Spring Integration DSL: How to add the HTTP.outboundGateway header?

Question

What is the easiest way to add the HTTP.outboundGateway header in my program?

What I want to do is:

I first do the HTTP GET for the URL

http://localhost:8050/session

then I get the JSON

{
   "session": "session8050"
}

I extract the value of the session variable and add that to the next HTTP GETas the session header variable.

Currently I have working code, but I was thinking could I do this easier? My implementation

1. Extracts the session variable from the JSON with the jsonPath method

2. Then the implementation adds the session variable to the integration flow message header with the enrichHeaders method

3. Then the implementation adds the session variable to the HTTP call header with the HeaderMapper class

   My implementation is

   integrationFlowBuilder
   .transform(p -> authenticationJson)
   .enrichHeaders(h -> h.header("Content-Type", "application/json"))
   .handle(Http.outboundGateway("http://localhost:8050/session").httpMethod(HttpMethod.POST)
           .expectedResponseType(String.class))
   .enrichHeaders(
           h -> h.headerExpression("session", "#jsonPath(payload, '$.session')", true)
   .handle(Http
           .outboundGateway(completeFromUrl)
           .httpMethod(HttpMethod.GET).mappedRequestHeaders("session").headerMapper(headerMapper())
           .expectedResponseType(String.class))
   

My headerMapper is

@Bean
HeaderMapper headerMapper() {
    final DefaultHttpHeaderMapper headerMapper = new DefaultHttpHeaderMapper();
    final String[] headerNames = { "session" };
    headerMapper.setOutboundHeaderNames(headerNames);

    headerMapper.setUserDefinedHeaderPrefix("");
    return headerMapper;
}

Is it possible to extract the session variable from the JSON and add it straight to the HTTP headers??

Why the HeaderMapper must be used? Why the integration flow message headers don't go straight to the HTTP.outboundGateway call as the payload goes?

Answer 1

First of all you need to understand that main goal of Spring Integration as any other EIP solution is to make components in the flow as isolated as possible, so in the future you can add some intermediate steps or remove without big impact for the whole solution and other components in your integration flow. This should be an answer to your questions about why HeaderMapper must be used.

As you see the contract of the HeaderMapper to remap MessageHeaders to the target protocol headers representation. There is nothing about payload, hence you need to map the value from the payload into the headers, first of all. And then say Http.outboundGateway() what should be remapped from the MessageHeaders into the HttpHeaders.

By default the DefaultHttpHeaderMapper (it is present there in the Http.outboundGateway()) maps only standard HTTP headers suitable for the HTTP request.

If you need to include some custom header, like in your case with that session, you really can use a custom configuration for the DefaultHttpHeaderMapper, or just configure a convenient option on the Http.outboundGateway():

.mappedRequestHeaders("session")

The setUserDefinedHeaderPrefix("") is not necessary from version 5.0. It is empty string by default now, since there is no requirements in the prefixes for custom headers in the HTTP protocol.