单例模式建立工具类

小灰灰 2022-08-09 00:45 145阅读 0赞

/* * *JdbcUtilsSing.java * * 这是一个工具类，一般工具类不能被继承，定义成final * 不需要构造实例，用私有的构造方法 * 单例模式或者用static静态代码块，本例用单例模式 * * */
    package cn.itcast.jdbc;
    
    import java.sql.Connection;
    import java.sql.DriverManager;
    import java.sql.ResultSet;
    import java.sql.SQLException;
    import java.sql.Statement;
    
    public final class JdbcUtilsSing { 
    
        private static String url = "jdbc:mysql://localhost:3306/jdbc";
        private static String user = "root";
        private static String password = "123456";
    
        private static JdbcUtilsSing instance = null;
    
        private JdbcUtilsSing() {
        }
    
        public static JdbcUtilsSing getInstance() {
            if (instance == null) { // 延迟加载
                synchronized (JdbcUtilsSing.class) { // 同步锁
                    if (instance == null)
                        instance = new JdbcUtilsSing();
                }
            }
            return instance;
        }
    
        // 注册驱动
        static {
            try {
                Class.forName("com.mysql.jdbc.Driver");
            } catch (Exception e) {
                throw new ExceptionInInitializerError(e);
            }
        }
    
        public Connection getConnection() throws SQLException {
            return DriverManager.getConnection(url, user, password);
        }
    
        public void free(ResultSet rs, Statement st, Connection conn) {
            try {
                if (rs != null)
                    rs.close();
            } catch (SQLException e) {
                e.printStackTrace();
            } finally {
                try {
                    if (st != null)
                        st.close();
                } catch (SQLException e) {
                    e.printStackTrace();
                } finally {
                    try {
                        if (conn != null)
                            conn.close();
                    } catch (SQLException e) {
                        e.printStackTrace();
                    }
                }
            }
        }
    }

使用这个类

package cn.itcast.jdbc;
    
    import java.sql.Connection;
    import java.sql.DriverManager;
    import java.sql.ResultSet;
    import java.sql.SQLException;
    import java.sql.Statement;
    
    public class Base { 
    
        public static void main(String[] args) {
            try {
                template();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    
        static void template() throws Exception {
            Connection conn = null;
            Statement st = null;
            ResultSet rs = null;
    
            try {
                // 2.建立连接
                conn = JdbcUtilsSing.getInstance().getConnection();
    
                // 3.创建语句
                st = conn.createStatement();
    
                // 4.执行语句
                rs = st.executeQuery("select * from user");
    
                // 5.处理结果
                while (rs.next()) {
                    System.out.println("id:" + rs.getObject(1) + "\tname:"
                            + rs.getObject(2) + "\tbirthday:" + rs.getObject(3)
                            + "\tmoney:" + rs.getObject(4));
                }
    
                // 6.释放资源
            } finally {
                JdbcUtilsSing.getInstance().free(rs,st,conn);
            }
        }
    }

比起static代码的方式稍微麻烦一点