Spring @Value empty list as default

Question

Is there a way to set an empty list as default value for a property in Spring, something like:

@Value("${my.list.of.strings :" + new ArrayList<>() + "}")
private List<String> myList;

Obviously not new ArrayList, but I need an empty list there instead.

Is `private List myList = new ArrayList<>()` not enough? – Ken Bekov Mar 21 '17 at 07:34 — Ken Bekov, Mar 21 '17 at 07:34

Andrei Roba · Accepted Answer · 2018-03-05T11:29:01.860

32

After taking a look at SpEL specification and combined with @javaguy's answer I came up with this:

@Value("${my.list.of.strings:}#{T(java.util.Collections).emptyList()}")
private List<String> myList;

edited Mar 05 '18 at 11:29

answered Mar 21 '17 at 08:07

Andrei Roba

your answer is helpful but incomplete, since the value annotation points to a property in the first place (which should be initialized when not present - otherwise it leads to an IllegalArgumentException) – Andrei Roba Mar 21 '17 at 08:27
I have used that for zero sized `list` in my project and did not get any `IllegalArgumentException` – Vasu Mar 21 '17 at 08:29
don't know about your code but mine breaks if I specify a property where there's no default value and does not come up in any property file – Andrei Roba Mar 21 '17 at 08:37
Seems it works, but no idea why. I would expect that empty list directly after colon and not after closing parenthesis }. – Ondřej Stašek Mar 22 '18 at 12:59
Whit what logic this work? How $ and # in the same annotation can work like this? It seems to work , but why? could you please explain? @RobyRodriguez – artas Oct 13 '21 at 16:16
It's really much simpler with @sf_ 's answer – Crystark Jun 17 '22 at 13:15

score 20 · Answer 2 · answered Jan 24 '20 at 12:43

20

Actually, in the current Spring versions works just one symbol : with an empty defaultvalue.

The full example that I'm using:

@Value("${my.custom.prop.array:}")
private List<Integer> myList;

To be sure and safer I also adding init to the List variable:

@Value("${my.custom.prop.array:}")
private List<Integer> myList = new ArrayList<>();

answered Jan 24 '20 at 12:43

sf_

1

This solution requires `ConversionServiceFactoryBean` as reported here: https://stackoverflow.com/a/29970335/685806 – Pino Mar 30 '20 at 10:38

Philipp Wirth · Answer 3 · 2018-11-09T16:47:32.067

4

@Value("#{T(java.util.Arrays).asList('${my.list.of.strings:}')}")
private List<String> myList;

works for me, using Spring 5.0.x (gives empty list, if your my.list.of.strings property is not configured in context)

afterwards you can easily do something like

CollectionUtils.isNotEmpty(myList)

edited Nov 09 '18 at 16:47

answered Nov 09 '18 at 16:34

Philipp Wirth

score 2 · Answer 4 · answered Mar 21 '17 at 07:49

2

You can use Collections.emptyList() to populate the empty list object with zero size as shown below:

@Value("#{T(java.util.Collections).emptyList()}")
private List<String> myList;

This will give you a zero sized myList

answered Mar 21 '17 at 07:49

Vasu

what's the significance of 'T' here ? – flash Dec 04 '20 at 19:04
@flash, it is allowing to reinterpret the parameter as a class object. Next the static method `.emptyList()` is called on `java.util.Collections.class` class – msangel Jan 29 '21 at 15:31

4 Answers4