使用单例写JDBC操作数据库的工具类

package cn.jdbctest;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

/**
 * 单例模式
 * @author zhangjl
 * @date   2013-12-12
 */
public final class JdbcUtils {
	private final String URL = "jdbc:mysql://localhost:3306/user";
	private final String USER = "root";
	private final String PASSWORD = "root";
	
	private Connection conn=null;
	private JdbcUtils instance=null;
	//private JdbcUtils instance=new JdbcUtils();在类创建时就加载。
	/**
	 * 防止构建实例
	 */
	private JdbcUtils(){
		
	}
	
	static{
		try {
			Class.forName("com.jdbc.mysql.Driver");
		} catch (ClassNotFoundException e) {
			throw new ExceptionInInitializerError(e);
		}
	}
	
	/**
	 * 延时加载（懒加载），在第一次调运时在加载
	 * @return
	 */
	public JdbcUtils getInstance(){
		if(instance==null){
			synchronized (instance) {
				if(instance==null){
					instance= new JdbcUtils();
				}
			}
		}
		return instance;
	}
	
	public  Connection getConn() {
		try {
			conn = DriverManager.getConnection(URL, USER, PASSWORD);
		} catch (SQLException e) {
			e.printStackTrace();
		}
		return conn;
	}

	public void free(ResultSet rs, Statement stat, Connection conn) {
		if (rs != null) {
			try {
				rs.close();
			} catch (SQLException e) {
				e.printStackTrace();
			} finally {
				if (stat != null) {
					try {
						stat.close();
					} catch (SQLException e) {
						e.printStackTrace();
					} finally {
						if (conn != null) {
							try {
								conn.close();
							} catch (SQLException e) {
								e.printStackTrace();
							}
						}
					}
				}
			}
		}
	}
}