public T getDefaultJoin() {
        getExtensionClassesEntity();
        if (StringUtils.isBlank(cachedDefaultName)) {
            return null;
        }
        return getJoin(cachedDefaultName);
    }
Gets default join.

@return the default join.
shenyu-spi/src/main/java/org/apache/shenyu/spi/ExtensionLoader.java