I've been trying to catch an exception in a Salesforce custom apex class but doesn't seems to work. 

In this case I have a controller constructor that initializes the environment and I am trying to catch exceptions in the constructor, but doesn't work, the exception is not catched.

```
public MyController(){
  try{
    this.myVar = ApexPages.currentPage().getParameters().get('myParam');
    ....

   }
   catch( System.StringException se ){
     ..
   }
   catch( System.NullPointerException ne ){
     ..
   }
   catch( Exception e ){
     ..
   }
  ....
}

```