冬眠中的魔法ê

问题描述

当我写作时，会话session = sessionFactory.getCurrentSession（）;
清单< Candidate>候选人=（List< Candidate>）session.createQuery（
select Candidate from Candidate as candidate left outer join
+candidate.skills as skill where skill.id =+ 1）.list ）;

我在第二行有npe

如果我写

  
从第一个查询中删除 select Candidate  $ c> Candidate 在候选人实体）？ 
完整实体选择的HQL语法是<映射实体>的 ... 。
 
 
   
 
 
关于您的问题： 
 
 
  NPE如何发生：一些调试或HQL解析器）
 HQL解析器 - 在AST构建过程中 - 寻找名为 Candidate 的属性引用，但找不到它因此请尝试将其解析为ClassName或函数（请参阅 org.hibernate.hql.internal.ast.util.LiteralProcessor.processConstant（） function）; 
候选是一个类名和解析器威胁它作为鉴别值（用于实体的子类）返回null。
在解析器之后，在select的显式列表的翻译过程中找到 Candidate  null 并尝试将其解析为函数，因为 null 不是有效的数据类型（实体，基元或其他有效的数据类型），所以尝试将它们解析为函数;值 null 我们用作函数 SQLFunctionRegistry.findSQLFunction（）的参数：
  public SQLFunction findSQLFunction（String functionName）{
 return sfi.getSqlFunctionRegistry（）。findSQLFunction（functionName.toLowerCase（））; 
 
  
和 functionName.toLowerCase（）导致NPE。
 
When I write
Session session = sessionFactory.getCurrentSession();
List<Candidate> candidates = (List<Candidate>) session.createQuery(
        "select Candidate from Candidate as candidate  left outer join"
        + " candidate.skills as skill    where skill.id =" + 1).list();
I have npe at second line
if I write
Session session = sessionFactory.getCurrentSession();
List<Candidate> candidates = (List<Candidate>) session.createSQLQuery(
        "select candidate.* from candidate inner join candidate_skill"
        + " on candidate.id = candidate_skill.candidate_id"
        + " inner join skill on candidate_skill.skill_id = skill.id"
        + " where skill.id = 1").list();
I have good result
why I have NPE in first example?
 解决方案 
Remove select Candidate from first query (do you have a field Candidate in Candidate entity)? HQL syntax for full entity select is from <mapped entity>....
About your question:
How magic NPE happens: (...After a bit of debugging or HQL parser)
HQL parser - during AST building - look for a property-reference called Candidate and doesn't find it so try to resolve as a ClassName or a function (look in org.hibernate.hql.internal.ast.util.LiteralProcessor.processConstant() function);  Candidate IS a classname and parser threat it as a discriminator value (for entity subclassing) returning null.
After that parser, during translation of select's explicit list find Candidate with value null and try to resolve as a function because null is not a valid dataType (entity, primitive or other valid datatatype) so try to resolve them as a function; value null us used as parameter for function SQLFunctionRegistry.findSQLFunction() which as this body:
public SQLFunction findSQLFunction(String functionName) {
        return sfi.getSqlFunctionRegistry().findSQLFunction( functionName.toLowerCase() );
    }
and functionName.toLowerCase() cause the NPE.
                        
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