Generating Your Own Metadata by Using the Annotation Processor

You can easily generate your own configuration metadata 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.

Configuring the Annotation Processor

To use the processor, include a dependency on spring-boot-configuration-processor.

With Maven the dependency should be declared as optional, as shown in the following example:

<dependency>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-configuration-processor</artifactId>
	<optional>true</optional>
</dependency>

With Gradle, the dependency should be declared in the annotationProcessor configuration, as shown in the following example:

dependencies {
	annotationProcessor "org.springframework.boot:spring-boot-configuration-processor"
}

If you are using an additional-spring-configuration-metadata.json file, the compileJava task should be configured to depend on the processResources task, as shown in the following example:

tasks.named('compileJava') {
	inputs.files(tasks.named('processResources'))
}

This dependency ensures that the additional metadata is available when the annotation processor runs during compilation.

If you are using AspectJ in your project, you need to make sure that the annotation processor runs only once. There are several ways to do this. With Maven, you can configure the maven-apt-plugin explicitly and add the dependency to the annotation processor only there. You could also let the AspectJ plugin run all the processing and disable annotation processing in the maven-compiler-plugin configuration, as follows:

<plugin>
	<groupId>org.apache.maven.plugins</groupId>
	<artifactId>maven-compiler-plugin</artifactId>
	<configuration>
		<proc>none</proc>
	</configuration>
</plugin>

If you are using Lombok in your project, you need to make sure that its annotation processor runs before spring-boot-configuration-processor. To do so with Maven, you can list the annotation processors in the right order using the annotationProcessors attribute of the Maven compiler plugin. If you are not using this attribute, and annotation processors are picked up by the dependencies available on the classpath, make sure that the lombok dependency is defined before the spring-boot-configuration-processor dependency.

Automatic Metadata Generation

The processor picks up both classes and methods that are annotated with @ConfigurationProperties.

If the class has a single parameterized constructor, one property is created per constructor parameter, unless the constructor is annotated with @Autowired. If the class has a constructor explicitly annotated with @ConstructorBinding, one property is created per constructor parameter for that constructor. Otherwise, properties are discovered through the presence of standard getters and setters with special handling for collection and map types (that is detected even if only a getter is present). The annotation processor also supports the use of the @Data, @Value, @Getter, and @Setter lombok annotations.

Consider the following example:

import org.springframework.boot.context.properties.ConfigurationProperties;

@ConfigurationProperties(prefix = "my.server")
public class MyServerProperties {

	/**
	 * Name of the server.
	 */
	private String name;

	/**
	 * IP address to listen to.
	 */
	private String ip = "127.0.0.1";

	/**
	 * Port to listener to.
	 */
	private int port = 9797;

	// getters/setters ...
	public String getName() {
		return this.name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public String getIp() {
		return this.ip;
	}

	public void setIp(String ip) {
		this.ip = ip;
	}

	public int getPort() {
		return this.port;
	}

	public void setPort(int port) {
		this.port = port;
	}
	// fold:off

}

This exposes three properties where my.server.name has no default and my.server.ip and my.server.port defaults to "127.0.0.1" and 9797 respectively. The Javadoc on fields is used to populate the description attribute. For instance, the description of my.server.ip is "IP address to listen to.".

You should only use plain text with @ConfigurationProperties field Javadoc, since they are not processed before being added to the JSON.

If you use @ConfigurationProperties with record class then record components' descriptions should be provided via class-level Javadoc tag @param (there are no explicit instance fields in record classes to put regular field-level Javadocs on).

The annotation processor applies a number of heuristics to extract the default value from the source model. Default values have to be provided statically. In particular, do not refer to a constant defined in another class. Also, the annotation processor cannot auto-detect default values for Enums and Collectionss.

For cases where the default value could not be detected, manual metadata should be provided. Consider the following example:

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

import org.springframework.boot.context.properties.ConfigurationProperties;

@ConfigurationProperties(prefix = "my.messaging")
public class MyMessagingProperties {

	private List<String> addresses = new ArrayList<>(Arrays.asList("a", "b"));

	private ContainerType containerType = ContainerType.SIMPLE;

	// getters/setters ...

	public List<String> getAddresses() {
		return this.addresses;
	}

	public void setAddresses(List<String> addresses) {
		this.addresses = addresses;
	}

	public ContainerType getContainerType() {
		return this.containerType;
	}

	public void setContainerType(ContainerType containerType) {
		this.containerType = containerType;
	}

	public enum ContainerType {

		SIMPLE, DIRECT

	}

}

In order to document default values for properties in the class above, you could add the following content to the manual metadata of the module:

{"properties": [
	{
		"name": "my.messaging.addresses",
		"defaultValue": ["a", "b"]
	},
	{
		"name": "my.messaging.container-type",
		"defaultValue": "simple"
	}
]}

Only the name of the property is required to document additional metadata for existing properties.

Nested Properties

The annotation processor automatically considers inner classes as nested properties. Rather than documenting the ip and port at the root of the namespace, we could create a sub-namespace for it. Consider the updated example:

import org.springframework.boot.context.properties.ConfigurationProperties;

@ConfigurationProperties(prefix = "my.server")
public class MyServerProperties {

	private String name;

	private Host host;

	// getters/setters ...

	public String getName() {
		return this.name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public Host getHost() {
		return this.host;
	}

	public void setHost(Host host) {
		this.host = host;
	}

	public static class Host {

		private String ip;

		private int port;

		// getters/setters ...
		public String getIp() {
			return this.ip;
		}

		public void setIp(String ip) {
			this.ip = ip;
		}

		public int getPort() {
			return this.port;
		}

		public void setPort(int port) {
			this.port = port;
		}
		// @fold:off // getters/setters ...

	}

}

The preceding example produces metadata information for my.server.name, my.server.host.ip, and my.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.

This has no effect on collections and maps, as those types are automatically identified, and a single metadata property is generated for each of them.

Adding Additional Metadata

Spring Boot’s configuration file handling is quite flexible, and it is often the case that properties may exist that are not bound to a @ConfigurationProperties bean. You may also need to tune some attributes of an existing key. To support such cases and let you provide custom "hints", the annotation processor automatically merges items from META-INF/additional-spring-configuration-metadata.json into the main metadata file.

If you refer to a property that has been detected automatically, the description, default value, and deprecation information are overridden, if specified. If the manual property declaration is not identified in the current module, it is added as a new property.

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 do not have any additional properties, do not add the file.