Develop Reference

How to consume Google In-App purchases Server-side

I need to be able to consume a purchase without an Android client being connected using a backend server. Is this possible? If not, why? I am trying to make it possible to process a purchase entirely without a user being connected in the case of a disconnection.
Do I need to send a message to my client from my server requesting consumption and then provide a response to the server when the purchase is consumed? This is what I am trying to avoid.
I see various other similar questions but they are for the AIDL API which is now discontinued for google-in-app billing.
Relative API(s);
https://mvnrepository.com/artifact/com.google.apis/google-api-services-androidpublisher/v3-rev103-1.25.0

SOLUTION: To my knowledge, at this current time, July 2019, it is not possible to consume a purchase from a server. It is only possible from the android client.
This means that the flow of the purchase depends on a request to a client and an response from the client, followed by a required GET to validate the purchase has actually been consumed.

How to send message from backend(java) to frontend(react)

Hi Im still learning how to use react and java. I am creating a project where the front end is in react and the backend is in java.
I was wondering if someone would be able to point me in the right direction or give me an example of java sending a message to the front end in react. What Im essentially trying to do is give the user a message after the backend receives a file they submit. I already have the part where the backend is processing the file.
Ive tried looking at a lot of sites but theres not a lot of documentation out there for react and java as the backend.
thank you

What you need is Websocket, he provide a full-duplex communication channels over a single TCP connection. So, when you start a comunication between client and server, a session is created, making possible to send and receive message for both side.
Some possibilities are, use javax:
https://docs.oracle.com/javaee/7/api/javax/websocket/package-summary.html
https://www.baeldung.com/java-websockets
Use spring-websocket(if are you already using spring, will be easy to use):
https://spring.io/guides/gs/messaging-stomp-websocket/
https://docs.spring.io/spring/docs/5.0.0.BUILD-SNAPSHOT/spring-framework-reference/html/websocket.html
Here is another example with javax https://www.pegaxchange.com/2018/01/28/websocket-server-java/
So, there are a lot of examples.

Websocket would open a channel between the client and the server, so you can send a server message at any time.
But, if you are already using a POST from the client to upload the file, you could just send a response to the POST.

Secured client server requests

I am looking for some guidelines as to how to secure requests from android client to server
How can i prevent un autenthic (users which initiate requests not from the android app) requests to be accepted and processed by the server?
Waybe generate token at user registration and use it somehow at each call?
My server is a lite instance and performance is top issue in the implementation of the server client communication.
Appreciate any help!

You could use Google Cloud Messaging as a via medium to make sure messages that reach your server are from the app, without getting too involved in details. Check the documentation here:
https://developer.android.com/google/gcm/index.html
Or else, as you said, create a unique id on registration and send it back to the client. Store it at both client and server. Now with each request from the client, add an extra field in the request and send the id also. Check this id serverside and make sure it is valid (this check could eventually become heavy on time).

Create a basic network app

I have a little question about an android app with a function "remote" use via internet.
So I have these ideas:
Create a webservice using php on the server, and refresh the client app all x time like 4-5 sec
Or make a java server (so I don't know how I can do that)
I just want make a remote connected via user/password to the server and the other app connect to the server and refresh his status.
Do you know the best way for do that?
Thanks

The question is not very precise, but as far as I understand it, both your ideas implement the 'pull' concept, that is the client app checking the status of the server every now and then.
If you want the app to be instantly notified of the server status change, I would suggest push notifications with GCM (Google Clound Messaging). You can find some basic descriptions and examples at: http://developer.android.com/google/gcm/index.html
Sending a server-to-client notification is simple regardless of the platform you use server side. E.g. for PHP integration with GCM, check out this thread: GCM with PHP (Google Cloud Messaging)

How to send an email to a java program

I'd like to write a program, probably a servlet or something to run on the a google app engine that I can send an email to. So not a program to send email, but one that can receieve it and parse it.
My question is, what code or API are out there that can receive an email?

Basically on your google app engine you can use an inbound mail service.
Please see this documentation for more information.
http://code.google.com/appengine/docs/java/mail/overview.html#Receiving_Mail_in_Java

You cant send an email to a program, you send an email to a server, so what you are looking for is a way to access an email server via your program. Unfortunately there is no single solution here, you need to configure your program for every different email account/server you want to access. (If you have ever set up an account in outlook or something like it you will get the idea)
For example here is a link to the gmail api, you could use this to access gmail accounts
http://code.google.com/apis/gmail/

You need to have a mailbox to send message there and you could read messages with the code like this: http://www.java2s.com/Code/Java/Network-Protocol/GetEmailMessageExample.htm

This can be done with a built in Java library.
javax.mail
Check out this link. It should be able to help you get started.
This won't work for every mail server, but depending on your setup it might help.

To send an email to a Java program, that program must be running. Generally that means a server style (aka service) receiver is favored.
For the email to be received, the Java service must understand an email protocol. There are a number of protocols, but SMTP is the standard for receiving email. Once you have a service that understands SMTP protocol for receiving email, you have written a mail server.
Note that most people don't care to write a mail server, as a mail client needs to connect to the server and pull the email to make it readable. Keep this in mind when designing the solution to your problem.