无法连接到mysql数据库jdbc

Question

这是DAO类，它由JDBC数据库连接

组成

package ca.sheridancollege.dao;

import ca.sheridancollege.beans.*;
import java.util.ArrayList;
import java.sql.*;

public class DAO {

    public String user;
    public String password;
    public String host;
    public String database = "homework22";
    public String table = "student";

    public DAO(String host, String user, String password) {
        this.user = user;
        this.password = password;
        this.host = host;

    }

    public void addContact(Student student) {
        try {

            Class.forName("com.mysql.jdbc.Driver");
            Connection con;
            //Try to connect to the database.  If the database does 
            //not exist then create it.
            //The database in this case is called "homework22"

            try {
                con = DriverManager.getConnection(host + database, user, password);

            } catch (Exception e) {
                con = DriverManager.getConnection(host + database, user, password);
                Statement st = con.createStatement();
                st.executeUpdate("CREATE DATABASE " + database + ";");
                st.executeUpdate("USE " + database + ";");
                con = DriverManager.getConnection(host + database, user, password);

            }

            String add = "INSERT INTO " + table + " VALUES "
                    + "('" + student.getName()
                    + "', '" + student.getId()
                    + "', '" + student.getGrade()
                    + "');";

            Statement st = con.createStatement();
            //Add the new contact into the SQL table.
            try {
                st.executeUpdate(add);
            } catch (Exception e) {
                System.out.println(e);
                String make = "CREATE TABLE " + table
                        + "( Name VARCHAR(50)  "
                        + ", " + " Id VARCHAR(20)  "
                        + ", " + " Grade VARCHAR(10)  "
                        + ");";

                st.executeUpdate(make);
                st.executeUpdate(add);
            }
            con.close();

        } catch (Exception e) {
            System.out.println(e);
        }
    }

    public ArrayList<Student> getContacts() {
        ArrayList<Student> contacts = new ArrayList();

        try {
            //Create a connection to our JDBC Driver
            Class.forName("com.mysql.jdbc.Driver");

            //Create a connection to the SQL Server
            //(Database, user, password)
            Connection con = DriverManager.getConnection(host + database, user, password);

            Statement st = con.createStatement();

            String Query = "SELECT * FROM " + table + ";"; //MySQL statement

            ResultSet rs = st.executeQuery(Query);
            //ResultSet rs = st.executeUpdate(Query);

            ResultSetMetaData rsmd = rs.getMetaData();

            int numColumns = rsmd.getColumnCount();

            while (rs.next()) {
                Student c = new Student(
                        rs.getString(1),
                        rs.getInt(2),
                        rs.getDouble(3)
                );

                // System.out.println(d.toString());
                contacts.add(c);
            }
            // so the fact is that the vlue 
            con.close();

        } catch (Exception e) {
            System.out.println(e);
        }
        return contacts;

    }

}

这是获取JDBC的用户名，密码和数据库连接的代码。

如果最初未创建的 homework22 数据库将使用此代码创建。

用户在 index.html 页面中输入详细信息，并且有一个视图链接，该链接会在表格中显示结果。

但是显示例外

com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: Unknown database 'homework22'

这是web.xml文件

<?xml version="1.0" encoding="UTF-8"?>

<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
         version="3.1">


    <context-param>
        <param-name>host</param-name>
        <param-value>jdbc:mysql://localhost/</param-value>
    </context-param>

    <context-param>
        <param-name>user</param-name>
        <param-value>root</param-value>
    </context-param>

    <context-param>
        <param-name>password</param-name>
        <param-value>root</param-value>
    </context-param>

    <error-page>
        <error-code>404</error-code>
        <location>/popup.jsp</location>
    </error-page>


    <error-page>
        <exception-type>java.lang.Throwable</exception-type>
        <location>/popup.jsp</location>
    </error-page>

    <session-config>
        <session-timeout>
            30
        </session-timeout>
    </session-config>
</web-app>