Spring Boot @RequestParam Annotation

➔ The @RequestParam annotation is used to extract form data and query parameters from the URL path of an HTTP request to the server and bind those values ​​to a method parameter in the controller.

Key characteristics of the @RequestParam annotation:

➔ The @RequestParam annotation is used to extract query parameters and values ​​from the URL path and attach them to the controller's method parameters.

➔ @RequestParam is mainly used to map values ​​from URL query strings to Java variables in method parameters and can be used multiple times in a method if needed.

➔ @RequestParam also binds the form data sent in the request body to Content-Type:application/x-www-form-urlencoded and maps it to a DTO or domain model.

Basic Usage Syntax

URL: https://localhost:8090/api/customers?customerid=100
@PostMapping(value="/api/customers")
public String customerHandler(@RequestParam(value="customerid") int cid) {
	return  "Customer ID: "+cid;
}

➔ A single handler method can handle multiple request parameters.

URL: https://localhost:8090/api/customers?customerid=100&orderid=500
@PostMapping(value="/api/customers")
public String customerHandler(@RequestParam(value="customerid") int cid, @RequestParam(name="orderid") int oid) {
	return  "Customer ID: "+cid +"\nOrder ID: "+oid;
}

➔ Spring boot automatically converts String input values ​​to the target method parameter type (such as String, int, long, Boolean, etc.).

➔ If the name of the Java method variable and the name of the URL parameter are different, the explicit name must be declared in the annotation.

➔ If the name of the Java method variable and the name of the URL parameter are the same, declaring the name explicitly in the annotation is optional.

URL: https://localhost:8090/api/customers?customerid=100&orderid=500
@PostMapping(value="/api/customers")
public String customerHandler(@RequestParam int customerid, @RequestParam int orderid) {
	return  "Customer ID: "+customerid +"\nOrder ID: "orderid;
}

Making Request Parameters Optional in URL:

➔ By default the parameters are mandatory and a 400 Bad Request error occurs if they are not provided with the request.

➔ @RequestParam is required and these values ​​can be made optional by setting required=false.

➔ Using `required=false` prevents Spring Boot from throwing an exception even if the parameter is missing.

URL: https://localhost:8090/api/customers
@PostMapping(value = "/api/customers")
public String customerHandler(@RequestParam(required = false) Integer customerid, 
        @RequestParam(value = "orderid", required = false) String oid) {
	return "Customer ID: " + customerid + "\nOrder ID: " + oid;
}
//Output
Customer ID: null
Order ID: null

Setting default values for Request Parameters:

➔ The @RequestParam annotation has a built-in defaultValue attribute, so the default value can be set using this attribute, for example: @RequestParam(defaultValue = "500").

➔ If the parameter is missing from the request URL, the defaultValue attribute provides a fallback value for the parameter, to avoid null values ​​or exceptions.

➔ Setting defaultValue automatically makes required false, so the required attribute is optional when using the defaultValue attribute.

➔ The name/value attributes in @RequestParam are aliases of each other.

URL: https://localhost:8090/api/customers
@PostMapping(value = "/api/customers")
public String customerHandler(@RequestParam(name="customerid", defaultValue = "101") int cid,
			@RequestParam(value = "orderid", defaultValue = "501") String oid) {
	return "Customer ID: " + cid + "\nOrder ID: " + oid;
}
//Output
Customer ID: 101
Order ID: 501

Collecting multiple parameters with the same name.

URL: localhost:8090/api/customers?customerid=100&customerid=102&customerid=105
@PostMapping(value = "/api/customers")
public String customerHandler(@RequestParam(name="customerid") List<String> ids) {
	return "Search for : " + ids;
}
//Output
Search for : [100, 102, 105]

Collecting multiple parameters under different names.

URL: localhost:8090/api/customers?customerid=100&orderid=500&name=maria
@PostMapping(value = "/api/customers")
public String customerHandler(@RequestParam Map<String, String> allParams) {
	return "Parameters map : " + allParams +"\nEntryset: "+allParams.entrySet()+
		"\nKeyset: "+allParams.keySet()+"\nvalues: "+allParams.values();
}
//Output
Parameters map : {customerid=100, orderid=500, name=maria}
Entryset: [customerid=100, orderid=500, name=maria]
Keyset: [customerid, orderid, name]
values: [100, 500, maria]