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How to handle JWT Token expiry in web API without logging out?

When the token is generated by sending a HTTP request to web API & the user has started working on the application that generated token is used in a particular session of the application. If during any HTTP request from the application to web API if JWT token expires WEB API won't return data. How is this situation handled in the application without any misbehavior or without troubling the user how that request will be continued?
And even if we generate a refresh token how to continue with the same HTTP request without troubling the user?
(If we store the generated token in the database then we know the token is valid but expired)

Try this :
Write backend refresh token API and allow an authenticated user to refresh their JWT token
In Frontend before requesting the API call decode the user's current JWT token and check whether it is expired or not.
https://www.npmjs.com/package/jwt-decode
If the token expired call the refresh token API before the actual request.
If you are using Angular or React library then there is a mechanism called HTTP_interceptor
https://www.bezkoder.com/angular-12-refresh-token/
https://www.bezkoder.com/react-refresh-token/

We also give a refresh token to the user along with the token, which has no claim and only has a username and a long expiration date. Every time the token expires, the security part of the applicationlooks at the refresh token and issues a new token for that username. You can manage the issuance of program tokens by setting the refresh token lifetime

OAuth2 in memory token loss due to server application restart

I encountered this problem in test stage of my application. Client set expiration date of token for very long time (19 years or so), so we wouldn't request new token often during tests. But After a while it came out that token has already expired (after random time).
The problem was server application being restarted/updated, resulting in in-memory tokens lost and rendering my simple check for expires_in not working:
if (_currentToken.ExpirationDate < DateTime.Now.AddMinutes(1))
{
_currentToken = GetToken();
}
How would I secure such scenario? It might as well happen on production, but hopefully more rarely due to less application restarts and shorter token time. Unfortunately I don't have an access to server side authorization settings and tokens won't be persited in any storage.
I would like avoid calling some dummy action on server to check if it returns 401 unauthorized before every action.

IMO it's server side's responsibility to validate token and decide whether the request with token could access specific protected resources . So as #armagedescu suggested , just send the token when performing token request , server side will check the claims like expire time , issuer ... and also check the signature . If token is expired , it will return 401 status code , and the OAuth 2.0 bearer token spec adds error, error_description, and error_uri attributes to the WWW-Authenticate header for reporting additional error information :
HTTP/1.1 401 Unauthorized
WWW-Authenticate: Bearer realm="example",
error="invalid_token",
error_description="The access token expired"
Then on client side you can check the errors , and refresh access token to perform another token request .

Token authentication for Web API renewal

I’m writing a web api that will be called from a background service to fetch some data. After some research I decided to use a Json web token to achieve that but I’m still a bit confused regarding when a new token should be requested.
Let’s say I start up my service, I request a token, the token expires after 15 minutes, then after 20 minutes I make an api call with the expired token. I will get an unauthorized error or something.
My question is: How will the client know when to request a new token? Should it request a new one before every api call? Seems like I’m missing something. Maybe I should make the token permanent and store it in the database?
Thanks

The answer to this is slightly application specific, but the OAuth specification has a mechanism for "refresh tokens", which can be used to grant new "access tokens" (the token typically included on each API request), without having to send the user to the UI authentication process to have them re-authenticate. So, once you request an access token, you will receive a refresh token and an access token. This methodology allows access tokens to be used for much shorter time frames.
This can also be done without refresh tokens, but in those cases the access token timeout would likely be longer, and then you would request that the user re-authenticate through the usual OAuth UI process. Note that even when you do have refresh tokens, the refresh token can also be set to expire, in which would then require a user re-authentication through UI again.
In some API's you just make the API request as usual, and if you get a response that is defined by the API to be one that indicates the access token has expired, you can then issue an API call to refresh the token (or fully request a new one if that is expired, or you the API doesn't have refresh tokens), and then make the original API call again with the new access token.
The API can also have a response that includes the timeout or expiration date/time of the access token as well. Then, the client can avoid sending the initial API call first, and simply send the refresh token call first.
In implementing your API, you could likely use any of these methodologies.
Here's some general discussion on the OAuth spec website, to provide more depth:
https://www.oauth.com/oauth2-servers/making-authenticated-requests/
https://www.oauth.com/oauth2-servers/access-tokens/access-token-lifetime/
https://www.oauth.com/oauth2-servers/access-tokens/refreshing-access-tokens/
And also, here's an example from the Twitter API regarding response codes showing one of the access token expiration techniques (see the "Error Codes" section, under error code 89, which implies the token has expired and you need to get a new one):
https://developer.twitter.com/en/docs/basics/response-codes

Since your client is background service , you can use the Oauth2 Client Credential Flow . Your background service can request an access token using only its client credentials when the client is requesting access to the protected resources under its control.
With this flow , you does't need to care much about the token expires , if client sends an expired token to web api , web api validate the token and create token expires response to your service , your service check the status code/response , directly send a new token request to web api to get new access token , there is no need to use refresh token which uses in other flows .

The fact is that your harness should be prepared to request any token when getting an Unauthorized status code. What I do in test is to check the expiration datetime, if close enough I refresh or get a new token whatever applies to your Auth. Also when getting an unauthorized status code my code does a refresh once and keep a count. If I get another unauthorized code then I return a false or throw an exception after I log the error on the second try. This works fine for me.

.NET Core API - does the middleware set the JWT bearer token in the header?

I've got all the code working to generate the JWT and I've wired up my ConfigureServices with the proper config code but I'm not seeing the header actually get set.
I assumed that the middleware would do this for you but maybe not, is it up to me to return the token from the login method of my controller and the client to then take it and set the header for subsequent requests?

No it does not.
The way it works is that you send your login credentials to a login server. In most cases its the same but in more secure applications this won't be the case.
The server then authenticates your credentials, creates a JWT token and sends that back to you.
You can then use that JWT in your header when making a request to the application server:
"Authorization":"Bearer xxxxx.yyyyy.zzzzz"
This needs to be done with every call to the server because the point of JWT is that it is stateless, meaning the server does not save the data at all. Instead in reads the JWT token with every call and grants access/functionality based on that.

OpenIdConnect token renewal

We have an MVC application that uses IdentityServer4 as a STS. We have set the access_token and identity_token lifetime to four hours on the Client properties in the STS. We have also added the scope offline_access in our MVC client.
How do we refresh the access_token and/or the identity_token from the STS in the MVC client?
What could work, but I don't see this as a good solution is request a new token on every request. Should we keep track of this time in the MVC client and call something like: https://github.com/IdentityServer/IdentityServer4.Samples/blob/293622b8438d27f4c9c2574e43fe92a22560ac6b/Clients/src/MvcHybrid/Controllers/HomeController.cs#L46
Or is there some event that we can hook up to and request the new token(s)?

You don't refresh identity tokens - only access tokens.
There is no event - either refresh tokens pro-actively based on expiration time - or wait until the API returns a 401 and refresh lazily.