求助！数据库Connection close一次 ，下次再连接就打不开了

/**

	 * 获取按钮事件

	 * 

	 * @return

	 * @throws SQLException

	 */

	public List<AlarmData> getNewButtonStat2() throws SQLException {

		List<AlarmData> list = new ArrayList<AlarmData>();

		Connection conn = MySQLDBUtil.getConn();

		

		Statement statement = conn.createStatement();

		// 通过查询返回结果

		ResultSet rs = statement

				.executeQuery("SELECT  id,bid,keyid,ttime,yn,status FROM(SELECT * FROM t_alarmdata WHERE bid BETWEEN 9 AND 12 ORDER BY ttime DESC LIMIT 2000) a GROUP BY a.bid,a.keyid");

		// 循环取出 rs 中的结果

		while (rs.next()) {

			AlarmData ad = new AlarmData();

			ad.setBid(rs.getString("bid"));

			ad.setId(rs.getInt("id"));

			ad.setKeyid(rs.getString("keyid"));

			ad.setStatus(rs.getInt("status"));

			ad.setYn(rs.getInt("yn"));

			list.add(ad);

		}

		rs.close();

		statement.close();

		conn.close(); // 如果这里的不关闭，则没有错误信息，加上conn.close(); 下次再进来这里就会报如下错误

		System.out.println(list.size());

		return list;

	}

import java.sql.Connection;

import java.sql.DriverManager;

import java.sql.SQLException;

import java.sql.Statement;



public class MySQLDBUtil {

	

	// 本机

	private static final String URL = "jdbc:mysql://127.0.0.1:3306/warning_light";

	

	private static final String USER = "root";

	

	private static final String PASSWORD = "888888";

	

	private static Connection conn = null;

	

	DBPool dbp = null;



	static {

		// 1.加载驱动（反射机制）

				try {

					Class.forName("com.mysql.jdbc.Driver");

					// 2.获得数据库连接

					conn = DriverManager.getConnection(URL, USER, PASSWORD);

				} catch (ClassNotFoundException e) {

					System.out.println("sorry,can't find the Driver!");

					e.printStackTrace();

				} catch (SQLException e) {

					System.out.println(conn);

					e.printStackTrace();

				}

	}

	

	public MySQLDBUtil(){

		

	}

	

	public static Connection getConn() {

		return conn;

	}



}

com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException: No operations allowed after connection closed.

	at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)

	at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)

	at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)

	at java.lang.reflect.Constructor.newInstance(Unknown Source)

	at com.mysql.jdbc.Util.handleNewInstance(Util.java:406)

	at com.mysql.jdbc.Util.getInstance(Util.java:381)

	at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:984)

	at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:956)

	at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:926)

	at com.mysql.jdbc.ConnectionImpl.checkClosed(ConnectionImpl.java:1098)

	at com.mysql.jdbc.ConnectionImpl.createStatement(ConnectionImpl.java:2380)

	at com.mysql.jdbc.ConnectionImpl.createStatement(ConnectionImpl.java:2362)

// 第一段代码的11行

	at com.lewei.dao.WarningLightDao.getNewButtonStat2(WarningLightDao.java:137)// 第一段代码的11行

	at com.lewei.www.WarningScreen$6.doInBackground(WarningScreen.java:983)

	at com.lewei.www.WarningScreen$6.doInBackground(WarningScreen.java:1)

	at javax.swing.SwingWorker$1.call(Unknown Source)

	at java.util.concurrent.FutureTask.run(Unknown Source)

	at javax.swing.SwingWorker.run(Unknown Source)

	at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)

	at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)

	at java.lang.Thread.run(Unknown Source)