JDBC: What is the concrete class that implements the jdbc interfaces

[prasad regula]

In a jdbc code, I am mentioning a sample code below for our reference, we come across
Connections, Statements, PreparedStatement, ResultSet. All these are interfaces. In which part of the jdbc code are we coming across a concrete class. We cant execute a code without a concrete class, by just using interfaces. I am not able to identify what class is the real concrete that is executing this jdbc code.

  

public class FirstExample {
   // JDBC driver name and database URL
   static final String JDBC_DRIVER = "com.mysql.jdbc.Driver";  
   static final String DB_URL = "jdbc:mysql://localhost/EMP";

   //  Database credentials
   static final String USER = "username";
   static final String PASS = "password";
   
   public static void main(String[] args) {
   Connection conn = null;
   Statement stmt = null;
   try{
      //STEP 2: Register JDBC driver
      Class.forName("com.mysql.jdbc.Driver");

      //STEP 3: Open a connection
      System.out.println("Connecting to database...");
      conn = DriverManager.getConnection(DB_URL,USER,PASS);

      //STEP 4: Execute a query
      System.out.println("Creating statement...");
      stmt = conn.createStatement();
      String sql;
      sql = "SELECT id, first, last, age FROM Employees";
      ResultSet rs = stmt.executeQuery(sql);

      //STEP 5: Extract data from result set
      while(rs.next()){
         //Retrieve by column name
         int id  = rs.getInt("id");
         int age = rs.getInt("age");
         String first = rs.getString("first");
         String last = rs.getString("last");

         //Display values
         System.out.print("ID: " + id);
         System.out.print(", Age: " + age);
         System.out.print(", First: " + first);
         System.out.println(", Last: " + last);
      }
      //STEP 6: Clean-up environment
      rs.close();
      stmt.close();
      conn.close();
   }catch(SQLException se){
      //Handle errors for JDBC
      se.printStackTrace();
   }catch(Exception e){
      //Handle errors for Class.forName
      e.printStackTrace();
   }finally{
      //finally block used to close resources
      try{
         if(stmt!=null)
            stmt.close();
      }catch(SQLException se2){
      }// nothing we can do
      try{
         if(conn!=null)
            conn.close();
      }catch(SQLException se){
         se.printStackTrace();
      }//end finally try
   }//end try
   System.out.println("Goodbye!");
}//end main
}//end FirstExample

  

[Bear Bibeault]

That's the whole point of interfaces -- you don't want or need to know!

[Ulf Dittmer]

Somewhere you have a jar file that contains the MySQL JDBC driver - the classes that implement those interfaces are in that jar file.

[Peter Johnson]

Why not add a line such as:

System.out.println("conn is a " + conn.getClass());

That should tell you the concrete class implementing that interface. You can do the same for the other interfaces.

The "magic" behind the way that JDBC works is that you ask the DriverManager for a connection using a URL. The DriverManager gives you a concrete class that implements the Connection interface. You then turn around and ask the concrete class that implements the Connection interface for a Statement, so that concrete class returns another concrete class that implements the Statement interface. Notice that in this whole scenario that you never "new" anything (that is, you never write "xxx = new XXX").