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/*******************************************************************
 * © 2019 SAP SE or an SAP affiliate company. All rights reserved. *
 *******************************************************************/
package com.sap.cloud.mt.subscription;

import com.sap.cloud.mt.subscription.exceptions.CommunicationError;
import com.sap.cloud.security.xsuaa.client.OAuth2TokenResponse;

import java.time.Instant;

/**
 * Class providing access to JWT tokens, a retrieved JWT is cached until it expires
 */
public class ClientCredentialJwtAccess {
    private final ClientCredentialJwtReader jwtReader;
    // response is buffered as long as JWT is valid
    //volatile on object level is ok, as parameters of response never change
    private volatile OAuth2TokenResponse oAuth2TokenResponse; //NOSONAR

    /**
     * @param jwtReader object that retrieves a new JWT from XSUAA
     */
    public ClientCredentialJwtAccess(ClientCredentialJwtReader jwtReader) {
        this.jwtReader = jwtReader;
    }

    /**
     * Returns a valid JWT
     *
     * @return the JWT as string
     * @throws CommunicationError
     */
    public String getJwt() throws CommunicationError {
        // see double checked locking for this pattern
        OAuth2TokenResponse localRef = oAuth2TokenResponse;
        if (isTokenExpired(localRef)) {
            synchronized (this) {
                localRef = oAuth2TokenResponse;
                if (isTokenExpired(localRef)) {
                    oAuth2TokenResponse = localRef = jwtReader.getJwt();
                }
            }
        }
        return localRef.getAccessToken();
    }

    private static boolean isTokenExpired(OAuth2TokenResponse tokenResponse) {
        return tokenResponse == null || Instant.now().isAfter(tokenResponse.getExpiredAt());
    }
}