java如何从主方法中获取要通过getConnection方法访问的值？

Question

我试图获取main方法将产生的值，并在getConnection方法中使用它们。但是，当我尝试访问getConnection方法时，返回的是null值

我想使用ConnectionManager类连接到数据库

代码如下

public class ConnectionManager {

    public static String database;    
    public static String dbuser;
    public static String dbpassword;

    public static void main(String args[])  {

        Properties prop = new Properties();
        InputStream input = null;

        try {
            input = new FileInputStream("config.properties");

            // load a properties file
            prop.load(input);

            database = prop.getProperty("database");
            dbuser = prop.getProperty("dbuser");
            dbpassword = prop.getProperty("dbpassword");

            System.out.println(database);
        } catch (IOException ex) {
            ex.printStackTrace();
        } finally {
            if (input != null) {
                try {
                    input.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }


    public static String url = "jdbc:mysql://localhost:3306/" + database;    
    private static String driverName = "com.mysql.jdbc.Driver";   
    private static String username = dbuser;   
    private static String password = dbpassword;
    private static Connection con;

    public static Connection getConnection() {

        try {
            Class.forName(driverName);
            try {
                con = DriverManager.getConnection(url, username, password);
            } catch (SQLException ex) {
                // log an exception. For example:
                System.out.println("Failed to create the database connection."); 
                System.out.println(url + " " + username + " " + password);
            }
        } catch (ClassNotFoundException ex) {
            System.out.println("Your driver has not been found."); 
        }
        return con;
    }
}

Answer 1

静态c字段在类加载时初始化一次。当连接字段仍然为空时，将其设置为ConnectionManager字段一次

要“修复”您的问题，请让连接中的代码使用ConnectionManager中的字段：

con = DriverManager.getConnection(url, ConnectionManager.dbuser, ConnectionManager.dbpassword);

Answer 2

您在DriverManager.getConnection(url, username, password)调用中得到空参数值，因为您已将它们声明为静态字段

因此，在从config.properties读取特定值之前，将它们初始化为nulls

让我们跟随流程：

静态初始化步骤：

private static String username = dbuser;      //username==null; dbuser==null;
private static String password = dbpassword;  //password==null; dbpassword==null
private static Connection con;                //con==null;

方法主执行：

database = (prop.getProperty("database"));
dbuser = (prop.getProperty("dbuser"));          //dbuser="smth"; username==null
dbpassword = (prop.getProperty("dbpassword"));  //dbpassword ="smth"; password==null

方法getConnection执行：

con = DriverManager.getConnection(url, username, password); //username==null; //password==null; 

注：如前所述，最好使用带有如下参数的函数：

public static Connection getConnection(String url, String username, String password) {
    /* Your code here */
}

Answer 3

只需使用参数调用getConnection（）方法

public static Connection getConnection(String url, String username, String password) {
    /* Your code here */
}

然后，调用这个方法

Connection connection = getConnection(url, username, password);