Java 和 SQLite：抛出 ResultSet 已关闭但仍在继续

Question

我遇到了以下问题：我正在尝试使用以下代码将数据（在本例中为用户名）插入到表中：

void AddNewUser(String name, Connection conn){

    if(ret == null){
        ret = new DB_Retriever(conn);
    }

    if(!ret.UserExists(name, conn)){
        try{
            Statement stm = conn.createStatement();
            stm.executeUpdate(DB_OperationalData.insert_new_user[0][0] + name + DB_OperationalData.insert_new_user[0][1]);
            stm.executeUpdate(DB_OperationalData.insert_new_user[1][0] + name + DB_OperationalData.insert_new_user[1][1]);
            stm.close();
        }
        catch(SQLException e){
            e.printStackTrace();
        }
    }
}

顺便说一句：我在 catch 子句中放什么绝对无关紧要，我放在那里的任何东西都不会被执行。为了说明一切，这里是 DB_OperationalData.insert_new_user 字符串数组的内容：

public static final String[][] insert_new_user = {
    {"INSERT INTO User (Username, Status) VALUES ('","','IN');"},
    {"INSERT INTO Statistics (Player_ID) SELECT ID FROM User WHERE Username='","';"}};

第二条语句应该复制插入的用户ID，并将其放入Statistics表的Player_ID字段中（表用户ID是一个自动编号的字段）。

我得到的例外是：

Error while processing the query: java.sql.SQLException: ResultSet closed

有趣的是，它可以正常工作并且正确添加了数据，但我根本不希望抛出任何异常。

这是我得到的控制台输出：

This is 'data' Package Testing class
Connection to the database established.
The number of tables existing in the database is: 0
All the queries have been processed successfully
Adding new users: 
Error while processing the query: java.sql.SQLException: ResultSet closed

异常上面的所有行都是我自己的打印输出，所以我知道实际发生了什么。

[编辑]

我已将代码更改为使用 PreparedStatement 而不是普通的 Statement，当前的 try 子句如下所示：

PreparedStatement pstm = conn.prepareStatement(DB_OperationalData.insert_new_user[0]);
pstm.setString(1, name);
pstm.addBatch();
conn.setAutoCommit(false);
pstm.executeBatch();
conn.setAutoCommit(true);
pstm.close();

并且输出是（仍然不管catch子句的内容）：

This is 'data' Package Testing class
Connection to the database established.
The number of tables existing in the database is: 0
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 0
at org.sqlite.PrepStmt.batch(PrepStmt.java:173)
at org.sqlite.PrepStmt.setString(PrepStmt.java:254)
at data.DB_Writer.AddNewUser(DB_Writer.java:28)
at data.DataHandler.AddNewUser(DataHandler.java:94)
at data.Tester.main(Tester.java:18)
All the queries have been processed successfully
Adding new users: 
Error while processing the query: java.sql.SQLException: ResultSet closed

[编辑 2] 关于原始版本，当我删除 stm.close();绝对没有区别，我仍然得到 'ResultSet closed' 异常。

[编辑 3] 以下是调用上述方法的代码：

public void AddNewUser(String username)throws IllegalUsernameException{
    if(username.length()==0 || username.length()>20){
        throw new IllegalUsernameException();
    }
    writer.AddNewUser(username, conn);

}

通过这个类建立与数据库的连接：

class DB_Connection {


public static Connection getConnection(){
    Connection conn = null;

    try{
        Class.forName("org.sqlite.JDBC");
    }
    catch(ClassNotFoundException e){
        log("Error while loading the database driver: " + e);
        return null;
    }
    try{
        conn = DriverManager.getConnection("jdbc:sqlite:database.db");
    }
    catch(SQLException e){
        log("Unable to connect to the database: " + e);
        return null;
    }
    return conn;
}

public static void log(String msg){
    System.out.println(msg);
}

}

正在检查现有用户名的 DB_Retriever 方法：

boolean UserExists(String name, Connection conn){

    String result = "";

    try{
        Statement stm = conn.createStatement();
        ResultSet rs = stm.executeQuery(DB_OperationalData.user_exists[0] + name + DB_OperationalData.user_exists[1]);
        result = rs.getString("Username");
    }
    catch(SQLException e){
        System.out.println("Error while processing the query: " + e);
    }

    if(result.equals(name)){
        return true;
    }

    return false;
}

Answer 1

Error while processing the query: java.sql.SQLException: ResultSet closed 可以打印到控制台的唯一位置是UserExists(..)，除非有其他方法具有类似的 catch 块。确实，ResultSet 在UserExists 中没有正确使用，可能会导致错误。

有关如何使用 JDBC 的更完整描述，请查看 this answer 或 JDBC documentation。现有UserExists 的可能替代方案是：

boolean userExists(String name, Connection conn) {
   PreparedStatement stmt = null;
   try{
      stmt = conn.prepareStatement("SELECT COUNT(Username) FROM User WHERE Username = ?");
      stmt.setString(1, name);
      ResultSet rs = stmt.executeQuery();
      rs.next();  // set cursor to first row
      int count = rs.getInt(1);
      rs.close();
      return count > 0;
   } catch(SQLException e) {
      // propagate error
      throw new RuntimeException(e);
   } finally {
      // clean up resources
      if (stmt != null) {
        try {
           stmt.close();
        } catch (SQLException ignore) {
           log("error on sql clean up", ignore);
        }
      }
   }
}