Spring @RequestParam Annotation

1. Overview

In this quick tutorial, we’ll explore Spring’s @RequestParam annotation.

Simply put, we can use @RequestParam to extract query parameters, form parameters and even files from the request.

We’ll discuss how to use @RequestParam and its attributes. We’ll also discuss the differences between @RequestParam and @PathVariable.

2. A Simple Mapping

Let’s say that we have an endpoint /api/foos that takes a query parameter called id:

@GetMapping("/api/foos")
@ResponseBody
public String getFoos(@RequestParam String id) {
    return "ID: " + id;
}

In this example, we used @RequestParam to extract the id query parameter.

A simple GET request would invoke getFoos:

http://localhost:8080/api/foos?id=abc
----
ID: abc

Next, let’s have a look at the annotation’s attributes: name, value, required and defaultValue.

3. Specifying the Request Parameter Name

In the previous example, both variable name and the parameter name are the same.

Sometimes we want these to be different, though. Or, if we aren’t using Spring Boot, we may need to do special compile-time configuration or the parameter names won’t actually be in the bytecode.

But what’s nice is that we can configure the @RequestParam name using the name attribute:

@PostMapping("/api/foos")
@ResponseBody
public String addFoo(@RequestParam(name = "id") String fooId, @RequestParam String name) { 
    return "ID: " + fooId + " Name: " + name;
}

We can also do @RequestParam(value = “id”) or just @RequestParam(“id”).

4. Making an Optional Request Parameter

Method parameters annotated with @RequestParam are required by default.

This means that if the parameter isn’t present in the request, we’ll get an error:

GET /api/foos HTTP/1.1
-----
400 Bad Request
Required String parameter 'id' is not present

We can configure our @RequestParam to be optional, though, with the required attribute:

@GetMapping("/api/foos")
@ResponseBody
public String getFoos(@RequestParam(required = false) String id) { 
    return "ID: " + id;
}

In this case, both:

http://localhost:8080/api/foos?id=abc
----
ID: abc

and

http://localhost:8080/api/foos
----
ID: null

will correctly invoke the method.

When the parameter isn’t specified, the method parameter is bound to null.

5. A Default Value for the Request Parameter

We can also set a default value to the @RequestParam by using the defaultValue attribute:

@GetMapping("/api/foos")
@ResponseBody
public String getFoos(@RequestParam(defaultValue = "test") String id) {
    return "ID: " + id;
}

This is like required=false, in that the user no longer needs to supply the parameter:

http://localhost:8080/api/foos
----
ID: test

Though, we are still okay to provide it:

http://localhost:8080/api/foos?id=abc
----
ID: abc

Note that when we set the defaultValue attribute, required is, indeed, set to false.

6. Mapping All Parameters

We can also have multiple parameters without defining their names or count by just using Map:

@PostMapping("/api/foos")
@ResponseBody
public String updateFoos(@RequestParam Map<String,String> allParams) {
    return "Parameters are " + allParams.entrySet();
}

Which will then reflect back any parameters sent:

curl -X POST -F 'name=abc' -F 'id=123' http://localhost:8080/api/foos
-----
Parameters are {[name=abc], [id=123]}

7. Mapping a Multi-Value Parameter

A single @RequestParam can have multiple values:

@GetMapping("/api/foos")
@ResponseBody
public String getFoos(@RequestParam List<String> id) {
    return "IDs are " + id;
}

And Spring MVC will map a comma-delimited id parameter:

http://localhost:8080/api/foos?id=1,2,3
----
IDs are [1,2,3]

Or a list of separate id parameters:

http://localhost:8080/api/foos?id=1&id=2
----
IDs are [1,2]

8. @RequestParam vs @PathVariable

@RequestParam and @PathVariable can both be used to extract values from the request URI, but they are a bit different.

8.1. Query Parameter vs URI Path

While @RequestParams extract values from the query string, @PathVariables extract values from the URI path:

@GetMapping("/foos/{id}")
@ResponseBody
public String getFooById(@PathVariable String id) {
    return "ID: " + id;
}

Then, we can map based on the path:

http://localhost:8080/foos/abc
----
ID: abc

And for @RequestParam, it will be:

@GetMapping("/foos")
@ResponseBody
public String getFooByIdUsingQueryParam(@RequestParam String id) {
    return "ID: " + id;
}

Which would give us the same response, just a different URI:

http://localhost:8080/foos?id=abc
----
ID: abc

8.2. Encoded vs Exact Value

Because @PathVariable is extracting values from the URI path, it’s not encoded. On the other hand, @RequestParam is.

Using the previous example, ab+c will return as-is:

http://localhost:8080/foos/ab+c
----
ID: ab+c

But for a @RequestParam request, the parameter is URL decoded:

http://localhost:8080/foos?id=ab+c
----
ID: ab c

8.3. Optional Values

Both @RequestParam and @PathVariable can be optional.

We can make @PathVariable optional by using the required attribute starting with Spring 4.3.3:

@GetMapping({"/myfoos/optional", "/myfoos/optional/{id}"})
@ResponseBody
public String getFooByOptionalId(@PathVariable(required = false) String id){
    return "ID: " + id;
}

Which, then, we can do either:

http://localhost:8080/myfoos/optional/abc
----
ID: abc

or:

http://localhost:8080/myfoos/optional
----
ID: null

For @RequestParam, we can also use the required attribute as we saw in a previous section.

Note that we should be careful when making @PathVariable optional, to avoid conflicts in paths.

9. Conclusion

In this article, we learned how to use @RequestParam and the difference between @RequestParam and @PathVariable.

The full source code for the examples can be found in the GitHub project.