我在数据库中创建表时遇到麻烦。该代码似乎没有任何错误,但没有输出任何内容。我尝试调试它,看来问题可能是连接还是语句对象,我认为...
如果有谁可以帮忙的话,谢谢。
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.sql.*;
import java.util.logging.Level;
import java.util.logging.Logger;
import sun.jdbc.odbc.JdbcOdbcDriver;
/**
* @author
*/
@WebServlet(urlPatterns = {"/FoodServlet"})
public class FoodServlet extends HttpServlet {
private Connection con;
private Statement stmt;
private ResultSet rs;
private PreparedStatement pstmt;
//Servlet initialization method creates con/stmt objects and loads jdbc drivers
@Override
public void init() throws ServletException {
try {
new JdbcOdbcDriver();
String url = "jdbc:odbc:Food";
String user = "";
String password = "";
con = DriverManager.getConnection(url, user, password);
stmt = con.createStatement();
} catch (SQLException e) {
e.printStackTrace();
System.exit(1);
}
}
/**
* Processes requests for both HTTP <code>GET</code> and <code>POST</code>
* methods.
*
* @param request servlet request
* @param response servlet response
* @throws ServletException if a servlet-specific error occurs
* @throws IOException if an I/O error occurs
* @throws java.sql.SQLException
*/
protected void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException, SQLException {
response.setContentType("text/html;charset=UTF-8");
try (PrintWriter out = response.getWriter()) {
//delete the database
stmt.executeUpdate("DROP TABLE IF EXISTS Foods");
//set up the database
stmt.executeUpdate("CREATE TABLE Foods (ItemID Integer PRIMARY KEY AUTOINCREMENT, ItemName VARCHAR(20), Likes Integer)");
stmt.executeUpdate("INSERT INTO Foods VALUES ('Ice Cream', 0)");
stmt.executeUpdate("INSERT INTO Foods VALUES ('Hamburger', 0)");
stmt.executeUpdate("INSERT INTO Foods VALUES ('Pizza', 0)");
//set likeValues for data storage
int likeValue;
//set strings for queries and update statements
String query = "SELECT Likes FROM Foods WHERE ItemName = ?";
String update = "UPDATE Foods SET Likes = ? WHERE ItemName = ?";
//set food to request parameter from html
String food = request.getParameter("food");
//get current likes and increase by one, store in 'likeValue'
pstmt = con.prepareStatement(query);
pstmt.setString(1, food);
rs = pstmt.executeQuery();
likeValue = rs.getInt(1) + 1;
//write updated 'likeValue' to the database
pstmt = con.prepareStatement(update);
pstmt.setInt(1, likeValue);
pstmt.setString(2, food);
pstmt.executeUpdate();
//output the data from the database
rs = stmt.executeQuery("SELECT ItemID, ItemName, Likes FROM Foods");
out.println("ItemID ItemName Likes");
while (rs.next()) {
out.print(rs.getInt(1) + "\t\t" + rs.getString(2) + "\t\t"
+ rs.getInt(3));
}
} finally {
}
}
// <editor-fold defaultstate="collapsed" desc="HttpServlet methods. Click on the + sign on the left to edit the code.">
/**
* Handles the HTTP <code>GET</code> method.
*
* @param request servlet request
* @param response servlet response
* @throws ServletException if a servlet-specific error occurs
* @throws IOException if an I/O error occurs
*/
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
try {
processRequest(request, response);
} catch (SQLException ex) {
Logger.getLogger(FoodServlet.class.getName()).log(Level.SEVERE, null, ex);
}
}
/**
* Handles the HTTP <code>POST</code> method.
*
* @param request servlet request
* @param response servlet response
* @throws ServletException if a servlet-specific error occurs
* @throws IOException if an I/O error occurs
*/
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
try {
processRequest(request, response);
} catch (SQLException ex) {
Logger.getLogger(FoodServlet.class.getName()).log(Level.SEVERE, null, ex);
}
}
/**
* Returns a short description of the servlet.
