android – 如何保持App Engine Servlet听Firebase

我正在按照教程：https://cloud.google.com/solutions/mobile/firebase-app-engine-android-studio

我有一切正常,电子邮件每2分钟发送一次.但是,我现在希望将其扩展为仅在Firebase节点上更改数据时触发发送电子邮件,而不是每2分钟发送一条消息.

测试我替换了cron.xml文件
从：

<?xml version="1.0" encoding="UTF-8"?>
<cronentries>
   <cron>
       <url>/hello</url>
       <description>Send me an email of outstanding items in the morning</description>
       <schedule>every 2 minutes</schedule>
   </cron>
</cronentries>

至：

<?xml version="1.0" encoding="UTF-8"?>
<cronentries/>

清除计划任务.

但是现在在对Firebase数据库进行更改时,电子邮件永远不会发送….

如何让我的应用引擎服务器“监听”firebase节点,然后实时生成onDataChanged上的操作？

MyServlet类：

public class MyServlet extends HttpServlet {
    static Logger Log = Logger.getLogger("com.example.username.myapplication.backend.MyServlet");

    @Override
    public void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws IOException {
        Log.info("Got cron message, constructing email.");

        //Create a new Firebase instance and subscribe on child events.
        Firebase firebase = new Firebase("[firebase ref]");
        firebase.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                // Build the email message contents using every field from Firebase.
                final StringBuilder newItemMessage = new StringBuilder();
                newItemMessage.append("This should arrive very closely after changing the data");


                //Now Send the email
                Properties props = new Properties();
                Session session = Session.getDefaultInstance(props, null);
                try {
                    Message msg = new MimeMessage(session);
                    //Make sure you substitute your project-id in the email From field
                    msg.setFrom(new InternetAddress("anything@[app-engine].appspotmail.com",
                            "Todo Nagger"));
                    msg.addRecipient(Message.RecipientType.TO,
                            new InternetAddress("[e-mail]", "Recipient"));
                    msg.setSubject("Feast Email Test");
                    msg.setText(newItemMessage.toString());
                    Transport.send(msg);
                } catch (MessagingException | UnsupportedEncodingException e) {
                    Log.warning(e.getMessage());
                }
            }

            public void onCancelled(FirebaseError firebaseError) {
            }
        });
    }

}

解决方法:

您的问题实际上是关于AppEngine以及如何创建自动启动并自动执行某些初始化的Servlet的问题.

您需要保持手动缩放,但请按照此处的步骤操作：
https://cloud.google.com/appengine/docs/java/config/appconfig#using_a_load-on-startup_servlet

用于在init()而不是http请求上设置侦听器.
你正在尝试的是绝对可能的,并且已经看到它在其他地方运行.

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