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How to make a Model attribute global?

I'm using Spring MVC Framework and I'd like all the .jsp pages of the View to have access to the User's attributes(name, sex, age...). So far, I use the addAttribute method of the Model(UI) in every Controller to pass the current User's attributes to the View. Is there a way to do this only once and avoid having the same code in every Controller?

You can use Spring's #ControllerAdvice annotation on a new Controller class like this:
#ControllerAdvice
public class GlobalControllerAdvice {
#ModelAttribute("user")
public List<Exercice> populateUser() {
User user = /* Get your user from service or security context or elsewhere */;
return user;
}
}
The populateUser method will be executed on every request and since it has a #ModelAttribute annotation, the result of the method (the User object) will be put into the model for every request through the user name, it declared on the #ModelAttribute annotation.
Theefore the user will be available in your jsp using ${user} since that was the name given to the #ModelAttribute (example: #ModelAttribute("fooBar") -> ${fooBar} )
You can pass some arguments to the #ControllerAdvice annotation to specify which controllers are advised by this Global controller. For example:
#ControllerAdvice(assignableTypes={FooController.class,BarController.class})
or
#ControllerAdvice(basePackages={"foo.bar.web.admin","foo.bar.web.management"}))

If it is about User's attributes, you can bind the model bean to session as an attribute which can be accessed on every view. This needs to be done only once.
Another option could be is to implement a HandlerInterceptor, and expose the model to every request.

What class or component of Spring MVC determines the object passed into a Controller's RequestMapping method parameter?

Raw beginner with Spring MVC -- that said,
What component of Spring MVC passes in objects to a method annotated with #RequestMapping within a controller object?
For example,
#RequestMapping
public String test(Model model) {
model.addAttribute("testMessage", "My Test");
return "test";
}
Where does Model come from?
Can my method take in any parameters I want?
Is there some bit of intuitive Spring framework dependency injection that I'm simply not understanding here?

This is all part of Spring dependency injection.
Spring injects the Model model
object to your controller method. Model model is simply a Map <String, Object>
that stores attributes. When you added the attribute to the model object using model.addAttribute("testMessage", "My Test") its adding one entry to the Map. This map is accessible from the view that you're interested in. So you can use this map in your view to access the attribute that you added from the controller (i.e. testMessage)
There are several other things that you can pass in these controller methods that Spring resolves automatically and injects appropriate objects/value. You can use things like:
ModelMap modelMap
#RequestParam
#PathVariable
#ModelAttribute
BindingResult bindingResult
and on and on. When you pass these things on your controller method, Spring knows how to resolve them and inject them to your method.
This might give you an better understanding of spring mvc: http://docs.spring.io/spring/docs/current/spring-framework-reference/html/mvc.html

Insert class in all spring model

I have on user registration form, a startup controler (with request method get) that loads the user class to that form and a post method to save the user. Everything uses spring. But now I want to move the registration form to a popup in the header file and every page will import that.
The problem is, I don't want to insert the user class in all Get methods. What I really want is to "inject" in all models the user class without having to do something in all other methods.
Is this possible to do? And how?

In Spring 3.2, there is a #ControllerAdvice class level annotation introduced. If you place your model attribute in a separate class with #ControllerAdvice, it will be available to all controllers. For example add getUserForm() in the #ControllerAdvice annotated class, instead of your original controller(s):
#ControllerAdvice
public class ModelAttributeAdvice {
#ModelAttribute
public SearchForm getUserForm(){
return new UserForm();
}
}

Where to store request specific values in Spring MVC?

I'm using Spring MVC and I want to store request specific values somewhere so that they can be fetched throughout my request context. Say I want to set a value into the context in my Controller (or some sort of handler) and then fetch that value from some other part of the Spring request/response cycle (could be a view, view resolver, interceptor, exception handler, etc)... how would I do that?
My question is:
Does Spring MVC already provide a method to do what I described above?
If Spring doesn't have this functionality, any ideas on the best way to do this (by extending something maybe)?
Thanks!

If you need to pass an object from your controller to view, you can use Spring's ModelMap.
#RequestMapping("/list")
public String list(ModelMap modelMap) {
// ... do foo
modelMap.addAttribute("greeting", "hello");
return viewName;
}
on your view:
<h1>${greeting}</h1>

You could use sessionAttributes.
Session Attributes
I took the latest version of the api (3.1) since you didn't mention your version of spring.

Passing an object from JSP to Spring controller

I created a class named Person. Then I pass an object of this class via Spring controller to JSP page, say abc.htm.
Now I want it to transfer back from abc.htm to another controller.
How could I do that?
Also tell me if any other class object (say Address class object) uses that person object as parameter, then how would I pass that Address class object to the controller.
I am very confused, please help me.

After the page is rendered you are no longer in the "Java realm", so you don't have your objects. You can rebuild them based on the parameters that are sent back in the next request.
This is called "binding". In Spring MVC this is done automatically (more or less) if you are using the <form:x> tags. Then in your controller your objects will be accessible as method attributes:
#RequestMapping(..)
public String foo(YourObject object) {..}
You might need a #ModelAttribute annotation if the name of your param and the one in the JSP are not the same. The MVC docs write:
Command or form objects to bind parameters to: as bean properties or fields, with customizable type conversion, depending on #InitBinder methods and/or the HandlerAdapter configuration. See the webBindingInitializer property on AnnotationMethodHandlerAdapter. Such command objects along with their validation results will be exposed as model attributes by default, using the non-qualified command class name in property notation. For example, "orderAddress" for type "mypackage.OrderAddress". Specify a parameter-level ModelAttribute annotation for declaring a specific model attribute name.
I'd suggest you review the PetClinic Sample Application to see how this works in practice.