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 A "401 Unauthorized" error when your OAuth2 client is calling your OAuth2 resource server in a Spring Security application typically indicates an authentication failure. Here are some steps to troubleshoot and resolve this issue: 1. **Check Your Configuration:**    Ensure that your Spring Security configuration for both the client and the resource server is correctly set up. 2. **Client Credentials:**    Double-check that the OAuth2 client credentials (client ID and client secret) are correctly configured in the client application. 3. **Access Tokens:**    Make sure that the OAuth2 client is obtaining a valid access token. The token should be included in the request to the resource server in the "Authorization" header. 4. **Resource Server Configuration:**    Verify that your resource server is correctly configured to validate access tokens and authorize requests. 5. **Token Endpoint:**    Check that the URL for the token endpoint is correct in your client's co

 In Spring Security 5.x and later, the `@EnableResourceServer` annotation is part of the OAuth2 functionality and typically used in conjunction with `@EnableWebSecurity`. To configure security for a resource server, you should extend the `ResourceServerConfigurerAdapter` class, which provides methods to configure resource server settings. If you are encountering an error like "does not have any WebSecurityConfigurerAdapter.class," it might indicate that you haven't correctly configured your Spring Security with the required classes and methods. Here's a step-by-step guide on how to configure Spring Security for a resource server using `@EnableResourceServer`: 1. Make sure you have the necessary Spring Security and OAuth2 dependencies in your `pom.xml` or `build.gradle`: ```xml <dependency>     <groupId>org.springframework.boot</groupId>     <artifactId>spring-boot-starter-security</artifactId> </dependency> <dependency>     &l