I want to develop an authentication mechanism for 3rd party applications using keycloak initial access tokens. But I want to do this only by using the access tokens that I have generated in the keycloak. For example, I will give a generated token to the user and allow him to log into the application. Is this possible? How can i do that? Initial Access Token
First, I'm not sure it is a good idea to use "initial access token" for authorizing requests to your own resources. It might even not be allowed. As per the doc:
An initial access token can only be used to create clients
Second, don't use keycloak libs for Spring as suggested in other answer. It is very deprecated.
Last, a REST API secured with Oauth2 is a resource-server (and not a client). As so, the dependency should be spring-boot-starter-oauth2-resource-server. Spring doc is pretty extensive on the subject. You can also have a look at those tutorials for resource-server configuration (and OAuth2 definitions).
Once the resource-server (Spring REST API) is configured, clients will have to authorize their requests as normal for OAuth2: provide with an Authorization header containing a Bearer access-token (a JWT issued by Keycloak to the client with one of standard flows: client-credentials or authorization-code depending you need to authorize the client itself or a user behind it).
I might have misunderstood your need. If what you want is having new clients register themself programmatically on your Keycloak instance, then those clients will just issue a REST request to Keycloak server API with the "initial access token" in Authorization header as normal.
Once registered, those clients will be able to call you resource-servers as described in my initial answer.
