Right now this serves no purpose because you're using reflection to set it.

Incorrectly attempts to read "token_file" from config. Should be "jwt_token_file". I'll also recommend to change the variable name to jwtTokenFile.

It is okay to use reflection but there is a simpler way. Reflection is inherently slower than direct field access because the JVM has to do extra work at runtime.

I can assume why you made this choice because enableXdsBootstrapCallCreds in BootstrapperImpl.java is not public, hence you chose to go with reflection. We can safely make it public there because it is anyway internal.

jwt_token_file* here and below in this file.

jwt_token_file*

Critical Bug: Valid cached token is ignored and requests fail during backoff

When a background refresh fails (e.g., due to a transient file read error), the credential enters the BACKOFF state. Under the current implementation, any request made during the backoff window will fail immediately because readState == ReadState.BACKOFF is checked first and causes an immediate failure, completely ignoring the fact that a perfectly valid cached token still exists in cachedToken.

This can cause severe availability issues where healthy, unexpired tokens are not used simply because a background refresh attempt failed.

We should check if a valid cache exists first. If it does, we should always use it and only trigger/skip background refreshes based on the backoff state. If no valid cache exists, only then should we fail-fast during the backoff window.

    synchronized (lock) {
      boolean hasValidCache = cachedToken != null && now < expirationTimeMillis;
      boolean expiringSoon = hasValidCache && (expirationTimeMillis - now <= 60000);

      if (hasValidCache) {
        tokenToApply = new TokenInfo(cachedToken, expirationTimeMillis);
        if (readState == ReadState.BACKOFF && now >= nextAttemptTimeMillis) {
          readState = ReadState.IDLE;
        }
        if (expiringSoon && readState == ReadState.IDLE) {
          readState = ReadState.READING;
          triggerRead = true;
        }
      } else {
        if (readState == ReadState.BACKOFF && now >= nextAttemptTimeMillis) {
          readState = ReadState.IDLE;
        }

        if (readState == ReadState.BACKOFF) {
          failStatus = lastReadFailureStatus != null ? lastReadFailureStatus : Status.UNAVAILABLE;
        } else {
          if (readState == ReadState.IDLE) {
            readState = ReadState.READING;
            triggerRead = true;
          }
          queuedAppliers.add(applier);
        }
      }
    }