Named Credentials as Callout Endpoints
Named Credentials also include an OutboundNetworkConnection field that you can use to route callouts through a private connection. By separating the endpoint URL and authentication from the callout definition, named credentials make callouts easier to maintain. For example, if an endpoint URL changes, you update only the named credential. All callouts that reference the named credential simply continue to work.
If you have multiple orgs, you can create a named credential with the same name but with a different endpoint URL in each org. You can then package and deploy—on all the orgs—one callout definition that references the shared name of those named credentials. For example, the named credential in each org can have a different endpoint URL to accommodate differences in development and production environments. If an Apex callout specifies the shared name of those named credentials, the Apex class that defines the callout can be packaged and deployed on all those orgs without programmatically checking the environment.
To reference a named credential from a callout definition, use the named credential URL. A named credential URL contains the scheme callout:, the name of the named credential, and an optional path. For example: callout:My_Named_Credential/some_path.
You can append a query string to a named credential URL. Use a question mark (?) as the separator between the named credential URL and the query string. For example: callout:My_Named_Credential/some_path?format=json.
Example
HttpRequest req = new HttpRequest();
req.setEndpoint('callout:My_Named_Credential/some_path');
req.setMethod('GET');
Http http = new Http();
HTTPResponse res = http.send(req);
System.debug(res.getBody());

The Apex code remains the same no matter what authentication you use. The authentication settings differ in the external credential, which references an authentication provider that’s defined in the org.

In contrast, let’s see what the Apex code looks like without a named credential. Notice that the code becomes more complex to handle authentication, even if we stick with basic password authentication. Coding OAuth is even more complex and is an ideal use case for named credentials.
HttpRequest req = new HttpRequest();
req.setEndpoint('https://my_endpoint.example.com/some_path');
req.setMethod('GET');
// Because we didn't set the endpoint as a named credential,
// our code has to specify:
// - The required username and password to access the endpoint
// - The header and header information
String username = 'myname';
String password = 'mypwd';
Blob headerValue = Blob.valueOf(username + ':' + password);
String authorizationHeader = 'BASIC ' +
EncodingUtil.base64Encode(headerValue);
req.setHeader('Authorization', authorizationHeader);
// Create a new http object to send the request object
// A response object is generated as a result of the request
Http http = new Http();
HTTPResponse res = http.send(req);
System.debug(res.getBody());