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Polish "Update placeholder docs to mention env vars and default values"
See gh-30012
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@ -444,17 +444,20 @@ For example, if a secret named `db.password` is mounted at location `/run/secret
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[[features.external-config.files.property-placeholders]]
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==== Property Placeholders
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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).
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The standard `$\{name:default}` property-placeholder syntax can be used anywhere within a value.
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The standard `$\{name}` property-placeholder syntax can be used anywhere within a value.
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Property placeholders can also specify a default value using a `:` to separate the default value from the property name, for example `${name:default}`.
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For example, the following file will set `app.description` to "`MyApp is a Spring Boot application written by john.doe`":
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The use of placeholders with and without defaults is shown in the following example:
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[source,yaml,indent=0,subs="verbatim",configblocks]
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----
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app:
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name: "MyApp"
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description: "${app.name} is a Spring Boot application written by ${USERNAME:john.doe}"
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description: "${app.name} is a Spring Boot application written by ${username:Unknown}"
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----
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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`.
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TIP: You can also use this technique to create "`short`" variants of existing Spring Boot properties.
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See the _<<howto#howto.properties-and-configuration.short-command-line-arguments>>_ how-to for details.
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