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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.

HTTP Client-Server resquest response

I am trying to write a simple client-server application in Java using HTTP request/response. I would like the client to be a desktop program which sends (posts) a request to a server. The server is a web page which will be hosted on Apache Tomcat server. The server must be able to read the information and display it on the browser and must be able to respond to the client with a status code 200. I am using eclipse and Apache tomcat server. I have so far tried various resources, but all I could find is a client which could request response from an already existing web server. Could someone please give me an example or some insight on how to make the client request the our own server which runs on the local machine.

Good question, though in your case, I won't recommend you implement a simple HTTP request/response approach as you will end up implementing a timer, heartbeat or Comet. You might wanna try javax or jetty WebSocket API. All you need is to create three parts:
a websocket.server
a websocket.client (desktop application)
a javascript websocket client (browser agent)
Your server and both clients will become full-duplex via onMessage and send events.
Here's an example which I believe is a bit relevant one.
https://dzone.com/articles/sample-java-web-socket-client

Java web server to process Android client request

Since I'm a newbie to Java web server, I don't know where to begin to create a server in Java to reply from Android devices request( A WEB SERVER NOT AN ACTUAL SITE).
Which technology should I use and which document should i read?. I have searched on the Internet about Java but i found out many technologies and i don't know which one i can use.
Spring MVC,JSF,JSP,Struts,Servlet..............
Hope this question clear enough for you to reply. Thank you

It doesn't matter what be process android devices requests on server side. In this case main idea is http communication - client sends request, server process this request and sends response to client, and you can use any technology on server side - java, php etc.
If you prefer java I can suggest you try servlets, it is simplest technology of those you mentioned above.

Changing tcp/ip packets c++ or java

Here is the situation. There are server and client in network. They communicate like this:
Client sends request for some function.
Server sends to client function parameters.
Client trying to perform function and sends answer to server.
Server sends to client data which it should show.
But sometimes client can't perform function and sends error. I want to catch all packets from step 2, analyze them (I've already have tools for that), prevent some of them to reach client, process them with my program and form packet like in step 3. This must be done on client side. I have no access neither to server nor to client.
So, the question is: Is there libraries for changing, injecting and removing tcp/ip packet in c++ or java? The solution should be working in both Win and Linux systems.
Also, may be you have better ideas to expand client functionality?
Thanks for any help!
I tried to google how to change packets, but all I got were unanswered questions and sniffers=(
Edit: Actually, I don't really need injecting and removing packets, I can manage it with only changing packet data. Also, there is no multiple requests in the same packet, and a single request across multiple packets is not a problem.

You have to build a Proxy for your server. The client connects to the proxy, and the proxy itself connects to the server. It just routes all the packages between client and server.
BUT it is now able to intercept specific messages and to modify them. Imagine a filtering HTTP proxy, it works the same way.

I have personal experience with libpcap on linux and freeBSD, a kind of lowlevel library that helps to catch or inject packets. I did use it in an IPV6 network bridge project... But i know there is a windows port for it.
http://sourceforge.net/projects/libpcap/
You can let the library to:
catch packets using a filter
extract data from packet
you can process the data (modify them)
reinject it again using the same library
But you would have to work with internal data in a quite raw matter. Best documentation for this library are comments inside its header file, that is the most up to date info. Maybe there are some more comfortable highlevel libraries.

java socket programming

Can I write a socket programming to provide services to the web clients?
I did it using servlet, but I want to do it using java.net API.
Please give me a sample code of some program, such that I can access that simply by mentioning URL at the address bar of any web browser.

I suggest you look at the source for jetty. It is the simplest web server I can think of. If you want an ultra basic web server, you can do this with plain sockets, however the HTTP protocol is quite complex and using a web server library to handle all the details is likely to be the best approach.

If you want to be able to receive requests typed into a web browser, you need to do a couple things.
-Set the socket to listen on port 80
-Receive/parse/process HTML requests
-Return an HTML response across the socket
Rather than write the code for you, here is some pseudocode
//setup socket on port 80
socket.lisen();
while(true)
{
newsocket = socket.accept();
new thread(process(newsocket));
}
The most complicated part, I think, will be handling the HTML, processing the request, and generating a response. After that, just send it back over the socket.
Considering how many libraries are out there for this sort of thing, I wouldn't reccommend writing one from scratch.

The problem is that "web client" is just a browser, so you don't have direct access to TCP/IP.
Few options:
HTML5 WebSockets (only modern browsers)
flash helper (there are javascript wrappers)
java applet helper (there are javascript wrappers)
some tricks based on ajax pooling