如何为消息发送器构建Fluent API？

方案1：让Builder直接持有Publisher，消除循环依赖 #

你可以将MessagePublisher直接注入MessageBuilder，让MessageSender仅负责创建Builder并传递Publisher，这样就避免了Builder对MessageSender的依赖：

public class MessageSender {
    private final MessagePublisher publisher;

    public MessageSender(MessagePublisher publisher) {
        this.publisher = publisher;
    }

    public MessageBuilder newMessage() {
        return new MessageBuilder(publisher);
    }
}

public class MessageBuilder {
    private final MessagePublisher publisher;
    private final Message message = new Message();

    public MessageBuilder(MessagePublisher publisher) {
        this.publisher = publisher;
    }

    public MessageBuilder withSomeField(String value) {
        message.setSomeField(value);
        return this;
    }

    public MessageBuilder withAnotherField(String value) {
        message.setAnotherField(value);
        return this;
    }

    public void publish() {
        publisher.publish(message);
    }
}

这种设计下，MessageBuilder只依赖MessagePublisher，和MessageSender没有绑定关系，彻底消除了你反感的循环依赖。

方案2：分离构建与发送，用终结器衔接 #

如果不想让Builder持有Publisher，可以把构建和发送拆分为两个独立步骤，通过MessageSender提供的发送方法完成最终操作：

public class MessageSender {
    private final MessagePublisher publisher;

    public MessageSender(MessagePublisher publisher) {
        this.publisher = publisher;
    }

    public MessageBuilder newMessage() {
        return new MessageBuilder();
    }

    public void publish(Message message) {
        publisher.publish(message);
    }

    public class MessageBuilder {
        private final Message message = new Message();

        public MessageBuilder withSomeField(String value) {
            message.setSomeField(value);
            return this;
        }

        public MessageBuilder withAnotherField(String value) {
            message.setAnotherField(value);
            return this;
        }

        public Message build() {
            return message;
        }
    }
}

调用方式会变成：

Message message = messageSender.newMessage()
    .withSomeField("some value")
    .withAnotherField("some value")
    .build();
messageSender.publish(message);

如果仍想保留链式调用的体验，也可以在Builder中添加publish方法，让它调用外部MessageSender的发送逻辑，这是单向依赖而非循环依赖：

// 在MessageBuilder中新增
public void publish() {
    MessageSender.this.publish(message);
}

方案3：静态Builder+全局Publisher入口（适合简单场景） #

如果你的MessagePublisher是单例或可全局获取，可以用静态Builder实现无实例化调用：

public class MessageSender {
    private static MessagePublisher publisher;

    public static void init(MessagePublisher publisher) {
        MessageSender.publisher = publisher;
    }

    public static MessageBuilder newMessage() {
        return new MessageBuilder();
    }

    public static class MessageBuilder {
        private final Message message = new Message();

        public MessageBuilder withSomeField(String value) {
            message.setSomeField(value);
            return this;
        }

        public MessageBuilder withAnotherField(String value) {
            message.setAnotherField(value);
            return this;
        }

        public void publish() {
            publisher.publish(message);
        }
    }
}

这种方式不需要实例化MessageSender，但依赖全局状态，仅适合小型或简单项目。

内容的提问来源于stack exchange，提问作者Jakub Znamenáček