Question

我有这个代码，它不会显示我在我的数据库中的东西。我创建了一个表并插入了一些数据。我知道一切都很好并且有效。但我遇到的问题是它不会显示任何数据，我不知道为什么......所以，如果有人可以帮助我，那将是伟大的。只是不承担我做错了什么或如果我错过了什么在这里是我的代码。我使用tomcat，如果这对servlet部分很重要。

import java.io.*;
import java.util.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.sql.*;

public class test extends HttpServlet{

 static final String JDBC_DRIVER="com.mysql.jdbc.Driver";  
 static final String DB_URL="jdbc:mysql://localhost/csc3610_Connection";

 //  Database credentials
  static final String USER = "root";
  static final String PASS = "";


public void doGet(HttpServletRequest request,
                HttpServletResponse response)
        throws ServletException, IOException
{


  // Set response content type
  response.setContentType("text/html");
  PrintWriter out = response.getWriter();
  String title = "Database Result";
  String docType =
    "<!doctype html public \"-//w3c//dtd html 4.0 " +
     "transitional//en\">\n";
     out.println(docType +
     "<html>\n" +
     "<head><title>" + title + "</title></head>\n" +
     "<body bgcolor=\"#f0f0f0\">\n" +
     "<h1 align=\"center\">" + title + "</h1>\n");

     Connection conn=null;
     Statement stmt=null;
  try{
     // Register JDBC driver
     Class.forName("com.mysql.jdbc.Driver");

     // Open a connection
     conn = DriverManager.getConnection(DB_URL,USER,PASS);

     // Execute SQL query
     stmt = conn.createStatement();
     String sql;
     sql = "SELECT FROM csc3610_Connection.Employees";
     ResultSet rs = stmt.executeQuery(sql);

     // Extract data from result set
     while(rs.next()){
        //Retrieve by column name
        int id  = rs.getInt("id");
        int age = rs.getInt("age");
        String first = rs.getString("first");
        String last = rs.getString("last");

        //Display values
        out.println("ID: " + id + "<br>");
        out.println(", Age: " + age + "<br>");
        out.println(", First: " + first + "<br>");
        out.println(", Last: " + last + "<br>");
     }
     out.println("</body></html>");

     // Clean-up environment
     rs.close();
     stmt.close();
     conn.close();
  }catch(SQLException se){
     //Handle errors for JDBC
     se.printStackTrace();
  }catch(Exception e){
     //Handle errors for Class.forName
     e.printStackTrace();
  }finally{
     //finally block used to close resources
     try{
        if(stmt!=null)
           stmt.close();
     }catch(SQLException se2){
     }// nothing we can do
     try{
        if(conn!=null)
        conn.close();
     }catch(SQLException se){
        se.printStackTrace();
     }//end finally try
  } //end try
 }
}

Answer 1

首先，SELECT语句是错误的。你必须使用

sql = "SELECT * FROM csc3610_Connection.Employees";

然后请重构检索数据库连接的代码段。您不应该在每个请求上打开新连接。此外，Java中的类名应以大写字母开头。看看Java Code Conventions

例如：

private Connection connection = null;

public void init(ServletConfig config) throws ServletException
{
    super.init(config);
    try 
    {
        // Load the MySQL driver
        Class.forName(JDBC_DRIVER);
        connection = DriverManager.getConnection(
        DB_URL, "root", "");
    }
    catch (ClassNotFoundException e)
    { 
        throw new UnavailableException(this, "Couldn't load database driver");
    }    
    catch (SQLException e) 
    { 
        throw new UnavailableException(this, "Couldn't get db connection");
    }    
}

然后在你的doGet-Method中使用这样的东西：

     // Execute SQL query
     stmt = connection.createStatement();
     String sql = "SELECT * FROM csc3610_Connection.Employees";
     ResultSet rs = stmt.executeQuery(sql);

不要在我的java servlet中显示我的数据库信息

1 个答案: