单例模式建立工具类

/*
 * 
 *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