code: public Class<? extends ThreadContextMap> loadThreadContextMap() {
          if (threadContextMapClass != null) {
              return threadContextMapClass;
          }
          if (threadContextMap == null) {
              return null;
          }
          final ClassLoader loader = classLoader.get();
          if (loader == null) {
              return null;
          }
          try {
              final Class<?> clazz = loader.loadClass(threadContextMap);
              if (ThreadContextMap.class.isAssignableFrom(clazz)) {
                  return clazz.asSubclass(ThreadContextMap.class);
              }
          } catch (final Exception e) {
              LOGGER.error("Unable to create class {} specified in {}", threadContextMap, url.toString(), e);
          }
          return null;
      }
docs: Loads the {@link org.apache.logging.log4j.spi.ThreadContextMap} class specified by this Provider.
  
  @return the ThreadContextMap implementation class or {@code null} if there was an error loading it