I want to implement some util class which manage tokens generated by any API. The idea is simple: It should store current token value. Token should be initialized just before first use and then re-initialied after some time elapsed. If API return some error related with token, it should be regenerated and job should be repeated. Here we have util class:

import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.TimeUnit;

public abstract class TokenManager {

    private volatile String token;
    private ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecutor();
    private ScheduledFuture<?> schedule;

    public void useToken(TokenConsumer tokenConsumer) {
        useToken(tokenConsumer, true);
    }

    private void useToken(TokenConsumer tokenConsumer, boolean retryWhenTokenExpired) {
        try {
            tokenConsumer.useToken(getToken());
        } catch (Exception e) {
            if (retryWhenTokenExpired && tokenExpired(e.getMessage())) {
                System.out.println("token expired, retry");
                resetToken();
                useToken(tokenConsumer, false);
            }
        }
    }

    synchronized private String getToken() {
        if (token == null) {
            resetToken();
        }
        return token;
    }

    synchronized private void resetToken() {
        if (schedule != null) {
            schedule.cancel(false);
        }
        System.out.println("reset token");
        token = generateToken();
        int oneMinute = 60 * 1000;
        long delay = getTokenValidityTimeMillis() - oneMinute;
        schedule = scheduler.schedule(this::resetToken, delay > 0 ? delay : getTokenValidityTimeMillis(), TimeUnit.MILLISECONDS);

        System.out.println("reset token finished");
    }

    protected abstract String generateToken();

    protected abstract boolean tokenExpired(String message);

    protected abstract long getTokenValidityTimeMillis();

    @FunctionalInterface
    public interface TokenConsumer {
        void useToken(String token) throws Exception;
    }
}

And here we have it's simple implementation with use example:

import java.util.Random;

public class TokenManagerImpl extends TokenManager {
    @Override
    protected String generateToken() {
        //here would go some api call which retrieves token
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return "token " + new Random().nextInt();
    }

    @Override
    protected boolean tokenExpired(String message) {
        return "Token expired".equals(message);
    }

    @Override
    protected long getTokenValidityTimeMillis() {
        return 1000;
    }

    public static void main(String[] args) {
        TokenManager tokenManager = new TokenManagerImpl();
        TokenConsumer tokenConsumer = token -> {
            //here would go some api call which uses token
            System.out.println("using token : " + token);
        };
        while (true) {
            tokenManager.useToken(tokenConsumer);
            try {
                Thread.sleep(50L);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}