JAVAEE_Servlet_19_重定向可以解决页面刷新问题(sendRedirect)

重定向可以解决页面刷新问题(sendRedirect)

• 在向数据库中添加数据的时候，如果使用转发(getRequestDispatcher),数据插入成功后，转发到提示插入成功页面，在数据插入成功页面刷新该页面，那么就会再次向数据库中添加一遍该数据，因为转发是一次请求，刷新一次添加一次数据。

• 如果使用重定向（sendRedirect），那么数据插入成功之后跳转到提示插入成功页面，此时刷新该页面，刷新的只是提示插入成功页面，因为重定向是两次请求。

• 代码：

  • 向数据库中插入数据Servlet

    import javax.servlet.ServletException;
    import javax.servlet.http.HttpServlet;
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;
    import java.io.IOException;
    import java.io.PrintWriter;
    import java.sql.*;
    
    public class SaveServlet  extends HttpServlet {
        @Override
        protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    
            //解决中文乱码问题
            request.setCharacterEncoding("UTF-8");
    
            //获取表单数据
            String userCode=request.getParameter("usercode");
            String userName=request.getParameter("username");
    
            //创建数据库连接对象
            Connection connection=null;
            PreparedStatement preparedStatement=null;
    
            try {
                // 连接数据库
                Class.forName("com.mysql.cj.jdbc.Driver");
    
                // 获取连接
                connection=DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/mydb?useUnicode=true&characterEncoding=utf8&serverTimezone=Asia/Shanghai",
                        "root","123456");
    
                // 获取预编译数据库操作对象
                String sql="insert into t_user(usercode,username)values(?,?)";
                preparedStatement=connection.prepareStatement(sql);
    
                //给预编译数据库操作对象插入值
                preparedStatement.setString(1,userCode);
                preparedStatement.setString(2,userName);
    
                //执行SQL语句
                int count=preparedStatement.executeUpdate();
    
                // 显示执行结果
                response.setContentType("text/html;charset=UTF-8");
                PrintWriter out=response.getWriter();
                if(count!=0){
                    out.print("数据插入成功,正在跳转...");
                    //转发
                    //request.getRequestDispatcher("/SearchServlet").forward(request,response);
                    //跳转
                    response.sendRedirect("/myWeb/SearchServlet");
                }
    
            } catch (ClassNotFoundException | SQLException e) {
                e.printStackTrace();
            }finally {
    
                //释放资源
                if(preparedStatement!=null){
                    try {
                        preparedStatement.close();
                    } catch (SQLException e) {
                        e.printStackTrace();
                    }
                }
    
                if(connection!=null){
                    try {
                        connection.close();
                    } catch (SQLException e) {
                        e.printStackTrace();
                    }
                }
            }
        }
    }
    

     - 提示成功页面，并把数据库中的数据显示出来的  Servlet

        import javax.servlet.ServletException;
        import javax.servlet.http.HttpServlet;
        import javax.servlet.http.HttpServletRequest;
        import javax.servlet.http.HttpServletResponse;
        import java.io.IOException;
        import java.io.PrintWriter;
        import java.sql.*;

        public class SearchServlet extends HttpServlet {
            @Override
            protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

                //解决中文乱码问题
                request.setCharacterEncoding("UTF-8");

                //创建数据库连接对象
                Connection connection=null;
                PreparedStatement preparedStatement=null;
                ResultSet resultSet=null;

                try {

                    // 连接数据库
                    Class.forName("com.mysql.cj.jdbc.Driver");

                    // 获取连接
                    connection= DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/mydb?useUnicode=true&characterEncoding=utf8&serverTimezone=Asia/Shanghai",
                            "root","123456");

                    // 获取预编译数据库操作对象
                    String sql="select * from t_user";
                    preparedStatement=connection.prepareStatement(sql);

                    //执行SQL语句
                    resultSet=preparedStatement.executeQuery();

                    //处理查询结果集
                    response.setContentType("text/html;charset=UTF-8");
                    PrintWriter out=response.getWriter();
                    //HTML代码
                    out.print("          <!DOCTYPE html>");
                    out.print("<meta http-equiv=\"content-type\" content=\"text/html\" charset=\"utf-8\"/> ");
                    out.print("  <head>");
                    out.print("      <meta charset='UTF-8'>");
                    out.print("      <title>用户信息</title>");
                    out.print("  </head>");
                    out.print("     <body>");
                    out.print("          <h3 align='center'>用户信息表</h3>");
                    out.print("          <hr width='60%'>");
                    out.print("          <table border='1' align='center' width='50%'>");
                    out.print("              <tr align='center'>");
                    out.print("                  <th>序号</th>");
                    out.print("                  <th>用户编号</th>");
                    out.print("                  <th>用户姓名</th>");
                    out.print("              </tr>");
                    int i=1;
                    while(resultSet.next()){
                      String userCode=resultSet.getString("usercode");
                      String userName=resultSet.getString("username");
                        out.print("              <tr align='center'>");
                        out.print("                  <th>"+(i++)+"</th>");
                        out.print("                  <th>"+userCode+"</th>");
                        out.print("                  <th>"+userName+"</th>");
                        out.print("              </tr>");
                    }

                    out.print("          </table>");
                    out.print("    </body>");
                    out.print("  </html>");

                } catch (ClassNotFoundException | SQLException e) {
                    e.printStackTrace();
                }finally {

                    //释放资源
                    if(resultSet!=null){
                        try {
                            resultSet.close();
                        } catch (SQLException e) {
                            e.printStackTrace();
                        }
                    }
                    if(preparedStatement!=null){
                        try {
                            preparedStatement.close();
                        } catch (SQLException e) {
                            e.printStackTrace();
                        }
                    }

                    if(connection!=null){
                        try {
                            connection.close();
                        } catch (SQLException e) {
                            e.printStackTrace();
                        }
                    }
                }

            }
        }