我尝试遵循在Akka演员之间发送消息的最佳实践，并阅读以下内容：
https://github.com/akka/akka/blob/v2.6.15/akka-docs/src/test/java/jdocs/actor/ActorDocTest.java#L101-L129
https://doc.akka.io/docs/akka/current/actors.html
最好的做法似乎是定义一个包含要发送的消息的静态类。
通过阅读文档，我定义了以下内容：

import akka.event.Logging;
import akka.event.LoggingAdapter;
import akka.persistence.AbstractPersistentActor;

class RecoveryActor extends AbstractPersistentActor {

    public static class Msg1 {}
    public static class Msg2 {}

    private final LoggingAdapter log = Logging.getLogger(getContext().getSystem(), this);

    /**
     * Handler that will be called on recovery.
     * @return
     */
    @Override
    public Receive createReceiveRecover() {
        return receiveBuilder()
                .match(Msg1.class, this::receiveMsg1)
                .match(Msg2.class, this::receiveMsg2)
                .build();
    }

    private void receiveMsg1(Msg1 msg) {
        log.info("in receive message 1");
    }

    private void receiveMsg2(Msg2 msg) {
        log.info("in receive message 2");
    }

    /**
     * The normal receive method.
     * @return
     */
    @Override
    public Receive createReceive() {
        return receiveBuilder()
                .match(String.class, s -> s.equals("cmd"), s -> persist("evt", this::handleEvent))
                .build();
    }

    private void handleEvent(String event) {
        System.out.println("in handle event");
    }

    @Override
    public String persistenceId() {
        return "simple-accountant"; //best practice, the id should be unique.
    }

}

但我不确定如何使用Msg1和Msg2发送消息
我定义了一个新类来发送消息：

import akka.actor.ActorRef;
import akka.actor.ActorSystem;
import akka.actor.Props;

public class ActorMessaging {

    public static void main(String args[]){
        final ActorSystem system = ActorSystem.create("example");
        final ActorRef actor = system.actorOf(Props.create(RecoveryActor.class));

        actor.tell(RecoveryActor.Msg1);

    }
}

但它失败，并出现编译器错误：

java: cannot find symbol
  symbol:   variable Msg1
  location: class recoverydemo.RecoveryActor

如何向类型为Msg1的执行元发送消息？
这是在参与者之间发送消息的最佳实践吗？将消息 Package 在自定义类中？