java.sql.SQLException: connection holder is null错误解决办法

javaCopy codeimport java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
public class DatabaseExample {
    private Connection connection;
    public void connectToDatabase() throws SQLException {
        String url = "jdbc:mysql://localhost:3306/mydatabase";
        String username = "root";
        String password = "mypassword";
        
        connection = DriverManager.getConnection(url, username, password);
    }
    
    public void executeQuery(String query) throws SQLException {
        if (connection == null) {
            throw new SQLException("Connection holder is null");
        }
        
        Statement statement = null;
        ResultSet resultSet = null;
        
        try {
            statement = connection.createStatement();
            resultSet = statement.executeQuery(query);
            
            while (resultSet.next()) {
                // Process the retrieved data
            }
        } finally {
            if (resultSet != null) {
                resultSet.close();
            }
            if (statement != null) {
                statement.close();
            }
        }
    }
    
    public void closeConnection() throws SQLException {
        if (connection != null) {
            connection.close();
        }
    }
    
    public static void main(String[] args) {
        DatabaseExample example = new DatabaseExample();
        
        try {
            example.connectToDatabase();
            
            // Perform database operations...
            example.executeQuery("SELECT * FROM users");
            
            example.closeConnection();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

javaCopy codeimport javax.sql.DataSource;
import java.sql.Connection;
import java.sql.SQLException;
public class ConnectionPoolExample {
    private DataSource dataSource;
    
    public ConnectionPoolExample(DataSource dataSource) {
        this.dataSource = dataSource;
    }
    
    public void performDatabaseOperation() {
        Connection connection = null;
        
        try {
            connection = dataSource.getConnection();
            
            // 使用连接执行数据库操作
            // ...
            
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            if (connection != null) {
                try {
                    connection.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                }
            }
        }
    }
    
    public static void main(String[] args) {
        // 创建数据源
        DataSource dataSource = createDataSource();
        
        // 创建连接池示例
        ConnectionPoolExample example = new ConnectionPoolExample(dataSource);
        
        // 执行数据库操作
        example.performDatabaseOperation();
    }
    
    private static DataSource createDataSource() {
        // 创建并配置数据源
        // ...
        return dataSource;
    }
}