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annotation-processor.adoc

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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:

compileJava.inputs.files(processResources)

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

Note

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>

Automatic Metadata Generation

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

If the class is also annotated with @ConstructorBinding, a single constructor is expected and one property is created per constructor parameter. 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, @Getter, and @Setter lombok annotations.

Consider the following example:

link:{docs-java}/configurationmetadata/annotationprocessor/automaticmetadatageneration/MyServerProperties.java[role=include]

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.".

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

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:

link:{docs-java}/configurationmetadata/annotationprocessor/automaticmetadatageneration/MyMessagingProperties.java[role=include]

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"
	}
]}
Note
 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:

link:{docs-java}/configurationmetadata/annotationprocessor/automaticmetadatageneration/nestedproperties/MyServerProperties.java[role=include]

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.

Tip
 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.