Connection关闭时报错怎么解决呢？

想要写一个定时访问数据库，读取数据里的数据但是我没法关闭Connection链接，请问怎么解决？
下面是我代码
public class UploadFileDao {

private Connection conn = null;
private PreparedStatement pstmt = null;
private ResultSet rs = null;

public List<UploadFile> listUploadFile() {
List<UploadFile> uploadFile_l = new ArrayList<UploadFile>();
String sql = "select * from uploadfile";
conn = DBConnection.getConn();
pstmt = DBConnection.getPstmt(conn, sql);
rs = DBConnection.getRest(pstmt);
try {
while (rs.next()) {
UploadFile uploadFile = new UploadFile();
uploadFile.setId(rs.getInt("id"));
uploadFile.setFileUrl(rs.getString("file_Url"));
uploadFile.setStatus(rs.getInt("status"));
uploadFile_l.add(uploadFile);
}
} catch (SQLException e) {
e.printStackTrace();
} finally{
if(rs != null){
try {
rs.close();
rs = null;
} catch (SQLException e) {
e.printStackTrace();
}
}
if(pstmt != null){
try {
pstmt.close();
pstmt = null;
} catch (SQLException e) {
e.printStackTrace();
}
}
if(conn != null){
try {
conn.close();
conn = null;
} catch (SQLException e) {
e.printStackTrace();
}
}
}
return uploadFile_l;
}

 

public class ReadDB implements Runnable {

public void run() {
List<UploadFile> uploadFiles = new UploadFileDao().listUploadFile();
System.out.println(uploadFiles.size());
}

public static void main(String[] args) {
ReadDB readDB = new ReadDB();
while (true) {
readDB.run();
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}

运行的时候报
com.mysql.jdbc.excepti**.MySQLNonTransientConnectionException: No operati** allowed after connection closed.
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:888)
at com.mysql.jdbc.Connection.checkClosed(Connection.java:1931)
at com.mysql.jdbc.Connection.prepareStatement(Connection.java:4705)
at com.mysql.jdbc.Connection.prepareStatement(Connection.java:4671)
at com.test.util.DBConnection.getPstmt(DBConnection.java:40)
at com.test.dao.UploadFileDao.listUploadFile(UploadFileDao.java:23)
at com.test.util.ReadDB.run(ReadDB.java:11)
at com.test.util.ReadDB.main(ReadDB.java:18)
Exception in thread "main" java.lang.NullPointerException
at com.test.util.DBConnection.getRest(DBConnection.java:50)
at com.test.dao.UploadFileDao.listUploadFile(UploadFileDao.java:24)
at com.test.util.ReadDB.run(ReadDB.java:11)
at com.test.util.ReadDB.main(ReadDB.java:18)
说我的于数据库断开之后还进行数据库操作
但是如果不断开链接会导致内存泄露。。。