在Java连接数据库时,有时候在ResultSet 调用getString (或其他类似的方法),有时候会抛出NullPointException,异常信息如下:
java.lang.NullPointerException com.mysql.jdbc.ResultSetImpl.findColumn(ResultSetImpl.java:1059) com.mysql.jdbc.ResultSetImpl.getInt(ResultSetImpl.java:2719) org.apache.jsp.delete_jsp._jspService(delete_jsp.java:110) org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70) javax.servlet.http.HttpServlet.service(HttpServlet.java:803) org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:374) org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:337) org.apache.jasper.servlet.JspServlet.service(JspServlet.java:266) javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
但是通过debug发现ResultSet 对象不为空,这个问题就比较头疼,网上也有很多人在提问,但是大多都没有找到问题的根本原因。出现这个问题是因为多个ResultSet 使用了一个Statement或者PrepareStatement对象而导致,比如下面的代码:
Statement stat = conn.createStatement(); try { ResultSet rs = stat.executeQuery("select * from BbsRecord where id=" + id); rs.next(); sid = rs.getInt("sid"); ResultSet rss = stat.executeQuery("select * from bbsrecord where sid=" + sid); rss.last(); if (rss.getRow() == 1) { if (rs.getInt("root") == 0) { //错误报告指示的是这里 stat.execute("update bbsrecord set leaf = 1 where id=" + sid); stat.execute("delete from bbsrecord where id =" + id); if (rs.getInt("leaf") == 0) { delete(conn,id); } } } else { stat.execute("delete from bbsrecord where id =" + id); if (rs.getInt("leaf") == 0) { delete(conn, id); } } rs.close(); rss.close(); } catch (SQLException e) { System.out.println("sql异常"); } finally { stat.close(); conn.close(); }
在上述代码中,由于第6行和第3行使用同一个Statement,导致了第9行抛出的NullPointerException异常