foo.java

public interface HandlerInterface{
    public void doSmt(...);
}

public class HandlerImpl implements HandlerInterface{
    public void doSmt(args){
        //real Method
        sink();
    }
}

public class HandlerUtil{
    public static HandlerInterface getHandler() {
		return _handler;
    }
    public void setHandler(
		HandlerInterface handler) {

        _handler = handler;
    }
    public static void doSmt(args){
        getHandler().doSmt(args);
    }
}

public class Foo{
    public void processRequest(){
        //...
        args = request.get(...);
        HandlerUtil.doSmt(args);
        //...
    }
}