No 'Access-Control-Allow-Origin' header is present on the requested resource.

To fix this error,  I used to enable spring security I have added below class in my spring boot application.

Here my API is running in 8080 and angular in 4200

@Configuration
@EnableWebSecurity
public class SpringSecurityConfig extends WebSecurityConfigurerAdapter {

    @Override
    protected void configure(HttpSecurity http) throws Exception {
        http.cors().and().authorizeRequests().
                antMatchers( "/**")
                .permitAll()
                .anyRequest()
                .authenticated();
    }
}

And my controller is like

@RestController
public class AuthenticationController {

    @RequestMapping(method = RequestMethod.GET, value = "/login")
    public String userValidation(){
        return "valid user";
    }
    @GetMapping(path = "/basicauth")
    public AuthenticationBean basicauth() {
        return new AuthenticationBean("You are authenticated");
    }
}

After this I got below error, 

Access to XMLHttpRequest at 'http://localhost:8080/basicauth' from origin 'http://localhost:4200' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.

GET http://localhost:8080/basicauth net::ERR_FAILED 401

HttpErrorResponse {headers: HttpHeaders, status: 0, statusText: 'Unknown Error', url: 'http://localhost:8080/basicauth', ok: false, …}

1. error: ProgressEvent {isTrusted: true, lengthComputable: false, loaded: 0, total: 0, type: 'error', …}
2. headers: HttpHeaders {normalizedNames: Map(0), lazyUpdate: null, headers: Map(0)}
3. message: "Http failure response for http://localhost:8080/basicauth: 0 Unknown Error"
4. name: "HttpErrorResponse"
5. ok: false
6. status: 0
7. statusText: "Unknown Error"
8. url: "http://localhost:8080/basicauth"
9. [[Prototype]]: HttpResponseBase

Resolve this error I added below code in my controller class.

@CrossOrigin(origins = "http://localhost:4200")
@RestController
public class AuthenticationController {
//your methods are here...!
}
After this change the same call give response like below,

Thank you!