这个是传入到数据库的时候没有对数据进行编码，数据库读不到数据，jdbc:mysql://127.0.0.1:3306/micro_message?characterEncoding=utf8","root","root"

是空格的问题， sql.append(" and COMMAND =?");

在and 的前面加一个空格。

package com.imooc.servlet;

import java.io.IOException;

import java.sql.Connection;

import java.sql.DriverManager;

import java.sql.PreparedStatement;

import java.sql.ResultSet;

import java.sql.SQLException;

import java.util.ArrayList;

import java.util.List;

import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import com.imooc.bean.Message;

/*

 *列表页面初始化控制 

 */

@SuppressWarnings("serial")

public class ListServlet extends HttpServlet {

@Override

protected void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException {

// TODO Auto-generated method stub

try {

req.setCharacterEncoding("UTF-8");

String command=req.getParameter("command");

String description=req.getParameter("description");

Class.forName("com.mysql.jdbc.Driver");

 Connection conn= DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/micro_message","root","admin");

 StringBuilder sql=new StringBuilder();

 sql.append("select ID,COMMAND,DESCRIPTION,CONTENT from message where 1=1");

List<String> paramlist=new ArrayList<String>();

 if(command!=null &&!"".equals(command.trim())){

 sql.append("and COMMAND =?");

 paramlist.add(command);

 }

 if(description!=null &&!"".equals(description.trim())){

 sql.append("and DESCRIPTION =?");

 paramlist.add(description);

 }

 PreparedStatement stat=conn.prepareStatement(sql.toString());

for (int i=0;i<paramlist.size();i++) {

stat.setString(i+1, paramlist.get(i));

}

ResultSet rs=stat.executeQuery();

List <Message> messagelist=new ArrayList<Message>(); 

System.out.println(sql);

while(rs.next()){

Message message=new Message();

   message.setId(rs.getString("ID"));

   message.setCommand(rs.getString("COMMAND"));

   message.setDescription(rs.getString("DESCRIPTION"));;

   message.setContent(rs.getString("CONTENT"));

messagelist.add(message);

}

         req.setAttribute("messagelist", messagelist);

} catch (ClassNotFoundException e) {

// TODO Auto-generated catch block

e.printStackTrace();

} catch (SQLException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

req.getRequestDispatcher("/WEB-INF/jsp/back/list.jsp").forward(req, res);

}

@Override

protected void doPost(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException {

// TODO Auto-generated method stub

doGet(req, res);

}

}