@Nullable
    protected final SSLSession sslSession() {
        checkState(!sessionProtocol.isTls() || sslSession != null,
                   "sslSession must be set for a TLS-enabled protocol: %s", sessionProtocol);
        return sessionProtocol.isTls() ? sslSession : null;
    }
Returns the {@link SSLSession} of the connection.

@return the {@link SSLSession}, or {@code null} if the {@link SessionProtocol} is not TLS.
core/src/main/java/com/linecorp/armeria/common/AbstractRequestContextBuilder.java