详解JDBC数据库链接及相关方法的封装

public class Consql {
 private static Consql cOnsql=null;//单例设计模式
 private Connection cOnn=null;//数据库链接
 private final String url;//数据库url
 private final String username;//数据库用户名
 private final String password;//数据库密码
 //驱动类的加载
 static{//以静态代码块的形式加载驱动类，静态代码块只在类加载的时候执行一次
  try {
   Class.forName("com.mysql.jdbc.Driver");
  } catch (ClassNotFoundException e) {
   e.printStackTrace();
  }
 }
 //构造函数
 private Consql(String url,String username,String password) throws SQLException{
  this.url = url;
  this.username = username;
  this.password = password;
  open();//创建连接
 }
 private Connection open() throws SQLException
 {
  try {//驱动器获取数据库链接
   cOnn=DriverManager.getConnection(url, username, password);
  } catch (SQLException e) {
   // TODO Auto-generated catch block
   //e.printStackTrace();
   throw e;
  }  
  return conn;  
 }
 /**
  * 带限制条件查找
  * @param sql 带占位符？的sql语句
  * @param t 返回相关类型对象的类（T.class）
  * @param params 替换占位符的数据，为动态数组
  * @return ArrayList
  * @throws SQLException 
  */
 public  ArrayList select(String sql,Class t,Object...params) throws SQLException
 {//获取T类所有public方法
  Method[] declaredMethods = t.getDeclaredMethods();
  //创建一个盛放该类型对象集合
  ArrayList arrayList=new ArrayList<>();
  try (PreparedStatement pStatement=conn.prepareStatement(sql);)
  {   
   for(int i=0;i
  * @throws SQLException 
  */
 public  ArrayList select(String sql,Class t) throws SQLException
 {
  Method[] declaredMethods = t.getDeclaredMethods();
  ArrayList arrayList=new ArrayList<>();
  try (PreparedStatement pStatement=conn.prepareStatement(sql);)
  {      
   try(ResultSet rSet=pStatement.executeQuery();) 
   {
    ResultSetMetaData rData=rSet.getMetaData();
    int columnCount = rData.getColumnCount();    
    while (rSet.next()) {
     T a=t.newInstance();
     for(int i=0;i