@Test
    public void testSPINoDefaultJoin() {
        JdbcSPI jdbcSPI = ExtensionLoader.getExtensionLoader(JdbcSPI.class).getDefaultJoin();
        assertNull(jdbcSPI);
    }
test SPI no default value case.
shenyu-spi/src/test/java/org/apache/shenyu/spi/ExtensionLoaderTest.java