How JDBC implementation works

Question

In JDBC, I see that Connection is an interface, which defines methods for interacting with Database.

I also know that interface contains only abstract methods, and cannot be instantiated

But In JDBC code, how does the following work,

Connection connection = DriverManager.getConnection("URL String");

Statement statement=connection.createStatement();

As per my doubt createStatement() should be a abstract method, i..e, without any body

But every thing works fine... with this

Can anyone explain?

Answer 1

DriverManager.getConnection returns an object which implements the Connection interface - behind the scenes there is a real object.

Answer 2

JDBC is like any driver mechanism - ahead of time the folks and Sun (now Oracle) defined the interface (or contract) that Java would use to interact with a database. It was then the responibility of the database vendors to provide implementations of that interface in order for their particular database to be used with Java.

The take away here is that the JDBC API defines the standard interface through which Java will interact with a database, the obvious benefit being that, providing your code only uses the JDBC API, it will be reasonably uniform regardless of the database you use, meaning you can therefore swap one implementation with another (although it's not always that simple in practice).

With regards to how implementations of the JDBC API are registered with the java.sql.DriverManager, what used to happen was that your code would need to explicitly load the class(es) implementing java.sql.Driver; the act of loading the class caused it to be registered with the java.sql.DriverManager. This mechanism has now be replaced with the service provider but the end result is still the same:

One or more implementations of java.sql.Driver are registered with the java.sql.DriverManager.

If you then look at the source code for java.sql.DriverManager.getConnection(String) you will see that it simply loops through the registered drivers until it finds one that accepts the connection url at which point the java.sql.DriverManager calls the java.sql.Driver.connect(String, Properties) method, returning a specific implementation of java.sql.Connection.