code: public static AuthleteApi getDefaultApi()
      {
          if (sDefaultApi != null)
          {
              return sDefaultApi;
          }
  
          synchronized (AuthleteApiFactory.class)
          {
              if (sDefaultApi != null)
              {
                  return sDefaultApi;
              }
  
              // Load an Authlete configuration file.
              AuthleteConfiguration ac = new AuthletePropertiesConfiguration();
  
              // Create an AuthleteApi instance using the configuration.
              sDefaultApi = create(ac);
  
              return sDefaultApi;
          }
      }
docs: Get the default instance of {@link AuthleteApi}.
  
  <p>
  This method loads a configuration file (using {@link
  AuthletePropertiesConfiguration}) on the first call, creates
  an instance of {@link AuthleteApi} and caches the instance.
  The second and subsequent calls return the cached instance.
  </p>
  
  <p>
  The default name of the configuration file is {@code
  authlete.properties}, but it can be changed by a system property
  {@code authlete.configuration.file}. The current directory and
  the classpath are searched for the configuration file in this order.
  </p>
  
  @return
  An instance of {@code AuthleteApi}.
  
  @since 1.29