[Java] Singleton pattern

1. Runtime Memory

• Method
  • static
  • 상수 ⇒ Unique한 값
  • 프로그램 ㅌ코드
    • Class → 실행 시, Stack → new 연산자를 통해 Instance가 됨.
  • Stack
    • Function
  • Heap
    • Instance

2. Singleton Pattern

• 클래스 하나 당 하나의 Instance를 만든다.
  • 재사용
• 단점
  • 사용하지 않으면 GC가 처리할 수 있다.

3. static

• Instance 생성없이 필드와 메소드를 생성하고자 할 때 활용
• 필드와 메소드를 객체마다 다르게 가져야 한다면 인스턴스로 생성
• 공통 변수, 인스턴스 필드를 포함하지 않는 메소드를 선언하고자 할 때 사용
• 객체없이 바로 사용 가능

public class PlusClass{
  static int field1 = 15;

  static int plusMethod(int x, int y){ return x+y; } 
}

int ans1 = PlusClass.plusMethod(15,2);
int ans2 = PlusClass.field1 + 2;

3. final

• 선언되면 수정이 불가능

1) static final

• 모든 영역에서 고정된 값으로 사용하는 상수

4. DB Pool Module

• JdbcUtil

import java.sql.*;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class JdbcUtil {
    private static String className = "com.mysql.jdbc.Driver";
    private static String url = "jdbc:mysql://localhost:3306/examplesdb";
    private static String id = "urstory";
    private static String pw = "u1234";

    private static Connection conn;
    // Instance를 한번만 생성하고 재사용
    private static JdbcUtil instance = new JdbcUtil();

    static {
        try {
            // MySQL 드라이버 생성
            Class.forName(className);
            // DB Connection 생성
            conn = DriverManager.getConnection(url, id, pw);
            System.out.println("DB Connection Success");
        }
        catch (ClassNotFoundException e) {
            System.out.println("MySQL Driver Error");
            e.printStackTrace();
        } 
        catch (SQLException e) {
            System.out.println("Connection Error");
            e.printStackTrace();
        }
        catch (Exception e) {
            System.out.println("Error" + e);
            e.printStackTrace();
        }	
    }

    // 외부에서 JdbcUtil 객체를 사용하기 위해
    public static  JdbcUtil getInstance() {
        return instance;
    }

    // 외부에서 connection 사용하기 위해 
    public Connection getConnection() {
        return conn;
    }

    // 연결해제
    public void closeAll (Connection conn, PreparedStatement pstmt, ResultSet rs) {
        if(rs != null) {
            try {rs.close();}
            catch (SQLException e) {e.printStackTrace();}
        }

        if(pstmt != null) {
            try {pstmt.close();} 
            catch (SQLException e) {e.printStackTrace();}
        }

        if(conn != null) {
            try {conn.close();}
            catch (SQLException e) {e.printStackTrace();}
        }
    }

    // select문
    public List<Map<String, Object>> SelectAll(String query) {
        List<Map<String, Object>> rstList = new ArrayList<Map<String, Object>>();

        PreparedStatement pstmt = null;
        ResultSet rs = null;

        try {
            pstmt = JdbcUtil.conn.prepareStatement(query);
            rs = pstmt.executeQuery();

            ResultSetMetaData rsmd = rs.getMetaData();
            int columnCnt = rsmd.getColumnCount();
            while(rs.next()) {
                Map<String, Object> map = new HashMap<>();

                for(int i = 1; i <= columnCnt; i++) {
                    String colNm = rsmd.getColumnName(i);
                    map.put(colNm, rs.getObject(colNm));
                }
                rstList.add(map);
            }

        }
        catch(Exception e) {
            System.out.println(e);
        }
        finally {
            this.closeAll(conn, pstmt, rs);
        }

        return rstList;
    }

    // Insert / Update
    public int InsertOrUpdate(String query) {
        int result = -1;

        PreparedStatement pstmt = null;
        ResultSet rs = null;

        try {
            pstmt = JdbcUtil.conn.prepareStatement(query);
            result = pstmt.executeUpdate();
            JdbcUtil.conn.commit();
        }
        catch(Exception e) {
            System.out.println(e);
        }
        finally {
            this.closeAll(conn, pstmt, rs);
        }
        return result;
    }
}

• Main

public class ex01_jdbcConn {
    public static void main(String[] args) {
        JdbcUtil ju = JdbcUtil.getInstance();

        String selectQuery = 
                """
                CREATE TABLE mytable (
                    id INT UNSIGNED NOT NULL AUTO_INCREMENT,
                    name VARCHAR(50) NOT NULL,
                    modelnumber VARCHAR(15) NOT NULL,
                    series VARCHAR(30) NOT NULL,
                    PRIMARY KEY(id)
                );
                """;
        String insertQuery = 
                """
                insert into mytable(name, modelnumber, series) values('name1', '11111', '111');
                """;

        String updateQuery = 
                """
                update mytable set modelnumber = '22222', series = '222' where id = 1;
                """;
        int rst = ju.InsertOrUpdate(insertQuery);
        System.out.println(rst);
    }
}

 

 

💡 Reference

[Java] static, final, static final의 차이