code: public synchronized static ApplicationClassLoader getNodeApplicationClassLoader() {
          if (nodeApplicationClassLoader != null) {
              return nodeApplicationClassLoader;
          }
          if (systemClassLoader == null) {
              throw new IllegalStateException("Can't create nodeClassLoader because systemClassLoader is null.");
          }
          nodeApplicationClassLoader = new ApplicationClassLoader(systemClassLoader, true);
          return nodeApplicationClassLoader;
      }
docs: 获取节点的类加载器,对于一个节点来说,该类加载器唯一
  注意:由于niubi-job的启动类和lib在一个目录下,因此对于node级别的类加载器,不需要指定lib目录为该类加载器的URL.
  
  @return 节点的类加载器