retrofit-converters/jackson/src/main/java/retrofit2/converter/jackson/JacksonConverterFactory.java
@SuppressWarnings("ConstantConditions") // Guarding public API nullability.
  public static JacksonConverterFactory create(ObjectMapper mapper) {
    if (mapper == null) throw new NullPointerException("mapper == null");
    return new JacksonConverterFactory(mapper);
  }
@SuppressWarnings("ConstantConditions") // Guarding public API nullability.
  public static JacksonConverterFactory create(ObjectMapper mapper) {
    if (mapper == null) throw new NullPointerException("mapper == null");
    return new JacksonConverterFactory(mapper);
  }
Create an instance using {@code mapper} for conversion.