public void setFallback(ApacheHttpClientFallback fallback) {
        AssertUtil.notNull(fallback, "fallback cannot be null");
        this.fallback = fallback;
    }
@Test(expected = IllegalArgumentException.class)
    public void testConfigSetFallback() {
        SentinelApacheHttpClientConfig config = new SentinelApacheHttpClientConfig();
        config.setFallback(null);
    }