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Merge pull request #30012 from 62mkv

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* gh-30012:
  Polish "Update placeholder docs to mention env vars and default values"
  Update placeholder docs to mention env vars and default values

Closes gh-30012
This commit is contained in:
Andy Wilkinson 2022-03-03 12:07:56 +00:00
commit 58e2adb895
1 changed files with 6 additions and 3 deletions
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9
spring-boot-project/spring-boot-docs/src/docs/asciidoc/features/external-config.adoc
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@ -443,18 +443,21 @@ For example, if a secret named `db.password` is mounted at location `/run/secret
[[features.external-config.files.property-placeholders]]
==== Property Placeholders
The values in `application.properties` and `application.yml` are filtered through the existing `Environment` when they are used, so you can refer back to previously defined values (for example, from System properties).
The values in `application.properties` and `application.yml` are filtered through the existing `Environment` when they are used, so you can refer back to previously defined values (for example, from System properties or environment variables).
The standard `$\{name}` property-placeholder syntax can be used anywhere within a value.
Property placeholders can also specify a default value using a `:` to separate the default value from the property name, for example `${name:default}`.
For example, the following file will set `app.description` to "`MyApp is a Spring Boot application`":
The use of placeholders with and without defaults is shown in the following example:
[source,yaml,indent=0,subs="verbatim",configblocks]
----
app:
name: "MyApp"
description: "${app.name} is a Spring Boot application"
description: "${app.name} is a Spring Boot application written by ${username:Unknown}"
----
Assuming that the `username` property has not be set elsewhere, `app.description` will have the value `MyApp is a Spring Boot application written by Unknown`.
TIP: You can also use this technique to create "`short`" variants of existing Spring Boot properties.
See the _<<howto#howto.properties-and-configuration.short-command-line-arguments>>_ how-to for details.