Alternative for synchronized block in java

I use following code for guarantee startTime variable set once only:

public class Processor
{
    private Date startTime;

    public void doProcess()
    {
        if(startTime == null)
            synchronized(this)
            {
                  if(startTime == null)
                  {
                     startTime = new Date();
                  }
            }

        // do somethings
    }
}

I will guarantee by this code for variable instantiated once only for any number of invoking process method call.

My question is:

Is there alternative approach for my code be more concise? (for sample remove if & synchronized statements)