Question

In JDBC literatur I often encounter snippets like

void databaseCall() {
    Connection con = null;
    Statement statement = null        
    try {      
          con = getConnection(...);    
          statement = con.createStatement(...);        
          // do some query
    } catch (SQLException e) {
         // bla bla
    } finally {
         try {con.close();} catch (Exception e1) {}
         try {statement.close();} catch (Exception e1) {}
    }
}

I know it’s best practice to close connections and statements explicity, but, in this case, apparently the resources con and statement will be closed when the method is finished, i.e., when the try block is finished. Is the close statements in the finally block really necessary? Even if we did not release the resources explicitly, wouldn’t they be closed any way when the method is finished?

Answer 1

No, they wouldn’t be closed.

If getConnection() creates a new Connection, then the only thing that will happen at the end of the method is that the Connection could be garbage collected. But the GC won’t call the close() method for you. And, anyway, you want to close as soon as possible.

Most of the time, getConnection() will simply get a connection from a pool of connections, that stay open for a very long time. If you don’t close() the connection, it will not be put back into the pool of available connections, and after a few seconds or minutes, you won’t have any connection available anymore.

The title asks about System.exit(), but the question body doesn’t. If you call System.exit(), then the connection will end up being closed because the database will notice that the communication is broken. But it’s extremely rare for a program to startup, execute a query, and exit. Most of the applications start and stay running for days or even months. So you really want to release the connection once you have finished using it.