====== Spring RestController ======
  - Create a package call controllers (could be anything, but this name make it clear) under your project root package.
  - Create a class call Home (could be anything, but this name make it clear) in the controllers package.
  - <code>package com.example.demo.controllers

import com.example.demo.domain.Cat
import org.springframework.web.bind.annotation.GetMapping
import org.springframework.web.bind.annotation.RequestParam
import org.springframework.web.bind.annotation.RestController

@RestController
class Home {
    @GetMapping("/")
    String index() {
        return "Return as plain text"
    }

    @GetMapping("/cat")
    Cat something() {
        return new Cat()
    }

    @GetMapping("/param")
    String param(@RequestParam(value = "param1", defaultValue = "param1") String param1) {
        return param1
    }
}
</code>
  - Above your class name, add ''@RestController'' to tell spring that it is a rest controller class
  - To map url get path to a method, put @GetMapping with the corresponding path eg:''@GetMapping("/")'' above the method. This example mapped the root path to the method index()
  - If the thing return is a string, it will return a plain text to the http client.
  - If the thing return is an object, it will return a JSON that represent that Object. It depends in the ''getter'' of that Object.
  - We can take parameter from user by using ''@RequestParam''. We can map it to the parameter with the method parameter, and give it a default value if the user do not supply one.
  - For this example, we have links http://127.0.0.1:8080/, http://127.0.0.1:8080/cat, and http://127.0.0.1:8080/param?param1=type%20anything

===== More =====
We have all kind of mapping, and we can map the same path with different http method to a different functions.
  - @GetMapping
  - @PostMapping
  - @PutMapping
  - @DeleteMapping
  - @PatchMapping 

We can add ''@RequestMapping("something")'' to the class so that any mapping under this class will become ''/something/...''