Spring MVC: Complex object as GET @RequestParam

Question

You can absolutely do that, just remove the @RequestParam annotation, Spring will cleanly bind your request parameters to your class instance:

public @ResponseBody List<MyObject> myAction(
    @RequestParam(value = "page", required = false) int page,
    MyObject myObject)

I will add some short example from me.

The DTO class:

public class SearchDTO {
    private Long id[];

    public Long[] getId() {
        return id;
    }

    public void setId(Long[] id) {
        this.id = id;
    }
    // reflection toString from apache commons
    @Override
    public String toString() {
        return ReflectionToStringBuilder.toString(this, ToStringStyle.SHORT_PREFIX_STYLE);
    }
}

Request mapping inside controller class:

@RequestMapping(value="/handle", method=RequestMethod.GET)
@ResponseBody
public String handleRequest(SearchDTO search) {
    LOG.info("criteria: {}", search);
    return "OK";
}

Query:

http://localhost:8080/app/handle?id=353,234

Result:

[http-apr-8080-exec-7] INFO  c.g.g.r.f.w.ExampleController.handleRequest:59 - criteria: SearchDTO[id={353,234}]

I hope it helps :)

UPDATE / KOTLIN

Because currently I'm working a lot of with Kotlin if someone wants to define similar DTO the class in Kotlin should have the following form:

class SearchDTO {
    var id: Array<Long>? = arrayOf()

    override fun toString(): String {
        // to string implementation
    }
}

With the data class like this one:

data class SearchDTO(var id: Array<Long> = arrayOf())

the Spring (tested in Boot) returns the following error for request mentioned in answer:

"Failed to convert value of type 'java.lang.String[]' to required type 'java.lang.Long[]'; nested exception is java.lang.NumberFormatException: For input string: \"353,234\""

The data class will work only for the following request params form:

http://localhost:8080/handle?id=353&id=234

Be aware of this!

Since the question on how to set fields mandatory pops up under each post, I wrote a small example on how to set fields as required:

public class ExampleDTO {
    @NotNull
    private String mandatoryParam;

    private String optionalParam;
    
    @DateTimeFormat(iso = ISO.DATE) //accept Dates only in YYYY-MM-DD
    @NotNull
    private LocalDate testDate;

    public String getMandatoryParam() {
        return mandatoryParam;
    }
    public void setMandatoryParam(String mandatoryParam) {
        this.mandatoryParam = mandatoryParam;
    }
    public String getOptionalParam() {
        return optionalParam;
    }
    public void setOptionalParam(String optionalParam) {
        this.optionalParam = optionalParam;
    }
    public LocalDate getTestDate() {
        return testDate;
    }
    public void setTestDate(LocalDate testDate) {
        this.testDate = testDate;
    }
}

//Add this to your rest controller class
@RequestMapping(value = "/test", method = RequestMethod.GET)
public String testComplexObject (@Valid ExampleDTO e){
    System.out.println(e.getMandatoryParam() + " " + e.getTestDate());
    return "Does this work?";
}

I have a very similar problem. Actually the problem is deeper as I thought. I am using jquery $.post which uses Content-Type:application/x-www-form-urlencoded; charset=UTF-8 as default. Unfortunately I based my system on that and when I needed a complex object as a @RequestParam I couldn't just make it happen.

In my case I am trying to send user preferences with something like;

 $.post("/updatePreferences",  
    {id: 'pr', preferences: p}, 
    function (response) {
 ...

On client side the actual raw data sent to the server is;

...
id=pr&preferences%5BuserId%5D=1005012365&preferences%5Baudio%5D=false&preferences%5Btooltip%5D=true&preferences%5Blanguage%5D=en
...

parsed as;

id:pr
preferences[userId]:1005012365
preferences[audio]:false
preferences[tooltip]:true
preferences[language]:en

and the server side is;

@RequestMapping(value = "/updatePreferences")
public
@ResponseBody
Object updatePreferences(@RequestParam("id") String id, @RequestParam("preferences") UserPreferences preferences) {

    ...
        return someService.call(preferences);
    ...
}

I tried @ModelAttribute, added setter/getters, constructors with all possibilities to UserPreferences but no chance as it recognized the sent data as 5 parameters but in fact the mapped method has only 2 parameters. I also tried Biju's solution however what happens is that, spring creates an UserPreferences object with default constructor and doesn't fill in the data.

I solved the problem by sending JSon string of the preferences from the client side and handle it as if it is a String on the server side;

client:

 $.post("/updatePreferences",  
    {id: 'pr', preferences: JSON.stringify(p)}, 
    function (response) {
 ...

server:

@RequestMapping(value = "/updatePreferences")
public
@ResponseBody
Object updatePreferences(@RequestParam("id") String id, @RequestParam("preferences") String preferencesJSon) {


        String ret = null;
        ObjectMapper mapper = new ObjectMapper();
        try {
            UserPreferences userPreferences = mapper.readValue(preferencesJSon, UserPreferences.class);
            return someService.call(userPreferences);
        } catch (IOException e) {
            e.printStackTrace();
        }
}

to brief, I did the conversion manually inside the REST method. In my opinion the reason why spring doesn't recognize the sent data is the content-type.