Proper use of close() and = null for Closable objects

Question

Say, I'm using connection to database named con (or socket or anything else which is Closable). What happens after close()? Does con become equal to null or there is still something in it? And what is the difference between con.close() and con = null?

Answer 1

When you call close, the object should free all the resources it uses behind the scenes. In case of an instance of java.sql.Connection, it will one of two things:

• Free any physical connection to the database, thus freeing resources. This happens when you open a database connection manually e.g. DriverManager.getConnection(...).
• Go into a SLEEPING state and wait to be invoked again. This happens when the Connection is handled by a DataSource which is handled by a database connection pool.

Setting the object con = null just assigns null value to a variable, the reference will still be alive until the Garbage Collector decides to remove it. Still, setting the Connection to null doesn't call close method, thus you can have memory leaks.

As a best practice, you should ALWAYS call close method on instances of Closeable or use try-with-resources (available since Java 7) to make sure the resource(s) is(are) always closed.