单例模式建立工具类

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/* *  *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

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