BAEL-2435 Update Aggregate in regards to latest axon version

-Replace old import statements for the new locations
-Mark the aggregate as @Aggregate to make it autowired through the
spring boot starter
-Move all command handlers/decision making functions to the top of the
file
-Rename the command handler to `handle` as a best practice for command
handlers
-Use @EventSourcingHandler i.o. @EventHandler as a best practice for
event handlers in Aggregates, as those only are used to 'source' the
aggregate
-Update test case according to latest standards

axon/src/main/java/com/baeldung/axon/aggregates/MessagesAggregate.java

@@ -1,16 +1,18 @@
 package com.baeldung.axon.aggregates;
 
+import static org.axonframework.modelling.command.AggregateLifecycle.apply;
+
+import org.axonframework.commandhandling.CommandHandler;
+import org.axonframework.eventsourcing.EventSourcingHandler;
+import org.axonframework.modelling.command.AggregateIdentifier;
+import org.axonframework.spring.stereotype.Aggregate;
+
 import com.baeldung.axon.commands.CreateMessageCommand;
 import com.baeldung.axon.commands.MarkReadMessageCommand;
 import com.baeldung.axon.events.MessageCreatedEvent;
 import com.baeldung.axon.events.MessageReadEvent;
-import org.axonframework.commandhandling.CommandHandler;
-import org.axonframework.commandhandling.model.AggregateIdentifier;
-import org.axonframework.eventhandling.EventHandler;
-
-import static org.axonframework.commandhandling.model.AggregateLifecycle.apply;
-
 
+@Aggregate
 public class MessagesAggregate {
 
     @AggregateIdentifier
@@ -24,13 +26,13 @@ public class MessagesAggregate {
         apply(new MessageCreatedEvent(command.getId(), command.getText()));
     }
 
-    @EventHandler
+    @CommandHandler
+    public void handle(MarkReadMessageCommand command) {
+        apply(new MessageReadEvent(id));
+    }
+
+    @EventSourcingHandler
     public void on(MessageCreatedEvent event) {
         this.id = event.getId();
     }
-
-    @CommandHandler
-    public void markRead(MarkReadMessageCommand command) {
-        apply(new MessageReadEvent(id));
-    }
 }

axon/src/test/java/com/baeldung/axon/MessagesAggregateIntegrationTest.java

@@ -1,29 +1,28 @@
 package com.baeldung.axon;
 
+import java.util.UUID;
+
+import org.axonframework.test.aggregate.AggregateTestFixture;
+import org.axonframework.test.aggregate.FixtureConfiguration;
+import org.junit.*;
+
 import com.baeldung.axon.aggregates.MessagesAggregate;
 import com.baeldung.axon.commands.CreateMessageCommand;
 import com.baeldung.axon.commands.MarkReadMessageCommand;
 import com.baeldung.axon.events.MessageCreatedEvent;
 import com.baeldung.axon.events.MessageReadEvent;
-import org.axonframework.test.aggregate.AggregateTestFixture;
-import org.axonframework.test.aggregate.FixtureConfiguration;
-import org.junit.Before;
-import org.junit.Test;
-
-import java.util.UUID;
 
 public class MessagesAggregateIntegrationTest {
 
     private FixtureConfiguration<MessagesAggregate> fixture;
 
     @Before
-    public void setUp() throws Exception {
-        fixture = new AggregateTestFixture<MessagesAggregate>(MessagesAggregate.class);
-
+    public void setUp() {
+        fixture = new AggregateTestFixture<>(MessagesAggregate.class);
     }
 
     @Test
-    public void giveAggregateRoot_whenCreateMessageCommand_thenShouldProduceMessageCreatedEvent() throws Exception {
+    public void giveAggregateRoot_whenCreateMessageCommand_thenShouldProduceMessageCreatedEvent() {
         String eventText = "Hello, how is your day?";
         String id = UUID.randomUUID().toString();
         fixture.given()
@@ -32,7 +31,7 @@ public class MessagesAggregateIntegrationTest {
     }
 
     @Test
-    public void givenMessageCreatedEvent_whenReadMessageCommand_thenShouldProduceMessageReadEvent() throws Exception {
+    public void givenMessageCreatedEvent_whenReadMessageCommand_thenShouldProduceMessageReadEvent() {
         String id = UUID.randomUUID().toString();
 
         fixture.given(new MessageCreatedEvent(id, "Hello :-)"))