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Generate an Output in an Endpoint with a JSON body as a input

My company has asked me to do a small project in Java Spring and I have never programmed in Spring nor Java before, so I am a bit lost.
I have been asked to make a series of endpoints, seeing the documentation does not seem difficult, where I find the difficulty is that I always receive as input a JSON with several key-value pairs, and with that, I have to search in an Oracle database using Hibernate and return another JSON with other key-value pairs.
Example:
Input:
{
"client_id": 123,
"shop_id": 22,
"id_contract": 233
}
Output:
{ "loan": "70%"
"percentage_point": "80%"
}
My question is, should I do it through a POST method? Is the output a status 200? How do I get from JSON, for example, the client id to map it in the database with hibernate / Spring?
Any advice will be welcome, even if it is simply articles, etc.

Well what you have been asking is not for a specific problem.It is an entire requirement. I can try helping with the steps that you may need to follow.
If you are not done creating a project, I would suggest you to use Spring Boot for your project.To create a spring boot app click here
And if you are using maven or gradle select it and then search for Spring Web,Spring Data JPA in search dependencies and generate a project.
Once you have a project ready and imported it into an IDE (for example Eclipse, IntelliJ Idea, Netbeans or any other Java IDE), and finally all you have to do is to write REST services to implement your requirements.
And to answer your questions.
should I do it through a POST method?
If you were told that the request you will be receiving is in a JSON format, then you need to use POST method.
What is post and why to use post method.check here
Is the output a status 200?
A status 200 in http means SUCCESS. It means when the data sent by the client is valid and the server has processed the request(in your case JSON body) successfully it returns 200.So incase of success your api will return 200.
More on http status codes
How do I get from JSON, for example, the client id to map it in the database with hibernate / Spring?
To understand more on how to create a rest controller, take JSON input, create a POST method, use Spring JPA for making database operations please follow the links below:
create a rest service
spring boot crud Database operations example from scratch
Spring boot Data JPA example
Hope this will help you to put you in right path.

Without code or a more specific post about requirements it's impossible to help. All I can do is provide links to potentially helpful articles. You need to look into the following;
Spring Boot RESTful service
Spring Boot Controller vs RestController
Spring Boot JPA
Spring Boot Consuming/Producing JSON

Creating a secure Liferay Servlet

I'd like to develop a servlet whichs works as a datalayer between a porlet written in JavaScript (ExtJS) and a database.
Thus I downloaded Eclipse JavaEE and created a sample servlet project. I also successfully deployed a servlet which receives a http-get-request, calls the DB and outputs the result.
But everyone who nows the servlet-uri can call it and sees the result. Thats where it gets tricky - how do I secure my servlet so that only a authenticated/logged in user can call my servlet or how can my servlet check wether the requester is a valid user?
There is a similar question about that Topic but I'm not sure if this is about the same problem: How to create a top-level servlet in liferay
Unfortunately I have no clue how to get the liferay-libraries (e.g. PortalUtil) into my servlet-project. How do I do this step by step?
Is this concept even a good way to secure the datalayer or are there better options?
Any help is appreciated!

How to access spring mvc properties value in websocket onopen event?

I am using spring mvc + websocket. I have created application.properties to store configuration data of project. Now I want to use that properties values in websockets #onOpent() event. I am able to access those properties in simple rest controller #RequestMapping() but unfortunately I am not able to access those properties in in websockets #onOpent() event, I am getting null value for the same. How can I achieve this?

This happens because Websocket server class in not spring component.
you can mark websocket handler class as component using #Component annotation on class. Then you will be able to access properties.
Hope this helps you.

Seems like you are using Java websockets. you can use spring 4 websockets here is sample

Applet and JSF Integration - example

Is there a tutorial or a simple applet example with JSF? How do I perform a request to a Managed Bean from an applet?

Don't use a JSF managed bean. It is not suitable for this job. Use a servlet or a webservice. To exchange data, use the session scope with an unique autogenerated key which you pass as parameter to the applet beforehand. This way the data will be available to JSF as well.

JSF (and hence managed beans) executes on the server to produce HTML; An applet executes on the client's machine - so you can't just pass a reference to a managed bean to an applet.
If you just need to pass a value from a managed bean to an Applet at start time, you can use the <param> sub-element of the tag to pass this value.
If you need some kind of dynamic access to the managed bean, it's going to be a lot harder - basically, you'll need to build some kind of web service that's backed by the managed bean so that the applet can make http requests back to the server to get the values it needs.

Importing secured content using the jstl import tag and have the OSIV filter working correctly

Anyone ever tried the following? (and was successful)
In a web application (A), I am using the <c:import> tag to get secured content from another web application (B) running on the same application server (WebSphere 7). Both apps use Hibernate and Spring's OSIV filter.
Looking at the import tag source, I see that the strategy is that if the url is relative then it includes the content using RequestDispatcher.include() .If the url is absolute, the code opens a URLConnection.
Since I need to keep track of the remote user, I can't do the following:
<c:import url="http://host:port/B/getContent">
Doing
<c:import url="/getContent" context="/B">
instead would work. But with this approach I am not hitting Spring's OSIV filter configured in B. The original (importing) request in A does go through the OSIV filter but it has no effect in B. Hence I am getting the usual "No session or session closed" error for lazy initializations of entities.
I am bit in a catch 22 here and I am wondering if what I am trying to do is actually feasable according to my requirements.
The bottom line is that I did manage to get what I wanted by aggregating my content directly from the client using Dojo, (I am using SSO so the identity of the user gets carried) but I would prefer the other way if it was possible.