Update the DeadLetterEventHandlerInvoker

Update the DeadLetterEventHandlerInvoker to contain a delegate
EventHandlerInvoker. It uses the delegate to delegate the event handling
 process too. This allows it to capture exceptions, making it the place
 to store dead letter in the queue too. As an EventHandlerInvoker
 implementation, it will delegate all other invoker calls to the
 delegate. Lastly, add a builder for configuration and a test case to
 validate it

#2021

messaging/src/main/java/org/axonframework/eventhandling/deadletter/DeadLetteringEventHandlerInvoker.java

@@ -0,0 +1,173 @@
+/*
+ * Copyright (c) 2010-2021. Axon Framework
+ *
+ * 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
+ *
+ *    http://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 org.axonframework.eventhandling.deadletter;
+
+import org.axonframework.common.AxonConfigurationException;
+import org.axonframework.eventhandling.EventHandlerInvoker;
+import org.axonframework.eventhandling.EventMessage;
+import org.axonframework.eventhandling.Segment;
+import org.axonframework.messaging.deadletter.DeadLetterQueue;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.lang.invoke.MethodHandles;
+
+import static org.axonframework.common.BuilderUtils.assertNonEmpty;
+import static org.axonframework.common.BuilderUtils.assertNonNull;
+
+/**
+ * @author Steven van Beelen
+ * @since 4.6.0
+ */
+public class DeadLetteringEventHandlerInvoker implements EventHandlerInvoker {
+
+    private static final Logger logger = LoggerFactory.getLogger(MethodHandles.lookup().lookupClass());
+
+    private final EventHandlerInvoker delegate;
+    private final DeadLetterQueue<EventMessage<?>> queue;
+    private final String processingGroup;
+
+    /**
+     * @param builder
+     */
+    protected DeadLetteringEventHandlerInvoker(Builder builder) {
+        builder.validate();
+        this.delegate = builder.delegate;
+        this.queue = builder.queue;
+        this.processingGroup = builder.processingGroup;
+    }
+
+    /**
+     * @return
+     */
+    public static Builder builder() {
+        return new Builder();
+    }
+
+    @Override
+    public boolean canHandle(EventMessage<?> eventMessage, Segment segment) {
+        return delegate.canHandle(eventMessage, segment);
+    }
+
+    @Override
+    public boolean canHandleType(Class<?> payloadType) {
+        return delegate.canHandleType(payloadType);
+    }
+
+    @Override
+    public void handle(EventMessage<?> message, Segment segment) throws Exception {
+        String sequenceIdentifier = Integer.toString(delegate.sequenceIdentifier(message).hashCode());
+        if (queue.enqueueIfPresent(sequenceIdentifier, processingGroup, message)) {
+            logger.info(
+                    "Event [{}] is added to the dead-letter queue since its processing id [{}-{}] was already present.",
+                    message, sequenceIdentifier, processingGroup
+            );
+        } else {
+            logger.debug("Event [{}] with processing id [{}-{}] is not present in the dead-letter queue present."
+                                 + "Handle operation is delegated to the wrapped EventHandlerInvoker.",
+                         message, sequenceIdentifier, processingGroup);
+            try {
+                // TODO: 03-12-21 how to deal with the delegates ListenerInvocationErrorHandler in this case?
+                //  It is mandatory to rethrow the exception, as otherwise the message isn't enqueued.
+                delegate.handle(message, segment);
+            } catch (Exception e) {
+                queue.enqueue(sequenceIdentifier, processingGroup, message, e);
+            }
+        }
+    }
+
+    @Override
+    public boolean supportsReset() {
+        return delegate.supportsReset();
+    }
+
+    @Override
+    public void performReset() {
+        delegate.performReset();
+    }
+
+    @Override
+    public <R> void performReset(R resetContext) {
+        delegate.performReset(resetContext);
+    }
+
+    @Override
+    public Object sequenceIdentifier(EventMessage<?> event) {
+        return delegate.sequenceIdentifier(event);
+    }
+
+    /**
+     *
+     */
+    public static class Builder {
+
+        private EventHandlerInvoker delegate;
+        private DeadLetterQueue<EventMessage<?>> queue;
+        private String processingGroup;
+
+        /**
+         * @param delegate
+         * @return
+         */
+        public Builder delegate(EventHandlerInvoker delegate) {
+            assertNonNull(delegate, "The delegate EventHandlerInvoker may not be null");
+            this.delegate = delegate;
+            return this;
+        }
+
+        /**
+         * @param queue
+         * @return
+         */
+        public Builder queue(DeadLetterQueue<EventMessage<?>> queue) {
+            assertNonNull(queue, "The DeadLetterQueue may not be null");
+            this.queue = queue;
+            return this;
+        }
+
+        /**
+         * Sets the name of this invoker.
+         *
+         * @param processingGroup the name of this {@link EventHandlerInvoker}
+         * @return the current Builder instance, for fluent interfacing
+         */
+        public Builder processingGroup(String processingGroup) {
+            assertNonEmpty(processingGroup, "The processing group may not be null or empty");
+            this.processingGroup = processingGroup;
+            return this;
+        }
+
+        /**
+         * @return
+         */
+        public DeadLetteringEventHandlerInvoker build() {
+            return new DeadLetteringEventHandlerInvoker(this);
+        }
+
+        /**
+         * Validates whether the fields contained in this Builder are set accordingly.
+         *
+         * @throws AxonConfigurationException if one field is asserted to be incorrect according to the Builder's
+         *                                    specifications
+         */
+        protected void validate() {
+            assertNonNull(delegate, "The delegate EventHandlerInvoker is a hard requirement and should be provided");
+            assertNonNull(queue, "The DeadLetterQueue is a hard requirement and should be provided");
+            assertNonNull(processingGroup, "The processing group is a hard requirement and should be provided");
+        }
+    }
+}