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MetadataExtractor

Responders must interpret metadata. Composite metadata allows independently formatted metadata values (e.g. for routing, security, tracing) each with its own mime type. Applications need a way to configure metadata mime types to support, and a way to access extracted values.

MetadataExtractor is a contract to take serialized metadata and return decoded name-value pairs that can then be accessed like headers by name, for example via @Header in annotated handler methods.

DefaultMetadataExtractor can be given Decoder instances to decode metadata. Out of the box it has built-in support for "message/x.rsocket.routing.v0" which it decodes to String and saves under the "route" key. For any other mime type you’ll need to provide a Decoder and register the mime type as follows:

Java

DefaultMetadataExtractor extractor = new DefaultMetadataExtractor(metadataDecoders);
extractor.metadataToExtract(fooMimeType, Foo.class, "foo");

Kotlin

import org.springframework.messaging.rsocket.metadataToExtract

val extractor = DefaultMetadataExtractor(metadataDecoders)
extractor.metadataToExtract<Foo>(fooMimeType, "foo")

Composite metadata works well to combine independent metadata values. However the requester might not support composite metadata, or may choose not to use it. For this, DefaultMetadataExtractor may needs custom logic to map the decoded value to the output map. Here is an example where JSON is used for metadata:

Java

DefaultMetadataExtractor extractor = new DefaultMetadataExtractor(metadataDecoders);
extractor.metadataToExtract(
	MimeType.valueOf("application/vnd.myapp.metadata+json"),
	new ParameterizedTypeReference<Map<String,String>>() {},
	(jsonMap, outputMap) -> {
		outputMap.putAll(jsonMap);
	});

Kotlin

import org.springframework.messaging.rsocket.metadataToExtract

val extractor = DefaultMetadataExtractor(metadataDecoders)
extractor.metadataToExtract<Map<String, String>>(MimeType.valueOf("application/vnd.myapp.metadata+json")) { jsonMap, outputMap ->
	outputMap.putAll(jsonMap)
}

When configuring MetadataExtractor through RSocketStrategies, you can let RSocketStrategies.Builder create the extractor with the configured decoders, and simply use a callback to customize registrations as follows:

Java

RSocketStrategies strategies = RSocketStrategies.builder()
	.metadataExtractorRegistry(registry -> {
		registry.metadataToExtract(fooMimeType, Foo.class, "foo");
		// ...
	})
	.build();

Kotlin

import org.springframework.messaging.rsocket.metadataToExtract

val strategies = RSocketStrategies.builder()
		.metadataExtractorRegistry { registry: MetadataExtractorRegistry ->
			registry.metadataToExtract<Foo>(fooMimeType, "foo")
			// ...
		}
		.build()