What is Model in ModelAndView from Spring MVC?

Having this basic function

protected ModelAndView handleRequestInternal(...) {
    ...
    return new ModelAndView("welcomePage", "WelcomeMessage", message);
}

I know that this will return modelandView. I know that welcomePage is my viewname so that means something like welcomepage.jsp will get called.

But I am confused with what is Model part. What is WelcomeMessage and message and how Model work in that scenario?

The model presents a placeholder to hold the information you want to display on the view. It could be a string, which is in your above example, or it could be an object containing bunch of properties.

Example 1

If you have...

return new ModelAndView("welcomePage","WelcomeMessage","Welcome!");

... then in your jsp, to display the message, you will do:-

Hello Stranger! ${WelcomeMessage} // displays Hello Stranger! Welcome!

Example 2

If you have...

MyBean bean = new MyBean();
bean.setName("Mike!");
bean.setMessage("Meow!");

return new ModelAndView("welcomePage","model",bean);

... then in your jsp, you can do:-

Hello ${model.name}! {model.message} // displays Hello Mike! Meow!

new ModelAndView("welcomePage", "WelcomeMessage", message);

is shorthand for

ModelAndView mav = new ModelAndView();
mav.setViewName("welcomePage");
mav.addObject("WelcomeMessage", message);

Looking at the code above, you can see the view name is "welcomePage". Your ViewResolver (usually setup in .../WEB-INF/spring-servlet.xml) will translate this into a View. The last line of the code sets an attribute in your model (addObject("WelcomeMessage", message)). That's where the model comes into play.

It is all explained by the javadoc for the constructor. It is a convenience constructor that populates the model with one attribute / value pair.

So ...

   new ModelAndView(view, name, value);

is equivalent to:

   Map model = ...
   model.put(name, value);
   new ModelAndView(view, model);

Here in this case, we are having 3 parameter's in the Method namely ModelandView.
According to this question, the first parameter is easily understood. It represents the View which will be displayed to the client.
The other two parameters are just like The Pointer and The Holder
Hence you can sum it up like this

ModelAndView(View, Pointer, Holder);

The Pointer just points the information in the The Holder

When the Controller binds the View with this information, then in the said process, you can use The Pointer in the JSP page to access the information stored in The Holder to display that respected information to the client.
Here is the visual depiction of the respected process.

return new ModelAndView("welcomePage", "WelcomeMessage", message);

Well, WelcomeMessage is just a variable name for message (actual model with data). Basically, you are binding the model with the welcomePage here. The Model (message) will be available in welcomePage.jsp as WelcomeMessage. Here is a simpler example:

ModelAndView("hello","myVar", "Hello World!");

In this case, my model is a simple string (In applications this will be a POJO with data fetched for DB or other sources.). I am assigning it to myVar and my view is hello.jsp. Now, myVar is available for me in hello.jsp and I can use it for display.

In the view, you can access the data though:

${myVar}

Similarly, You will be able to access the model through WelcomeMessage variable.

ModelAndView: The name itself explains it is data structure which contains Model and View data.

Map() model=new HashMap(); 
model.put("key.name", "key.value");
new ModelAndView("view.name", model);

// or as follows

ModelAndView mav = new ModelAndView();
mav.setViewName("view.name");
mav.addObject("key.name", "key.value");

if model contains only single value, we can write as follows:

ModelAndView("view.name","key.name", "key.value");

@RequestMapping(value="/register",method=RequestMethod.POST)
   public ModelAndView postRegisterPage(HttpServletRequest request,HttpServletResponse response,
		   @ModelAttribute("bean")RegisterModel bean)
   {
	   RegisterService service = new RegisterService();
	   boolean b = service.saveUser(bean);
	   
	   if(b)
	   {
		   return new ModelAndView("registerPage","errorMessage","Registered Successfully!");
	   }
	   else
	   {
		   return new ModelAndView("registerPage","errorMessage","ERROR!!");
	   }
   }



/*  "registerPage" is the .jsp page -> which will viewed.
/* "errorMessage" is the variable that could be displayed in page using -> **${errorMessage}**
/* "Registered Successfully!" or "ERROR!!" is the message will be printed based on **if-else condition**