Java Style Guide: Constants example (Joiner vs Logger)

I've seen many discussions about if a `static final Logger` instance should be considered a constant or not (i.e. uppercase letters), and there are references to this style guide in which is suggested to use the logger as a non-constant value:

```
static final Logger logger = Logger.getLogger(MyClass.getName());
```

However, in most of the cases (at least all that I know), the `Logger` instances are immutables because there are no methods to change the internal status or to retrieve a mutable object, which seems to be the same case of the `Joiner`:
```
static final Joiner COMMA_JOINER = Joiner.on(','); // because Joiner is immutable
```

What is the fundamental difference between a `Logger` and a `Joiner` to differentiate them as a constant or non-constant?

What would be the case of a `Gson` instance if declared as a `static final`?

Thanks in advance