java - 自定义log4j输出至数据库，项目启动时的错误

在项目中添加了log4j输出至数据库，自定义了一个Log2OrclObject，结构如下：

public class Log2OrclObject
{
  private int log_id;
  private String log_user;
  private String log_ip;
  private String log_sys_name;
  private String log_fun_model;
  private String log_message;
  private String log_date;
  private String log_level;
  private String log_identity;
  private String log_exception;
  private String log_logger;
  private String log_source;
}

扩展了JdbcAppender,CustomJdbcAppender，如下：

   public class CustomJdbcAppender extends AppenderSkeleton
{
  private String user;
  private String pass;
  private String driver;
  private String url;
  private PreparedStatement pst;
  protected void append(LoggingEvent event)
  {
    this.sql = "insert into Y05_SYSTEMLOGS values(Y05_SYSTEMLOGS_SEQ.nextval,?,?,?,?,?,?,?,?,?,?,?)";
    try
    {
      Class.forName(this.driver);
      Connection conn = DriverManager.getConnection(this.url, this.user, this.pass);
      this.pst = conn.prepareStatement(this.sql);
      

      Log2OrclObject log2OracleObject = (Log2OrclObject)event.getMessage();
      ...
      }

这么写出来，单独用测试类测试的时候时没有问题的，但是当项目启动时，就会报错：

java.lang.ClassCastException:java.lang.String cannot be cast to com.log4j.Log2OracObject

位置就是在这一行中：

Log2OrclObject log2OracleObject = (Log2OrclObject)event.getMessage();

求解这是什么问题啊？ 本来log4j中 event.getMessage()获取到的就是Object类型了，为什么初始化的时候会报是String类型的错误呢？