java - Mysql 表对象列表到字符串数组通过 System.arraycopy 得到 ArrayStoreException？

Question

我正在尝试在 Mysql、Hibernate 的帮助下用 java 编写示例。我在Mysql中创建了2个表，分别命名为table1、table2。每个有 3 列表1.行索引，表1。列1，表1。列2table2.rowindex、table2.column1、table2.column2

场景是1. 有时table1.column1值等于table2.column2。2、通过table1.column2的值来查找。

所以我使用的查询

SELECT a.column1, b.column1, a.column2, b.column2 FROM table1 a, table2 b where a.column1 = b.column1 and a.column2 ='Mandar';

它在 Mysql 工作台中向我显示了准确的结果。然后我编写了java程序来获取这个结果并尝试打印它。

我编写的取表函数如下:-

protected static List get_combined_two_tables(String string) {
    if (resultList != null) {
        resultList.clear();
    }
    flag_file = filecheckhibernate1(false);
    if (flag_file) {
        session = HibernateUtil.getSessionFactory("hibernate1.cfg.xml").openSession();
        if (session != null) {
            if (session.isOpen() == true) {
                try {
                    session.beginTransaction();
                } catch (TransactionException e) {
                    e.printStackTrace();

                } catch (JDBCConnectionException e) {
                    e.printStackTrace();

                }
                String selectQuery = "SELECT a.column1, b.column1, a.column2, b.column2 FROM table1 a, table2 b where a.column1 = b.column1 and a.column2 =:name";
                System.out.println("selectQuery = " + selectQuery);
                Query q = null;
                try {
                    q = session.createQuery(selectQuery);
                    q.setString("name", "Mandar");
                } catch (org.hibernate.QueryException e) {
                    e.printStackTrace();
                }
                if (q != null) {
                    resultList = q.list();
                    session.getTransaction().commit();
                }
                if (session.isOpen()) {
                    session.close();
                }

            }
        }
    }
    return resultList;
}

对于打印，我编写了以下函数:-

                        List<?> combined_table_List = null;
                    combined_table_List = get_combined_two_tables("Mandar");
                    if(combined_table_List != null) {
                        String[] dest = null;
                        for (int i = 0; i < combined_table_List.size(); i++) {
                                dest = new String[combined_table_List.get(i).toString().length()];
                                if(dest != null) {
                                    try {
                                        System.arraycopy(combined_table_List,0,dest,0,combined_table_List.get(i).toString().length());
                                    } catch (Exception e) {
                                        e.printStackTrace();
                                    }
                                        if(Arrays.toString(dest) != null) {
                                             System.out.println("Arrays.toString(dest) ="+Arrays.toString(dest));
                                        }   
                                }

                        }   
                    }

当我运行程序时，出现如下异常:-

java.lang.ArrayStoreException
at java.lang.System.arraycopy(Native Method)
at test_db$1.run(test_db.java:132)
at java.awt.event.InvocationEvent.dispatch(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$500(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)
Arrays.toString(dest) =[null, null, null, null, null, null, null, null, 
null, null, null, null, null, null, null, null, null, null, null, null, 
null, null, null, null, null, null, null, null]

需要帮助来解决这个问题。

Answer 1

get_combined_two_tables 返回的列表将是一个对象数组列表，每个数组将代表查询返回的一行。

Hibernate Query list docs

当您尝试将此列表复制到“dest”String [] 时，这将失败，因为这两种类型不兼容。即您正在尝试将对象数组列表插入到字符串数组中。

ArrayStoreException Docs

您需要将列表中的对象数组解析为字符串数组。