apollo-core/src/main/java/com/ctrip/framework/foundation/internals/DefaultProviderManager.java/DefaultProviderManager/toString
class DefaultProviderManager:  @Override
  public String toString() {
    StringBuilder sb = new StringBuilder(512);
    if (null != m_providers) {
      for (Map.Entry<Class<? extends Provider>, Provider> entry : m_providers.entrySet()) {
        sb.append(entry.getValue()).append("\n");
      }
    }
    sb.append("(DefaultProviderManager)").append("\n");
    return sb.toString();
  }