/**
 * Returns a single entity matching the given {@link Example} or {@literal null} if none was found.
 *
 * @param example must not be {@literal null}.
 * @return a single entity matching the given {@link Example} or {@link Optional#empty()} if none was found.
 * @throws org.springframework.dao.IncorrectResultSizeDataAccessException if the Example yields more than one result.
 
 
*/
<S extends T> Optional<S> findOne(Example<S> example);
 
 
/**
 * Returns a reference to the entity with the given identifier.
 *
 * @param id must not be {@literal null}.
 * @return a reference to the entity with the given identifier.
 * @see EntityManager#getReference(Class, Object)
 * 
 
@throws javax.persistence.EntityNotFoundException if no entity exists for given {@code id}.
 */
T getOne(ID id);

findone：返回實體的optional對象

getone：返回實體的引用，代理對象

實體轉json的過程中會出錯，用debug查看實體其實是空，數據是放在代理對象中的，但jackson將實體轉json沒有拿到代理對象，然後產生以下錯誤，這時候json轉換失敗再導致輸出流報錯。

getOne示例：

jpa2.0以上findOne和getOne的區別