How to calculate checksum/digest of a transferred file in spring-integration - spring-integration

Question

How/where can I compute md5 digest for a file I need to transfer to a samba location in spring-integration in order to validate it against the digest I receive at the beginning of the flow. I get the file from a rest service and I have to make sure file is safely landing to samba location. The middle flow looks like this: (the digest to be compared against is stored somewhere in the messages)
GenericHandler smbUploader;
HttpRequestExecutingMessageHandler httpDownloader;
from(inbound()) //here I receive a notification with url where to download file + a checksum to be validated against
...
.handle(httpDownloader) //here I get file effectively
.handle(smbUploader) //here I upload the file to samba
...
and httpDownloader is defined like this:
public HttpRequestExecutingMessageHandler httpDownloader(){
HttpRequestExecutingMessageHandler h = new HttpRequestExecutingMessageHandler ("payload.url");
h.setExpectedResponseType(String.class);
h.setHttpMethod(GET);
return h;
}
and smbUploader is defined like this:
public GenericHandler smbUploader (MessageHandler smbMessageHandler){
return new GenericHandler<Message>(){
#Override
public Message handle(Message m, MessageHeaders h){
smbMessageHandler.handleMessage(m);
return m;
}
}
and smbMessageHandler is defined like this:
public MessageHandler smbMessageHandler (SmbRemoteFileTemplate template, FileNameGenerator g){
SmbMessageHandler h = new smbMessageHandler (template, REPLACE);
h.setAutoCreateDirectory(true);
h.setRemoteDirectoryExpression(getExpression("headers['msg'].smbFolder"));
h.setFileNameGenerator(g);
return h;
}
the inbound (starting the flow) is defined like this:
public HttpRequestHandlerEndpointSpec inbound(){
return Http.inboundChannelAdapter ("/notification")
.requestMapping(m->m.methods(POST))
.requestPayloadType(String.class)
.validator(notificationValidator);
}

Answer 1

First of all you should store a digest in the message headers in the beginning of the flow.
Then you need to write a service method to calculate a checksum of the file you got downloaded. And insert a new handle() in between:
.handle(httpDownloader) //here I get file effectively
.handle(smbUploader) //here I upload the file to samba
to call your service method. The input for that method must be a whole Message, so you got access to the downloaded file in the payload and digest in the headers. The result of this method could be just your file to proceed into an SMB handler for uploading.
How to calculate a checksum you can find in this SO thread: Getting a File's MD5 Checksum in Java