在使用Axon框架中，可以通过使用聚合的无参数构造函数来实例化另一个聚合。以下是一个示例代码，演示了如何在Axon中隐式使用无参数构造函数实例化另一个聚合：

首先，定义聚合类AggreagateA和AggregateB：

public class AggregateA {

    private String id;
    private AggregateB aggregateB;

    public AggregateA() {
        // 无参数构造函数
    }

    // Getter和Setter方法省略
}

public class AggregateB {

    private String id;
    
    public AggregateB() {
        // 无参数构造函数
    }

    // Getter和Setter方法省略
}

接下来，定义聚合的命令和事件：

public class CreateAggregateACommand {

    @TargetAggregateIdentifier
    private String id;

    public CreateAggregateACommand(String id) {
        this.id = id;
    }

    public String getId() {
        return id;
    }
}

public class AggregateACreatedEvent {

    private String id;

    public AggregateACreatedEvent(String id) {
        this.id = id;
    }

    public String getId() {
        return id;
    }
}

然后，创建聚合的命令处理器：

public class AggregateACommandHandler {

    @CommandHandler
    public void handle(CreateAggregateACommand command) {
        AggregateA aggregateA = new AggregateA();
        aggregateA.setId(command.getId());
        aggregateA.setAggregateB(new AggregateB());

        // 发布聚合创建事件
        AggregateACreatedEvent event = new AggregateACreatedEvent(command.getId());
        apply(event);
    }
}

最后，配置Axon的聚合和命令处理器：

@Configuration
public class AxonConfig {

    @Bean
    public AggregateACommandHandler aggregateACommandHandler() {
        return new AggregateACommandHandler();
    }

    @Bean
    public CommandBus commandBus() {
        SimpleCommandBus commandBus = SimpleCommandBus.builder().build();
        commandBus.registerDispatchInterceptor(new BeanValidationInterceptor<>());
        return commandBus;
    }

    @Bean
    public EventStorageEngine eventStorageEngine() {
        return new InMemoryEventStorageEngine();
    }

    @Bean
    public EventBus eventBus() {
        return SimpleEventBus.builder().build();
    }

    @Bean
    public EventSourcingRepository aggregateARepository(EventStore eventStore) {
        return EventSourcingRepository.builder(AggregateA.class)
                .eventStore(eventStore)
                .build();
    }

    @Bean
    public EventStore eventStore(EventStorageEngine eventStorageEngine) {
        return EmbeddedEventStore.builder()
                .storageEngine(eventStorageEngine)
                .build();
    }
}

通过以上配置和代码，当发送CreateAggregateACommand命令时，聚合AggregateA将被实例化，并且聚合AggregateB也会被隐式实例化。