Figured it out.
org.jberet.support.io.JsonItemWriter, in fact, any ItemWriter, takes java.util.List<Object> as input. So in case of JsonItemWriter, any type that can be properly converted to JSON format by Jackson, which is the library used by jberet-support underneath. The data type class is either pretty simple to convert, or contain some JAXB or Jackson annotations to help the mapping. An example class: