Client side :

To communicate with a server over HTTP. Angular makes requests and receive responses using HttpClient and the data comes in the format of JSON by default.

JSON (JavaScript Object Notation) is a format for data sent to and from APIs.

Consider we have a JavaScript object student and a number idD for example.
We want to send both the object and the number to the server side.

To do this, first create an object with these 2 elements

{student: student, idD: idD}

Now convert the object into a JSON string using JSON.stringify()

Our code is going to look like this:

public saveStudent(student : Student, idD:number):Observable<Object>{
    var data=JSON.stringify({student: student, idD: idD})
    return this.http.post<Object>(environment.backendHost+"/admin/students" ,data );
  }

Now your data is going to be sent to the server as a string.

Server side:

First Add the following dependencies to your POM.XML file:

        <dependency>
            <groupId>org.json</groupId>
            <artifactId>json</artifactId>
            <version>20220320</version>
        </dependency>

        <dependency>
            <groupId>com.google.code.gson</groupId>
            <artifactId>gson</artifactId>
            <version>2.9.1</version>
        </dependency>

Remember that we sent a String from the client side to the server side.

Your controller class should be expecting a String

@PostMapping(value = "/admin/students")
    public Student saveStudent(@RequestBody String myObject ) {

To convert the String to a JSON Object, we will be using JSONObject which is a subclass of java.util.HashMap.

First we need an implementation of a JSON package in Java.

Github repository:

Import the following in your Rest Controller:

import org.json.JSONObject;

Inside your controller pass you object as a parameter to the JSONObject's constructor

JSONObject jsonObject = new JSONObject(myObject);

In this example we're going to be using two JSONObject methods:
getJSONObject(String name)

That returns the object value to which the specified name is mapped.
And

getInt(String name)

That returns an integer from JSON number.

JSONObject myobj = jsonObject.getJSONObject("student");
int idD= jsonObject.getInt("idD");

The last step is to transform JSON object into a Java Object

I'm going to be using Gson which is a Java library that is used to serialize and deserialize Java objects to JSON

Gson is a Java library that can be used to convert Java Objects into their JSON representation. It can also be used to convert a JSON string to an equivalent Java object. Gson can work with arbitrary Java objects including pre-existing objects that you do not have source-code of.

Github repository:

Create a Gson object using the constructor:

Gson gson= new Gson();

In this example we're going to be using the fromJson() method

fromJson(java.lang.String json, java.lang.reflect.Type typeOfT)

This method deserializes the specified Json into an object of the specified type.

Convert your JSONObject myobj into a string using toString()

Then pass it as the first parameter to the fromJson() method.
The second parameter is going to be your java class.

Student student = gson.fromJson(myobj.toString(),Student.class);