Synchronization of non-final field

A warning is showing every time I synchronize on a non-final class field. Here is the code:

public class X  
{  
   private Object o;  

   public void setO(Object o)  
   {  
     this.o = o;  
   }  

   public void x()  
   {  
     synchronized (o) // synchronization on a non-final field  
     {  
     }  
   }  
 } 

so I changed the coding in the following way:

 public class X  
 {  

   private final Object o;       
   public X()
   {  
     o = new Object();  
   }  

   public void x()  
   {  
     synchronized (o)
     {  
     }  
   }  
 }  

I am not sure the above code is the proper way to synchronize on a non-final class field. How can I synchronize a non final field?