code: public static ClassLoader getDefaultClassLoader() {
          ClassLoader loader = Thread.currentThread().getContextClassLoader();
          if (loader == null) {
              loader = ClassUtils.class.getClassLoader();
              if (loader == null) {
                  loader = ClassLoader.getSystemClassLoader();
              }
          }
          return loader;
      }
docs: 获取默认classloader
  
  @return 当前默认classloader（先从当前线程上下文获取，获取不到获取加载该类的ClassLoader，还获取不到就获取系统classloader）