连接数据库时遇到问题

Question

我在连接数据库MySQL方面遇到了麻烦。我用databese编写了一个控制台项目，我在这里有一个类ConnectionManager。现在我开始写网站了。我想连接数据库ang只需将我的工作类Connection conn = ConnectionManager.getConnection();复制到新的动态项目中。但是这个类返回NULL而不是连接。也许你知道哪里有问题。 先感谢您！ 连接器已添加。 NullPointer异常由import java.sql.Connection; import java.sql.DriverManager; import java.sql.SQLException; public class ConnectionManager { private static String jdbcUrl = "jdbc:mysql://localhost:3306/registration"; private static String user = "root"; private static String password = "root"; private static Connection connection = null; public static Connection getConnection() { if (connection == null) { initializeConnection(); } return connection; } private static void initializeConnection() { Connection conn; try { conn = DriverManager.getConnection(jdbcUrl,user,password); connection = conn;// doesn't execute and I don't know why } catch (SQLException e) { e.printStackTrace(); } }}

调用

Connection conn = ConnectionManager.getConnection();

使用：var danu = ["nu", "da", "nu", "da", "da", "da", "da", "da", "da", "nu", "nu"], abc2 = ["2", "3", "2", "3", "3", "2", "2"]; danu.reduce(function (r, a, i, o) { o[i] = a === 'da' ? abc2[r++] : a; return r; }, 0); document.write('<pre>' + JSON.stringify(danu, 0, 4) + '</pre>');

Answer 1

尝试制作构造函数并在其中添加代码。我正在做同样的事情和它的工作。像这样： -

    public Connection()  {
    // TODO Auto-generated constructor stub
    try {
        if (connection == null)
        {
            initializeConnection();
        }
       return connection;       
    } 
    catch (ClassNotFoundException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
     catch (SQLException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
     }
}

Answer 2

可能不止一个原因

1-检查您的用户权限

GRANT [type of permission] ON [database name].[table name] TO ‘[username]’@'localhost’;

GRANT ALL PRIVILEGES ON * . * TO 'root'@'localhost';

2-检查你的jdbc网址及其端口。确保3306端口可用于使用mysql

jdbc:mysql://localhost:3306/registration

3-检查你的jar文件

• 下载.jar文件（mysql-connector-java-5.1.37-bin.jar）并将其移至libs文件夹
• 右键单击您的项目＆gt;属性＆gt;图书馆＆gt; ADD jar / Folder在该文件夹中选择您的jar文件。

测试您的连接

String dbUrl= "jdbc:mysql://localhost:3306/javabase";
String username = "root";
String password = "root";

try (Connection connection = DriverManager.getConnection(dbUrl, username, password)) {
    System.out.println("Connected to DB!");
} catch (SQLException e) {
    throw new IllegalStateException("Error: ", e);
}

4-检查您的用户密码。 如果您不确定更改密码。

SET PASSWORD FOR 'root'@'localhost' = PASSWORD('root');