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                        /通过自动回复机器人学Mybatis---基础版

模糊查询使用占位符出现问题

public class ListServlet extends HttpServlet{
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        // TODO Auto-generated method stub
        try {
            req.setCharacterEncoding("UTF-8");
            //首先不管是不是第一次查看列表页面，先得到查询条件
            String command=(String) req.getParameter("command");
            String description=(String) req.getParameter("description");
            req.setAttribute("command", command);
            req.setAttribute("description", description);
            Connection connection=ConnectionDB.getConnection();
            StringBuilder sql=new StringBuilder("select ID,COMMAND,DESCRIPTION,CONTENT from message where 1=1");
            //判断得到的参数是不是空，还有是不是空字符串,把条件存放在集合中，因为使用了占位符，下边要进行赋值
            List<String> list=new ArrayList<String>();
            if(command!=null && !command.trim().equals("")){
                sql.append(" and COMMAND = ?");
//                sql.append(" and COMMAND = "+command);
                list.add(command);
            }
            if(description!=null && !description.trim().equals("")){
                sql.append(" and DESCRIPTION like '%'？'%'");
//                sql.append(" and DESCRIPTION like '%"+description+"%'");
                list.add(description);
            }
//            System.out.println(sql);
            PreparedStatement statement=connection.prepareStatement(sql.toString());
            for(int i=0;i<list.size();i++){
                statement.setNString(i+1,list.get(i));
            }
            ResultSet rs=statement.executeQuery();
            List<Message> messages=new ArrayList<Message>();
            while(rs.next()){
                Message message=new Message();
                message.setId(rs.getInt("ID"));
                message.setCommand(rs.getString("COMMAND"));
                message.setDescription(rs.getString("DESCRIPTION"));
                message.setContent(rs.getString("CONTENT"));
                messages.add(message);
            }
            //跳转界面
            req.setAttribute("messageList", messages);
            req.getRequestDispatcher("/WEB-INF/jsp/list.jsp").forward(req, resp);
        } catch (SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

不明白为什么这样使用占位符会报错！！！！！

java.sql.SQLException: Parameter index out of range (1 > number of parameters, which is 0).
   at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1073)
   at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:987)
   at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:982)
   at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:927)
   at com.mysql.jdbc.PreparedStatement.checkBounds(PreparedStatement.java:3729)
   at com.mysql.jdbc.PreparedStatement.setInternal(PreparedStatement.java:3713)
   at com.mysql.jdbc.PreparedStatement.setString(PreparedStatement.java:4553)
   at com.mysql.jdbc.PreparedStatement.setNString(PreparedStatement.java:5262)
   at com.company.servlet.ListServlet.doGet(ListServlet.java:55)
   at com.company.servlet.ListServlet.doPost(ListServlet.java:80)
   at javax.servlet.http.HttpServlet.service(HttpServlet.java:650)
   at javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
   at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
   at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
   at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
   at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
   at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
   at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:218)
   at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
   at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:505)
   at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:169)
   at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
   at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:956)
   at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
   at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:442)
   at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1083)
   at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:640)
   at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:318)
   at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
   at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
   at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
   at java.lang.Thread.run(Thread.java:745)