Detailed explanation of JDBC database link and related method encapsulation

public class Consql {
 private static Consql consql=null;//Singleton design pattern private Connection conn=null;//Database link private final String url;//Database url
 private final String username;//Database username private final String password;//Database password //Driver class loading static{//Load the driver class in the form of a static code block. The static code block is only executed once when the class is loaded try {
   Class.forName("com.mysql.jdbc.Driver");
  } catch (ClassNotFoundException e) {
   e.printStackTrace();
  }
 }
 //Constructor private Consql(String url,String username,String password) throws SQLException{
  this.url = url;
  this.username = username;
  this.password = password;
  open(); //Create a connection}
 private Connection open() throws SQLException
 {
  try {//Driver obtains database link conn=DriverManager.getConnection(url, username, password);
  } catch (SQLException e) {
   // TODO Auto-generated catch block
   //e.printStackTrace();
   throw e;
  }  
  return conn;  
 }
 /**
  * Search with restriction conditions * @param sql with placeholders? sql statement * @param t returns the class of the related type object (T.class)
  * @param params replaces the placeholder data, which is a dynamic array * @return ArrayList<T>
  * @throws SQLException 
  */
 public <T> ArrayList<T> select(String sql,Class<T> t,Object...params) throws SQLException
 {//Get all public methods of class T Method[] declaredMethods = t.getDeclaredMethods();
  //Create an ArrayList<T> collection to hold objects of this type arrayList=new ArrayList<>();
  try (PreparedStatement pStatement=conn.prepareStatement(sql);)
  {   
   for(int i=0;i<params.length;i++)
   {
    pStatement.setObject(i+1, params[i]);
   }   
   try(ResultSet rSet=pStatement.executeQuery();) 
   {
    ResultSetMetaData rData=rSet.getMetaData();
    //Get the number of columns in the query result table int columnCount = rData.getColumnCount();    
    while (rSet.next()) {
     T a=t.newInstance(); //Create a generic class instance for(int i=0;i<columnCount;i++)
     {//Get the set method in the square array. This causes limitations. Only the database table column name can be consistent with the object name, and only the set method String aString="set"+rData.getColumnName(i+1);
      for (Method method : declaredMethods) {
       if(method.getParameterCount()==1&&method.getReturnType().toString().equals("void")&&method.getName().equalsIgnoreCase(aString))
       {//There is a problem here. The first two judgment conditions are basically useless. The main reason is that I didn’t want to use the string method above to determine whether to call the method of the parameter method.setAccessible(true);
        //Call this method using reflection method.invoke(a, rSet.getObject(i+1));
        break;
       }
      }
     }
     arrayList.add(a);
    }
   } catch (InstantiationException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
   } catch (IllegalAccessException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
   } catch (IllegalArgumentException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
   } catch (InvocationTargetException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
   } 
  } catch (SQLException e) {
   // TODO Auto-generated catch block
   throw e;
  }
  return arrayList;  
 }
 /**
  *Data insertion* @param sql with placeholder? SQL statement * @param params replaces the placeholder data, dynamic array * @throws SQLException
  */
 public void insert(String sql,Object...params) throws SQLException
 {
  try(PreparedStatement pStatement=conn.prepareStatement(sql);) {
   
   for(int i=0;i<params.length;i++)
   {
    pStatement.setObject(i+1, params[i]);
   }
   pStatement.executeUpdate();
  } catch (SQLException e) {
   // TODO Auto-generated catch block
   throw e;
  }
 }
 /**
  *Data update* @param sql with placeholder? SQL statement * @param params replaces the placeholder data, dynamic array * @throws SQLException
  */
 public void update(String sql,Object...params) throws SQLException
 {
  try(PreparedStatement pStatement=conn.prepareStatement(sql);) {
   
   for(int i=0;i<params.length;i++)
   {
    pStatement.setObject(i+1, params[i]);
   }
   pStatement.executeUpdate();
  } catch (SQLException e) {
   // TODO Auto-generated catch block
   throw e;
  }
 }
 /**
  * Delete with restriction conditions * @param sql with placeholder? SQL statement * @param params replaces the placeholder data, dynamic array * @throws SQLException
  */
 public void delete(String sql,Object...params) throws SQLException
 {
  try(PreparedStatement pStatement=conn.prepareStatement(sql);) {
   
   for(int i=0;i<params.length;i++)
   {
    pStatement.setObject(i+1, params[i]);
   }
   pStatement.executeUpdate();
  } catch (SQLException e) {
   // TODO Auto-generated catch block
   throw e;
  }
 }
 /**
  * Delete all, without restriction * @param sql
  * @throws SQLException
  */
 public void deleteall(String sql) throws SQLException
 {
  try(PreparedStatement pStatement=conn.prepareStatement(sql);) {      
   pStatement.executeUpdate();
  } catch (SQLException e) {
   // TODO Auto-generated catch block
   throw e;
  }
 }
 /**
  * Unrestricted search * @param sql 
  * @param t generic class T.class
  * @return ArrayList<T>
  * @throws SQLException 
  */
 public <T> ArrayList<T> select(String sql,Class<T> t) throws SQLException
 {
  Method[] declaredMethods = t.getDeclaredMethods();
  ArrayList<T> 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<columnCount;i++)
     {
      String aString="set"+rData.getColumnName(i+1);
      for (Method method : declaredMethods) {
       if (method.getName().equalsIgnoreCase(aString))
       {
        method.setAccessible(true);
        method.invoke(a, rSet.getObject(i+1));
        break;
       }
      }
     }
     arrayList.add(a);
    }
   } catch (InstantiationException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
   } catch (IllegalAccessException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
   } catch (IllegalArgumentException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
   } catch (InvocationTargetException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
   } 
  } catch (SQLException e) {
   // TODO Auto-generated catch block
   throw e;
  }
  return arrayList; 
 }
 /**
  * Return the number of rows in the table * @param tableName database table name * @return number of rows * @throws SQLException
  */
 public int count(String tableName) throws SQLException
 {
  String sql="select count(*) from "+tableName;
  try(PreparedStatement pStatement=conn.prepareStatement(sql);
    ResultSet rsSet = pStatement.executeQuery(); )
  {  
   if(rsSet.next())
   {
    return rsSet.getInt(1);
   }   
  } catch (SQLException e) {
   // TODO Auto-generated catch block
   throw e;
  }
  return 0;
 }
 /**
  * Determine whether the data exists * @param sql contains placeholders? SQL statement * @param params replaces the placeholder data, dynamic array * @return boolean
  * @throws SQLException
  */
 public boolean isExist(String sql,Object...params) throws SQLException
 {  
  try(PreparedStatement pStatement=conn.prepareStatement(sql);)
  {
   for(int i=0;i<params.length;i++)
   {
    pStatement.setObject(i+1, params[i]);
   }
   try(ResultSet rsSet=pStatement.executeQuery();) {
    if(rsSet.next())
    {
     return true;
    }
   finally
    
   }
  } catch (SQLException e) {
   // TODO Auto-generated catch block
   throw e;
  }
  return false;  
 }
 /**
  * Create an instance * @param url database url
  * @param username username * @param password password * @return consql object * @throws SQLException
  */
 public static Consql getnewInstance(String url,String username,String password) throws SQLException
 {
  if(consql==null)
   consql = new Consql (url, username, password);
  return consql;  
 }
 //Garbage collection, it seems that it cannot achieve the effect of the destructor protected void finalize() throws Throwable
 {
  if(conn!=null)
  {
   conn.close();  
  }
  super.finalize();
 }
}