*
* @return a String containing servlet description
*/
@Override
public String getServletInfo() {
return "Short description";
}// </editor-fold>
}
最佳答案
因此,经过大量的工作,我发现连接对象是第一个问题,我不确定如何解决它……然后,我遇到了一个空指针异常的问题,我将exceuteUpdate()放入了if语句来解决从html页面检查了一个值。然后出现一个错误,该错误涉及我的表值之一没有正确的拼写。
这是现在可以使用的更新代码,虽然不漂亮,但是可以使用。谢谢大家的帮助!
import java.io.IOException;
import java.io.PrintWriter;
import java.sql.DriverManager;
import java.sql.*;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import sun.jdbc.odbc.*;
/**
*
* @author
*/
@WebServlet(urlPatterns = {"/FoodServlet"})
public class FoodServlet extends HttpServlet {
private Connection con;
private Statement stmt;
private ResultSet rs;
private PreparedStatement pstmt;
private DatabaseMetaData dbmd;
//Servlet initialization method creates con/stmt objects and loads jdbc drivers
public void init() throws ServletException {
try {
new JdbcOdbcDriver();
String url = "jdbc:odbc:Java3DB";
String user = "";
String pword = "";
con = DriverManager.getConnection(url, user, pword);
stmt = con.createStatement();
} catch (SQLException e) {
e.printStackTrace();
System.exit(1);
}
}
/**
* Processes requests for both HTTP <code>GET</code> and <code>POST</code>
* methods.
*
* @param request servlet request
* @param response servlet response
* @throws ServletException if a servlet-specific error occurs
* @throws IOException if an I/O error occurs
* @throws java.sql.SQLException
*/
protected void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException, SQLException {
response.setContentType("text/html;charset=UTF-8");
PrintWriter out = response.getWriter();
try {
//open html tags
out.println("<!DOCTYPE html>");
out.println("<html>");
out.println("<head>");
out.println("<title>FoodServlet</title>");
out.println("</head>");
out.println("<body>");
//set up the table
createTable();
//set strings for queries and update statements
String update = "UPDATE Foods SET likes = (likes + 1) WHERE itemName = ?";
//set foodValue to request parameter from html
String foodValue = null;
foodValue = request.getParameter("food");
//update only if there is a value stored in the food parameter
if (foodValue != null) {
//out.println(foodValue);
pstmt = con.prepareStatement(update);
pstmt.setString(1, foodValue);
pstmt.executeUpdate();
}
//output the data from the database
rs = stmt.executeQuery("SELECT itemID, itemName, likes FROM Foods");
out.println("ItemID  ItemName  Likes <BR>");
while (rs.next()) {
out.print(+rs.getInt(1) + "       "
+ rs.getString(2) + "       "
+ rs.getInt(3) + "<br>");
}
out.println("</body>");
out.println("</html>");
} catch (SQLException e) {
e.printStackTrace();
out.print(e.getMessage());
}
}
//destroy method to close the servlet
public void destroy() {
try {
stmt.close();
con.close();
} catch (SQLException ex) {
Logger.getLogger(FoodServlet.class.getName()).log(Level.SEVERE, null, ex);
}
}
public void createTable() throws SQLException {
dbmd = con.getMetaData();
rs = dbmd.getTypeInfo();
//checks for the Foods table, create only if not table is found
if (rs.next() == false) {
//set up the database
stmt.executeUpdate("CREATE TABLE Foods (itemID INTEGER, itemName VARCHAR(20), likes Integer)");
stmt.executeUpdate("INSERT INTO Foods VALUES (1, 'IceCream', 0)");
stmt.executeUpdate("INSERT INTO Foods VALUES (2, 'Hamburger', 0)");
stmt.executeUpdate("INSERT INTO Foods VALUES (3, 'Pizza', 0)");
}
}
// <editor-fold defaultstate="collapsed" desc="HttpServlet methods. Click on the + sign on the left to edit the code.">
/**
* Handles the HTTP <code>GET</code> method.
*
* @param request servlet request
* @param response servlet response
* @throws ServletException if a servlet-specific error occurs
* @throws IOException if an I/O error occurs
*/
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
try {
processRequest(request, response);
} catch (SQLException ex) {
Logger.getLogger(FoodServlet.class.getName()).log(Level.SEVERE, null, ex);
}
}
/**
* Handles the HTTP <code>POST</code> method.
*
* @param request servlet request
* @param response servlet response
* @throws ServletException if a servlet-specific error occurs
* @throws IOException if an I/O error occurs
*/
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
try {
processRequest(request, response);
} catch (SQLException ex) {
Logger.getLogger(FoodServlet.class.getName()).log(Level.SEVERE, null, ex);
}
}
/**
* Returns a short description of the servlet.
*
* @return a String containing servlet description
*/
@Override
public String getServletInfo() {
return "Short description";
}// </editor-fold>
}