first improvements

[frankolschewski]

Do not Don't apply all my changes if you want to stay with MySQL and Maven (I haven't updated the pom.xml).

[yijiulove]

if you mean below error:

{
    "error": "invalid_client",
    "error_description": "Unauthorized grant type: password"
}

and please follow below way to fix authenticationManager null problem

1. init bean on WebSecurityConfiguration

@Bean
@Override
public AuthenticationManager authenticationManagerBean() throws Exception {
    return super.authenticationManagerBean();
}

2. inject into AuthorizationServerConfiguration

@Autowired
private AuthenticationManager authenticationManager;

@Override
public void configure(AuthorizationServerEndpointsConfigurer endpoints) throws Exception {
    endpoints.approvalStore(approvalStore())
            .authorizationCodeServices(authorizationCodeServices())
            .tokenStore(tokenStore())
            .authenticationManager(authenticationManager);
}

and please do remember to add grant type password to client_id.
table oauth_client_details.authorized_grant_types value include client_credentials,password

and then

curl -X POST \
  http://localhost:8081/oauth/token \
  -H 'authorization: Basic Y3VybF9jbGllbnQ6cGFzc3dvcmQ=' \
  -H 'cache-control: no-cache' \
  -H 'content-type: multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW' \
  -H 'postman-token: 6815f1d7-995a-e894-fd48-50942b395c8b' \
  -F grant_type=password \
  -F username=resource_admin \
  -F password=password

response

{
    "access_token": "def5b27c-3684-454c-b70a-16c6fd6db9dc",
    "token_type": "bearer",
    "expires_in": 6199,
    "scope": "read write"
}