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Document relaxed binding from the environment
Update the reference documentation with more details about how relaxed binding works against environment variables. Closes gh-18215
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@ -504,6 +504,7 @@ You can provide default values for your application in `application.properties`
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These default values can then be overridden at runtime with a different file located in one of the custom locations.
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NOTE: If you use environment variables rather than system properties, most operating systems disallow period-separated key names, but you can use underscores instead (for example, `SPRING_CONFIG_NAME` instead of `spring.config.name`).
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See <<boot-features-external-config-relaxed-binding-from-environment-variables>> for details.
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NOTE: If your application runs in a container, then JNDI properties (in `java:comp/env`) or servlet context initialization parameters can be used instead of, or as well as, environment variables or system properties.
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@ -978,9 +979,8 @@ NOTE: The `prefix` value for the annotation _must_ be in kebab case (lowercase a
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| Standard YAML list syntax or comma-separated values
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| Environment Variables
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| Upper case format with underscore as the delimiter.
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`_` should not be used within a property name
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| Numeric values surrounded by underscores, such as `MY_ACME_1_OTHER = my.acme[1].other`
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| Upper case format with underscore as the delimiter (see <<boot-features-external-config-relaxed-binding-from-environment-variables>>).
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| Numeric values surrounded by underscores (see <<boot-features-external-config-relaxed-binding-from-environment-variables>>)`
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| System properties
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| Camel case, kebab case, or underscore notation
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@ -989,6 +989,10 @@ NOTE: The `prefix` value for the annotation _must_ be in kebab case (lowercase a
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TIP: We recommend that, when possible, properties are stored in lower-case kebab format, such as `my.property-name=acme`.
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[[boot-features-external-config-relaxed-binding-maps]]
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===== Binding Maps
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When binding to `Map` properties, if the `key` contains anything other than lowercase alpha-numeric characters or `-`, you need to use the bracket notation so that the original value is preserved.
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If the key is not surrounded by `[]`, any characters that are not alpha-numeric or `-` are removed.
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For example, consider binding the following properties to a `Map`:
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@ -1008,6 +1012,32 @@ The properties above will bind to a `Map` with `/key1`, `/key2` and `key3` as th
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NOTE: For YAML files, the brackets need to be surrounded by quotes for the keys to be parsed properly.
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[[boot-features-external-config-relaxed-binding-from-environment-variables]]
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===== Binding from Environment Variables
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Most operating systems impose strict rules around the names that can be used for environment variables.
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For example, Linux shell variables can contain only letters (`a` to `z` or `A` to `Z`), numbers (`0` to `9`) or the underscore character (`_`).
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By convention, Unix shell variables will also have their names in UPPERCASE.
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Spring Boot's relaxed binding rules are, as much as possible, designed to be compatible with these naming restrictions.
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To convert a property name in the canonical-form to an environment variable name you can follow these rules:
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* Replace dots (`.`) with underscores (`_`).
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* Remove any dashes (`-`).
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* Convert to uppercase.
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For example, the configuration property `spring.main.log-startup-info` would be an environment variable named `SPRING_MAIN_LOGSTARTUPINFO`.
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NOTE: Underscores cannot be used to replace the dashes in property names.
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If you attempt to use `SPRING_MAIN_LOG_STARTUP_INFO` with the example above, no value will be bound.
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Environment variables can also be used when binding to object lists.
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To bind to a `List`, the element number should be surrounded with underscores in the variable name.
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For example, the configuration property `my.acme[0].other` would use an environment variable named `MY_ACME_0_OTHER`.
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[[boot-features-external-config-complex-type-merge]]
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==== Merging Complex Types
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When lists are configured in more than one place, overriding works by replacing the entire list.
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