How to Read Files and trigger http rest multipart endpoint using Spring Integration - spring-integration

Question

I am following this spring integration example - https://github.com/iainporter/spring-file-poller
#Bean
public IntegrationFlow writeToFile(#Qualifier("fileWritingMessageHandler") MessageHandler fileWritingMessageHandler) {
return IntegrationFlows.from(ApplicationConfiguration.INBOUND_CHANNEL)
.transform(m -> new StringBuilder((String)m).reverse().toString())
.handle(fileWritingMessageHandler)
.log(LoggingHandler.Level.INFO)
.get();
}
#Bean (name = FILE_WRITING_MESSAGE_HANDLER)
public MessageHandler fileWritingMessageHandler(#Qualifier(OUTBOUND_FILENAME_GENERATOR) FileNameGenerator fileNameGenerator) {
FileWritingMessageHandler handler = new FileWritingMessageHandler(inboundOutDirectory);
handler.setAutoCreateDirectory(true);
handler.setFileNameGenerator(fileNameGenerator);
return handler;
}
Controller example
#PostMapping(value ="/data/{id}")
public String load( #RequestParam("jsonFile") MultipartFile jsonFile,
#PathVariable("id") Long id) throws JsonMappingException, JsonProcessingException{
//some business logic
return "Controller is called";
}
Instead of simple handling, I want to call a Rest endpoint that expects a file.
i.e. calling a rest api in handler similar to fileWritingMessageHandler
https://github.com/spring-projects/spring-integration-samples/blob/261648bed136a076f76ed15b1017f5e5b6d8b9ae/intermediate/multipart-http/src/main/resources/META-INF/spring/integration/http-outbound-config.xml
How can I create Map
Map<String, Object> multipartMap = new HashMap<String, Object>();
multipartMap.put("jsonFile", ????);
and call a getway method like
HttpStatus postMultipartRequest(Map<String, Object> multipartRequest);

Answer 1

To send a multi-part request you need to have a payload as a Map<String, Object>. You can read files from a directory using FileReadingMessageSource and respective poller configuration: https://docs.spring.io/spring-integration/docs/current/reference/html/file.html#file-reading. This one emits messages with java.io.File as a payload. To create a Map for it you just need a simple transformer in Java DSL:
.<File, Map<String, File>>transform(file -> Collections.singletonMap("jsonFile", file))
and then you use standard .handle(Http.outboundChannelAdapter("/data/{id}").uriVariable("id", "headers.someId")): https://docs.spring.io/spring-integration/docs/current/reference/html/http.html#http-java-config