How do I retrieve query parameters in a Spring Boot controller?

I am developing a project using Spring Boot. I've a controller which accepts GET requests.

Currently I'm accepting requests to the following kind of URLs:

http://localhost:8888/user/data/002

but I want to accept requests using query parameters:

http://localhost:8888/user?data=002

Here's the code of my controller:

@RequestMapping(value="/data/{itemid}", method = RequestMethod.GET)
public @ResponseBody
item getitem(@PathVariable("itemid") String itemid) {   
    item i = itemDao.findOne(itemid);              
    String itemname = i.getItemname();
    String price = i.getPrice();
    return i;
}

Use @RequestParam

@RequestMapping(value="user", method = RequestMethod.GET)
public @ResponseBody Item getItem(@RequestParam("data") String itemid){

    Item i = itemDao.findOne(itemid);              
    String itemName = i.getItemName();
    String price = i.getPrice();
    return i;
}

While the accepted answer by afraisse is absolutely correct in terms of using @RequestParam, I would further suggest to use an Optional<> as you cannot always ensure the right parameter is used. Also, if you need an Integer or Long just use that data type to avoid casting types later on in the DAO.

@RequestMapping(value="/data", method = RequestMethod.GET)
public @ResponseBody
Item getItem(@RequestParam("itemid") Optional<Integer> itemid) { 
    if( itemid.isPresent()){
         Item i = itemDao.findOne(itemid.get());              
         return i;
     } else ....
}

To accept both @PathVariable and @RequestParam in the same /user endpoint:

@GetMapping(path = {"/user", "/user/{data}"})
public void user(@PathVariable(required=false,name="data") String data,
                 @RequestParam(required=false) Map<String,String> qparams) {
    qparams.forEach((a,b) -> {
        System.out.println(String.format("%s -> %s",a,b));
    }
  
    if (data != null) {
        System.out.println(data);
    }
}

Testing with curl:

• curl 'http://localhost:8080/user/books'
• curl 'http://localhost:8080/user?book=ofdreams&name=nietzsche'