code: public static String toJsonString(Object obj, ClassLoader classLoader) {
          if (obj == null) {
              return "null";
          }
          ClassLoader oldClassLoader = Thread.currentThread().getContextClassLoader();
          if (classLoader != null) {
              Thread.currentThread().setContextClassLoader(classLoader);
          }
          try {
              ObjectMapper mapper = poolMapper.borrowObject();
              if (mapper != null) {
                  try {
                      return mapper.writeValueAsString(obj);
                  } finally {
                      poolMapper.returnObject(mapper);
                  }
              }
              throw new SerializationException("No ObjectMapper instance avaialble!");
          } catch (Exception e) {
              throw e instanceof SerializationException ? (SerializationException) e
                      : new SerializationException(e);
          } finally {
              Thread.currentThread().setContextClassLoader(oldClassLoader);
          }
      }
docs: Serialize an object to JSON string, with a custom class loader.
  
  @param obj
  @param classLoader
  @return