Passing query parameter in Spring - @RequestParam

@GetMapping("/api/users")
@ResponseBody
public String getUserByEmail(@RequestParam String email) {
return "Provided email is: " + email;
}​

What we have here is a description of a service that processes GET requests. It should return the value of type String that contains "name" - provided as @RequestParam. This means that when you want to pass the parameter this way, you'll need to add ?paramName=paramValue on the end of the URL string.

If we call this service with a request containing "johndoe@gmail.com" as a value of email parameter we'll get the following result:

https://localhost:3030/api/users?email=johndoe@gmail.com
----
Provided email is: johndoe@gmail.com

You can also add the following parameters to the @RequestParam annotation:

• name
• value
• required
• defaultValue

Name parameter

It represents the name of the query variable to be provided to service. When you don't have a name parameter, the default expected name for @RequestParam will be the name of the controller method variable. You'll usually use the name parameter when you want to name @RequestParam differently than your method variable. It's not a problem even if they are called the same, but then you have unnecessary code. This might sound confusing, but let's check it out on example:

@GetMapping("/api/users")
@ResponseBody
public String getUserByEmail(@RequestParam (name="emailAddress") String email) {
return "Provided email is: " + email;
}​

So, what we see here is that in this request @RequestParam is named emailAddress, but the method variable is called email. We will invoke service this way:

https://localhost:3030/api/users?emailAddress=johndoe@gmail.com
----
Provided email is: johndoe@gmail.com

Value parameter

The value parameter is used for the same purpose as name parameter, it's a name parameter alias. You can use one of them, and if you like - even both, but always give them the same value. Otherways your application will have strange, unpredicted behavior concerning that @RequestParam.

@GetMapping("/api/users")
@ResponseBody
public String getUserByEmail(@RequestParam (name="emailAddress", value="emailAddress") String email) {
return "Provided email is: " + email;
}​

This way you'll have the following response:

https://localhost:3030/api/users?emailAddress=johndoe@gmail.com
----
Provided email is: johndoe@gmail.com

Required parameter

Method variables annotated with @RequestParam are required by default. This means we will get an error if the parameter isn’t present in the request. To enable providing null parameter values you should put the required parameter to false. 

@GetMapping("/api/users")
@ResponseBody
public String getUserByEmail(@RequestParam (required=false) String email) {
return "Provided email is: " + email;
}​

And when you invoke this service, you'll get:

https://localhost:3030/api/users
----
Provided email is: null

You can also pass null @RequestParam values if you're using Java 8 Optional, without even providing the required keyword:

@GetMapping("/api/users")
@ResponseBody
public String getUserByEmail(@RequestParam Optional <String> email) {
 return "Provided email is : " + email.orElseGet(() -> null);
}​

DefaultValue parameter

The defaultValue parameter is used in situations when you want to cover the situation where the user does not provide @RequestParam in a request. So, even if you didn't set required to false, this request will not throw an error. Instead, the @RequestParam value will be replaced with defaultValue.  

@GetMapping("/api/users")
@ResponseBody
public String getUserByEmail(@RequestParam (defaultValue="noEmail") String email) {
 return "Provided email is : " + email;
}​

The response will be:

https://localhost:3030/api/users
----
Provided email is: null

That's it about Spring @RequestParam. Happy coding!