[appendix]
[[configuration-metadata]]
== Configuration meta-data
Spring Boot jars are shipped with meta-data files that provide details of all supported
configuration properties. The files are designed to allow IDE developers to offer
contextual help and "`code completion`" as users are working with `application.properties`
or `application.yml` files.

The majority of the meta-data file is generated automatically at compile time by
processing all items annotated with `@ConfigurationProperties`.



[[configuration-metadata-format]]
=== Meta-data format
Configuration meta-data files are located inside jars under
`META-INF/spring-configuration-metadata.json` They use a simple JSON format with items
categorized under either "`groups`" or "`properties`":

[source,json,indent=0]
----
	{"groups": [
		{
			"name": "server",
			"type": "org.springframework.boot.autoconfigure.web.ServerProperties",
			"sourceType": "org.springframework.boot.autoconfigure.web.ServerProperties"
		}
		...
	],"properties": [
		{
			"name": "server.port",
			"type": "java.lang.Integer",
			"sourceType": "org.springframework.boot.autoconfigure.web.ServerProperties"
		},
		{
			"name": "server.servlet-path",
			"type": "java.lang.String",
			"sourceType": "org.springframework.boot.autoconfigure.web.ServerProperties"
			"defaultValue": "/"
		}
		...
	]}
----

Each "`property`" is a configuration item that the user specifies with a given value.
For example `server.port` and `server.servlet-path` might be specified in
`application.properties` as follows:

[source,properties,indent=0]
----
	server.port=9090
	server.servlet-path=/home
----

The "`groups`" are higher level items that don't themselves specify a value, but instead
provide a contextual grouping for properties. For example the `server.port` and
`server.servlet-path` properties are part of the `server` group.

NOTE: It is not required that every "`property`" has a "`group`", some properties might
just exist in their own right.



[[configuration-metadata-group-attributes]]
==== Group Attributes
The JSON object contained in the `groups` array can contain the following attributes:

[cols="1,1,4"]
|===
|Name | Type |Purpose

|`name`
| String
| The full name of the group. This attribute is mandatory.

|`type`
| String
| The class name of the data type of the group. For example, if the group was based
  on a class annotated with `@ConfigurationProperties` the attribute would contain the
  fully qualified name of that class. If it was based on a `@Bean` method, it would be
  the return type of that method. The attribute may be omitted if the type is not known.

|`description`
| String
| A short description of the group that can be displayed to users. May be omitted if no
  description is available. It is recommended that descriptions are a short paragraphs,
  with the first line providing a concise summary. The last line in the description should
  end with a period (`.`).

|`sourceType`
| String
| The class name of the source that contributed this group. For example, if the group
  was based on a `@Bean` method annotated with `@ConfigurationProperties` this attribute
  would contain the fully qualified name of the `@Configuration` class containing the
  method. The attribute may be omitted if the source type is not known.

|`sourceMethod`
| String
| The full name of the method (include parenthesis and argument types) that contributed
  this group. For example, the name of a `@ConfigurationProperties` annotated `@Bean`
  method.  May be omitted if the source method is not known.
|===



[[configuration-metadata-property-attributes]]
==== Property Attributes
The JSON object contained in the `properties` array can contain the following attributes:

[cols="1,1,4"]
|===
|Name | Type |Purpose

|`name`
| String
| The full name of the property. Names are in lowercase dashed form (e.g.
  `server.servlet-path`). This attribute is mandatory.

|`type`
| String
| The class name of the data type of the property. For example, `java.lang.String`. This
  attribute can be used to guide the user as to the types of values that they can enter.
  For consistency, the type of a primitive is specified using its wrapper counterpart,
  i.e. `boolean` becomes `java.lang.Boolean`. Collection types are harmonized to their
  interface counterpart and define the actual generic types, i.e.
  `java.util.HashMap<java.lang.String,java.lang.Integer>` becomes
  `java.util.Map<java.lang.String,java.lang.Integer>`. Note that this class may be a
  complex type that gets converted from a String as values are bound. May be omitted if
  the type is not known.

|`description`
| String
| A short description of the group that can be displayed to users. May be omitted if no
  description is available. It is recommended that descriptions are a short paragraphs,
  with the first line providing a concise summary. The last line in the description should
  end with a period (`.`).

|`sourceType`
| String
| The class name of the source that contributed this property. For example, if the
  property was from a class annotated with `@ConfigurationProperties` this attribute
  would contain the fully qualified name of that class. May be omitted if the source type
  is not known.

|`defaultValue`
| Object
| The default value which will be used if the property is not specified. Can also be an
  array of value(s) if the type of the property is an array. May be omitted if the default
  value is not known.

|`deprecated`
| boolean
| Specify if the property is deprecated. May be omitted if the field is not deprecated
  or if that information is not known.
|===



[[configuration-metadata-repeated-items]]
==== Repeated meta-data items
It is perfectly acceptable for "`property`" and "`group`" objects with the same name to
appear multiple times within a meta-data file. For example, Spring Boot binds
`spring.datasource` properties to Hikari, Tomcat and DBCP classes, with each potentially
offering overlap of property names. Consumers of meta-data should take care to ensure
that they support such scenarios.



[[configuration-metadata-annotation-processor]]
=== Generating your own meta-data using the annotation processor
You can easily generate your own configuration meta-data file from items annotated with
`@ConfigurationProperties` by using the `spring-boot-configuration-processor` jar.
The jar includes a Java annotation processor which is invoked as your project is
compiled. To use the processor, simply include `spring-boot-configuration-processor` as
an optional dependency, for example with Maven you would add:

[source,xml,indent=0,subs="verbatim,quotes,attributes"]
----
	<dependency>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-configuration-processor</artifactId>
		<optional>true</optional>
	</dependency>
----

The annotation will pickup both classes and methods that are annotated with
`@ConfigurationProperties`. The Javadoc for field values within configuration classes
will be used to populate the `description` attribute.

NOTE: You should only use simple text with `@ConfigurationProperties` field Javadoc since
they are not processed before being added to the JSON.



[[configuration-metadata-nested-properties]]
==== Nested properties
The annotation processor will automatically consider inner classes as nested properties.
For example, the following class:

[source,java,indent=0,subs="verbatim,quotes,attributes"]
----
	@ConfigurationProperties(prefix="server")
	public class ServerProperties {

		private String name;

		private Host host;

		// ... getter and setters

		private static class Host {

			private String ip;

			private int port;

			// ... getter and setters

		}

	}
----

Will produce meta-data information for `server.name`, `server.host.ip` and
`server.host.port` properties. You can use the `@NestedConfigurationProperty`
annotation on a field to indicate that a regular (non-inner) class should be treated as
if it were nested.



[[configuration-metadata-additional-metadata]]
==== Adding additional meta-data
Spring Boot's configuration file handling is quite flexible; and it often the case that
properties may exist that are not bound to a `@ConfigurationProperties` bean. To support
such cases, the annotation processor will automatically merge items from
`META-INF/additional-spring-configuration-metadata.json` into the main meta-data file.

The format of the `additional-spring-configuration-metadata.json` file is exactly the same
as the regular `spring-configuration-metadata.json`. The additional properties file is
optional, if you don't have any additional properties, simply don't add it.
