@Test
    public void testSPIGetDefaultJoin() {
        HasDefaultSPI spi = ExtensionLoader.getExtensionLoader(HasDefaultSPI.class).getDefaultJoin();
        assert Objects.nonNull(spi);
        assertThat(spi.getClass().getName(), is(SubHasDefaultSPI.class.getName()));
    }
test SPI has default value case.
shenyu-spi/src/test/java/org/apache/shenyu/spi/ExtensionLoaderTest.java