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A library used at otto.de to implement Spring Boot based event-sourcing microservices.
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package de.otto.synapse.endpoint;

import com.google.common.collect.ImmutableList;
import de.otto.synapse.endpoint.sender.AbstractMessageSenderEndpoint;
import de.otto.synapse.message.TextMessage;
import jakarta.annotation.Nonnull;
import jakarta.annotation.Nullable;
import org.slf4j.Logger;

import static org.slf4j.LoggerFactory.getLogger;

/**
 * Implementation of the Composite pattern for {@link MessageInterceptor message interceptors} that
 * can be used to build chains of interceptors.
 * 
 *     Just like any other {@link MessageInterceptor}, the InterceptorChain is used to process
 *     a message before it is consumed by {@link de.otto.synapse.consumer.MessageConsumer message consumers}
 *     on the receiver-side, or before it is sent by a {@link AbstractMessageSenderEndpoint message sender}
 *     to the channel infrastructure.
 * 
 * 
 *     The interceptors of the InterceptorChain will be called in order. The result of the first
 *     interceptor is propagated to the next interceptor, and so on.
 * 
 * 
 *     If an interceptor returns null, the chain will return null without further processing of the message.
 * 
 *
 * @see Composite Pattern
 */
public final class InterceptorChain implements MessageInterceptor {

    private static final Logger LOG = getLogger(InterceptorChain.class);

    private final ImmutableList interceptors;

    /**
     * Creates an empty InterceptorChain.
     */
    public InterceptorChain() {
        this.interceptors = ImmutableList.of();
    }

    public InterceptorChain(final ImmutableList messageInterceptors) {
        this.interceptors = messageInterceptors;
    }

    /**
     * Returns the immutable list of registered {@link MessageInterceptor message interceptors}.
     *
     * @return registered message interceptors
     */
    public ImmutableList getInterceptors() {
        return interceptors;
    }

    /**
     * Intercepts a message using all registered interceptors and returns the resulting message.
     * 
     *     The interceptors are called in order. The result of one interceptor is propagated to the
     *     next interceptor in the chain, until the end of the chain is reached, or one interceptor
     *     has returned null.
     * 
     * 
     *     If {@code null} is returned, the message must be dropped by the {@link AbstractMessageEndpoint}.
     * 
     * 
     *     Every interceptor may transform the message, or may take additional actions like, for example,
     *     logging, monitoring or other things.
     * 
     *
     * @param message the message to intercept
     * @return the (possibly modified) message, or null if the message should be dropped.
     */
    @Nullable
    public TextMessage intercept(final @Nonnull TextMessage message) {
        TextMessage interceptedMessage = message;
        for (final MessageInterceptor interceptor : interceptors) {
            if (interceptedMessage == null) {
                break;
            }
            interceptedMessage = interceptor.intercept(interceptedMessage);
        }
        if (interceptedMessage != null) {
            LOG.debug("Intercepted message '{}' converted to {}", message, interceptedMessage);
        } else {
            LOG.debug("Intercepted message '{}' converted to  - dropping message", message);
        }
        return interceptedMessage;
    }

}