一个表里面拿想要的字段

   使用JPA的时候，在使用它和手机端进行交互的时候常常遇到只要某几个字段，但是会全部拿到的情况， 如：

拿到某个用户的全部信息， 如果这个用户还有其他的相关内容都会被抓出来发给手机端

   
String jpql = "select admin from Admin admin where lower(admin.username) = lower(:username)";

考虑到和手机端的交互用JSON当然内容越少越好，提供二种解决办法：

1. 使用Tuple, 并将结果放入自己定义的AdminWrapper

public List<AdminWrapper> getAdminListByTuple() {
//1:
CriteriaBuilder criteriaBuilder = entityManager.getCriteriaBuilder();
//2: One of the JPA ways for getting only particular columns is to ask for a Tuple object.
CriteriaQuery<Tuple> criteriaQuery = criteriaBuilder.createTupleQuery();
//3:
Root<Admin> root = criteriaQuery.from(Admin.class);
//4:
criteriaQuery.multiselect( root.get("id"), root.get("username"), root.get("name") );

//5:条件
Predicate restrictions = criteriaBuilder.conjunction();
restrictions = criteriaBuilder.and(restrictions, criteriaBuilder.equal(root.get("id"), 1));
criteriaQuery.where(restrictions);	
criteriaQuery.orderBy(criteriaBuilder.desc(root.get("id")));

//5:
TypedQuery<Tuple> query = entityManager.createQuery(criteriaQuery).setFlushMode(FlushModeType.COMMIT);

//6
List<Tuple> tuples = query.getResultList();

AdminWrapper adminWrapper = null;
List<AdminWrapper> lists = new ArrayList<AdminWrapper>();
for (Tuple t : tuples) {
	adminWrapper = new AdminWrapper();
	adminWrapper.setId((Long)t.get(0));
	adminWrapper.setUsername((String)t.get(1));
	adminWrapper.setName((String)t.get(2));
	lists.add(adminWrapper);
}
return lists;
}

2. 直接使用AdminWrapper，前提是有构造方法

	public AdminWrapper() {
		
	}
	
	public AdminWrapper (Long id, String username, String name) {
		this.id = id;
		this.username = username;
		this.name = name;
	}

   然后
public List<AdminWrapper> getAdminListByT() {
//1:
CriteriaBuilder criteriaBuilder = entityManager.getCriteriaBuilder();
//2: One of the JPA ways for getting only particular columns is to ask for a Tuple object.
CriteriaQuery<AdminWrapper> criteriaQuery = criteriaBuilder.createQuery(AdminWrapper.class);
//3
Root<Admin> root = criteriaQuery.from(Admin.class);
//4
criteriaQuery.multiselect( root.get("id"), root.get("username"), root.get("name") );

//5
TypedQuery<AdminWrapper> query = entityManager.createQuery(criteriaQuery).setFlushMode(FlushModeType.COMMIT);

List<AdminWrapper> lists = query.getResultList();

return lists;
}