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Integration between Micronaut and Kafka Messaging

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/*
 * Copyright 2017-2021 original authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package io.micronaut.configuration.kafka.bind;

import io.micronaut.core.annotation.AnnotationMetadata;
import io.micronaut.core.convert.ArgumentConversionContext;
import io.micronaut.core.convert.ConversionService;
import io.micronaut.messaging.annotation.MessageHeader;
import jakarta.inject.Singleton;
import org.apache.kafka.clients.consumer.ConsumerRecord;
import org.apache.kafka.common.header.Headers;

import java.util.Optional;

/**
 * Binds message headers.
 * @param  The generic type
 */
@Singleton
public class KafkaMessageHeaderBinder implements AnnotatedConsumerRecordBinder {

    @Override
    public Class annotationType() {
        return MessageHeader.class;
    }

    @Override
    public BindingResult bind(ArgumentConversionContext context, ConsumerRecord source) {
        Headers headers = source.headers();
        AnnotationMetadata annotationMetadata = context.getAnnotationMetadata();

        // use deprecated versions as that is what is stored in metadata
        String name = annotationMetadata.stringValue(MessageHeader.class, "name")
                .orElseGet(() -> annotationMetadata.stringValue(MessageHeader.class)
                        .orElse(context.getArgument().getName()));
        Iterable value = headers.headers(name);

        if (value.iterator().hasNext()) {
            Optional converted = ConversionService.SHARED.convert(value, context);
            return () -> converted;
        } else if (context.getArgument().getType() == Optional.class) {
            //noinspection unchecked
            return () -> (Optional) Optional.of(Optional.empty());
        } else {
            //noinspection unchecked
            return BindingResult.EMPTY;
        }
    }
}