SpringBoot Improve property placeholder documentation to mention environment variables and default values

I've figured that way too often I try to find an example in official reference documentation, and invariably end up on this SO question: https://stackoverflow.com/questions/23027315/does-application-yml-support-environment-variables

given it's popularity, I think this deserves to be added in the documentation

Comment From: 62mkv

@wilkinsona I've tried..

Comment From: 62mkv

speaking about "explaining the name:default syntax" I would love to understand it in more details.. but can not seem to find where is it defined

Comment From: wilkinsona

It's mentioned, very briefly, in the Spring Framework documentation that I linked to above. In those docs it says:

It is possible to provide a default value as following:

```java @Component public class MovieRecommender {

private final String catalog;

public MovieRecommender(@Value("${catalog.name:defaultCatalog}") String catalog) {
    this.catalog = catalog;
}

}

It's also described in the javadoc of PlaceholderConfigurerSupport where it says the following:

Default property values can be defined […] on a property-by-property basis using the value separator which is ":" by default and customizable via setValueSeparator(String).

Comment From: 62mkv

Indeed.. still am not sure I understand how could we provide more details in this section, other than mentioning the syntax and providing an example. any suggestions?

Comment From: wilkinsona

I think that’s all that we should do in Boot’s docs.

Comment From: 62mkv

I'd think that changes required were addressed, or I might be missing something?

Comment From: wilkinsona

@62mkv Thanks very much for making your first contribution to Spring Boot.

Comment From: 62mkv

thanks @wilkinsona !!