ConfigurationProperties overwrites with list behaves oddly

[cleiter]

I have an application.yml:

service:
  hosts:
    - a

And the configuration:

@Component
@ConfigurationProperties("service")
class Config {
    private List<String> hosts;

    public List<String> getHosts() { return hosts; }

    public void setHosts(List<String> hosts) { this.hosts = hosts; }
}

When i set the environment variable SERVICE_HOSTS=b,c the result in my Config is actually [a, c] when I expected [b, c]. I tried if this is specific to lists but the same happens with arrays. Same behavior in 1.4.0.RELEASE and current 1.4.1.BUILD-SNAPSHOT.

[cleiter]

I had a closer look, the list gets initially set to [b, c] and then later element 0 is overwritten in AbstractNestablePropertyAccessor#setPropertyValue:378 by calling list.set(index, convertedValue).

[snicoll]

It's another incarnation of #4313 - @philwebb I think the PropertySource check did not take system properties into account maybe?

[philwebb]

Possibly also related to #6620

[snicoll]

@philwebb you closed #6620. How about this one?

[philwebb]

@snicoll I think it's different so can't be closed just yet.

[philwebb]

I have a feeling this is going to be pretty hard for us to fix until we can cleanup how flexible binding works (schedule for Spring Boot 2.0). The problem is PropertySourcesPropertyValues is not able to apply the same rules that it uses to detect collection.

What you effectively end up with are the value:

SERVICE_HOSTS=b,c
service.hosts[0]=a

The first property is applied and converted to a list, then the second one replaces the first item.

The horrible workaround for now is to change your yaml to:

service:
  hosts: a

[cleiter]

Thanks for your response. My workaround is to use a simple string and split it myself.

I understand this is probably not possible to fix without breaking things for someone else. I'd recommend though that there should be a big warning in the docs that elements of lists/arrays get merged element wise and are not overwritten as a whole. Depending on the nature of the lists this effect could be really bad and it will only show up when you actually overwrite list properties, worst case in production.

[philwebb]

@cleiter Good point, I've raised #6911 to address that.