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How to implement a messaging service in react-native with a java backend?

I am creating an app using React-Native for the front-end, Java for the back-end and PostgreSQL for the database.
One feature of this app is to allow users to add each other as friends and instant message them - just like other social networks. At the moment, security is not a priority but I don't want to create something that I will have to completely redo when I do have to make it secure!
The goal here is to give each user the ability to send/receive messages to/from their friends. They also need to be able to view the message history with each contact.
I just need some advice on the following points to get me going as I haven't gotten very far on my own:
How should I send messages from the front-end, to the Java server and then to the receiver? This will involve the app having to listen for new messages so would the Socket.IO library be the way to go for this?
How I should store the messages in the database? E.g. have a table for messages with user_ids to be used for retrieval?
I should point out that I am very new to React-Native, databases and networking so I am finding this very challenging!
I will massively appreciate any help and any examples would be fantastic!

You should use WebSocket on the server. Spring supports it
Here's a tutorial I wrote about sending messages from PostgreSQL to the browser using WebSocket, which isn't exactly what you're going to do, but can hopefully get you started:
http://blog.databasepatterns.com/2014/04/postgresql-nofify-websocket-spring-mvc.html

Send android data to XAMPP server Java file input and then send back to android device

I am creating an authentication system where I get some value( let us say "x") after processing the android application. I want to send the value "x" to the server where I have a java file and it needs to be run on the server end(may be using php by passing the argument as the value "x"). After running, I need the output to be sent back to the android device to display it.
Leads how to implement such short structure will help me complete the system.

You need to create a class that extends AsyncTask, and then use your php source that u would normally use for login. Here's a link that might help you out: Android, PHP authentication

Try using HTML request. I am not sure how is this implemented. Any one can comment regarding it.

Encrypting/hiding portions of the source code

I've made a simple mailing app that takes in email credentials and uses it to send emails of certain kinds to selected addresses. Problem is, I've had to input the credentials right into the code, so anyone who uses dex2jar can get the source code and get the email used for forwarding and easily make the app obsolete.
I imagine I'm not the only one facing this issue, so what are some ways to make my code secure?

No matter how good of a technique you use to hide the credentials, if it's in the code then it can always be found.
Instead of hard coding them in, you could perhaps let the user specify them when he starts the app? If that can't be avoided you could instead have a remote service that will do the sending and forward your request to that.

You can not both connect to an e-mail account and keep those same users out of said e-mail account. Consider using a hosted server as part of the project to securely connect to the e-mail account from the server level and process these e-mails remotely.

Using Fax4J With Multiple Fax Modems?

Can anyone give me a quick rundown on how to send faxes in Java using Fax4J? The tutorial provided by the javadocs is sketchy at best. In particular, it doesn't teach you how to specify which fax modem you are calling; it only says FaxClient faxClient=FaxClientFactory.createFaxClient(); but how does it work?
I downloaded the full fax4j code, and I noticed some files called FaxModem.java in there. How are those used?

For someone who actually authored fax4j, cant agree with the comment about the tutorial because i am not sure you read it.
The factory is explained returns a client which is backed by an engine called spi.
There are many different spi types.
Some would use windows native api, some would run a process, others would send emails and http requests to remote providers and it is all based on your fax4j configuration.
All cofiguration options possible are detailed in the tutorial so for example if you want to send an email to spcific mail server that converts it to fax ypu would set email address and other needed properties in your fax4j.properties file.
So based on the way you are sending requires different config and its all in the tutorial.
However you didnt write how you want to send and of course fax4j doesnt support everything.
I think the main bulk of users use the windows native api spi so they actually dont configure anything for fax4j and instead configure the fax settings in their windows machine and thats it

outlook calendar connectivity with java web service

We currently have a java/jsp online web service that includes it's own custom calendar. I am trying to do some research into the possibility of connecting it to a users outlook. Our basic needs that are most simple is some way to sent the person a meeting request that can be added to their outlook from our service.
I know the ideal solution is to sync back and forth but simply being able to import the data from our calendar into someone's outlook would be sufficient.
Does anyone have any resources they can point me to that might help with information gathering, or any example/comments?

Here is a sample to sync calendars using javascript,ie and ActiveX
http://www.codeproject.com/KB/office/OutlookDemo.aspx

I found a simple solution to getting what we needed. Outlook can email an event invite, I had one emailed to me... then copied the format of the email and just have the system sending event invites as emails. That is sufficient for our needs.