SpringDataJpa查询返回空集合

朱雀 2023-06-06 14:28 284阅读 0赞

当我们使用SpringDataJpa写sql查询数组时，如果数据库查询出来的数据为空，结果会是怎样的呢

接口类

public interface Dao extends PagingAndSortingRepository { 
        @Query("select eneity from table")
        List<String> getList();
    }

测试类：

public class Demo01 { 
        @Resource
        private static Dao dao;
    
        public static void main(String[] args) { 
            List<String> list = dao.getList();
            System.out.println(list == null);
        }
    }

这时候运行，打印出来的结果是false，违背了我们一直以来的思想，为什么数据库数据为空的时候，我们查询出来的不是Null，而是一个空集合呢。  
我在网上翻阅了很久，也没有找到一篇合理的解释，最后只能换一个角度去思考，jpa的底层实现是对Hibrenate进行了封装，所以我开始找Hibernate的源码，因为Hibernate源码太过复杂，多层调用，我给大家分享一下查找的入口和最终结果：  
入口类：

package org.hibernate;
    public interface Query { 
    		List list() throws HibernateException;
    ｝

最终实现类：

package org.hibernate.loader;
    public abstract class Loader { 
    	private List doQuery(SessionImplementor session, QueryParameters queryParameters, boolean returnProxies) throws SQLException, HibernateException { 
            RowSelection selection = queryParameters.getRowSelection();
            int maxRows = hasMaxRows(selection) ? selection.getMaxRows() : 2147483647;
            int entitySpan = this.getEntityPersisters().length;
            ArrayList hydratedObjects = entitySpan == 0 ? null : new ArrayList(entitySpan * 10);
            PreparedStatement st = this.prepareQueryStatement(queryParameters, false, session);
            ResultSet rs = this.getResultSet(st, queryParameters.hasAutoDiscoverScalarTypes(), queryParameters.isCallable(), selection, session);
            EntityKey optionalObjectKey = getOptionalObjectKey(queryParameters, session);
            LockMode[] lockModesArray = this.getLockModes(queryParameters.getLockOptions());
            boolean createSubselects = this.isSubselectLoadingEnabled();
            List subselectResultKeys = createSubselects ? new ArrayList() : null;
            ArrayList results = new ArrayList();
    
            try { 
                this.handleEmptyCollections(queryParameters.getCollectionKeys(), rs, session);
                EntityKey[] keys = new EntityKey[entitySpan];
                if (log.isTraceEnabled()) { 
                    log.trace("processing result set");
                }
    
                int count;
                for(count = 0; count < maxRows && rs.next(); ++count) { 
                    if (log.isTraceEnabled()) { 
                        log.debug("result set row: " + count);
                    }
    
                    Object result = this.getRowFromResultSet(rs, session, queryParameters, lockModesArray, optionalObjectKey, hydratedObjects, keys, returnProxies);
                    results.add(result);
                    if (createSubselects) { 
                        subselectResultKeys.add(keys);
                        keys = new EntityKey[entitySpan];
                    }
                }
    
                if (log.isTraceEnabled()) { 
                    log.trace("done processing result set (" + count + " rows)");
                }
            } finally { 
                session.getBatcher().closeQueryStatement(st, rs);
            }
    
            this.initializeEntitiesAndCollections(hydratedObjects, rs, session, queryParameters.isReadOnly(session));
            if (createSubselects) { 
                this.createSubselects(subselectResultKeys, queryParameters, session);
            }
    
            return results;
        }
    }

在这里，我们能清楚的看到，查询的底层是先new了一个ArrayList，然后去查询往里面赋值，如果查询出来没有数据的话，那么会直接将这个ArrayList返回，就会导致返回结果是一个空集合，所以这里我们在判断有没有查询出数据的时候，千万不要单纯用是否为Null去判断，推荐方法为 ：  
list.isEmpty() 和 CollectionUtils.isEmpty(list)