Use JSON transformer in Spring Integration - spring

Question

I have a problem that seems unaddressed in any of the examples I can find.
My application reads an ActiveMQ topic of JSON messages. It will build a completely new outbound REST call based on this data. Note that this is not a "transformation". It is given "X" produce "Y" i.e. ServiceActivator.
My flows thus far are
public IntegrationFlow splitInputFlow() {
return IntegrationFlows.from("inboundJmsChannel")
.split()
.log(LoggingHandler.Level.DEBUG)
.route(Message.class, m -> m.getHeaders().get("x-bn-class").equals("Healthcheck.class") ? "healthcheckChannel" : "metricChannel")
.get();
}
public IntegrationFlow healthcheckFlow() {
return IntegrationFlows.from("healthcheckChannel")
.log(LoggingHandler.Level.DEBUG)
.transform(Transformers.fromJson(Healthcheck.class))
.handle("healthcheckActivator", "process")
.get();
}
There are dozens of examples on how to use spring transformers. I have even considered trying a MessageConverter. But I don't see why it would help and it doesn't seem to be the normal approach.
The main problem here is that Integration calls healthcheckActivator.process(String payload). The payload itself is the expected valid JSON string.
I am a little surprised it does not call healtcheckActivator.process(Message payload) but But that wouldn't help so it doesn't much matter.
The real question is why does it not call healtcheckActivator.process(Healthcheck healthcheck)?
Well actually I understand "why". It is because DSL generates an internal channel to tie the steps together and as far as I understand anything on a channel is a spring.messaging.Message.
I can easily instantiate my Healthcheck object once I get inside the SA. But that leaves the nagging question: What possible good is the entire transform step? If it always "serializes" the object back into a Message -- what's the point.
Like I said I think I'm missing something fundamental here.
EDIT
My new (and probably last) idea is maybe I'm publishing it wrong.
To publish it I am using
jmsTemplate.convertAndSend(topicName, healthcheck, messagePostProcessor -> {
messagePostProcessor.setJMSType("TextMessage");
messagePostProcessor.setStringProperty("x-bn-class", "Healthcheck.class");
messagePostProcessor.setStringProperty("x-bn-service-name", restEndpoint.getServiceName());
messagePostProcessor.setStringProperty("x-bn-service-endpoint-name", restEndpoint.getEndpointName());
messagePostProcessor.setLongProperty("x-bn-heathcheck-timestamp", queryDate);
messagePostProcessor.setStringProperty("x-bn-healthcheck-status", subsystemStatus.getStatus(subsystemStatus));
messagePostProcessor.setIntProperty("httpStatus", httpStatus.value());
return messagePostProcessor;
});
What arrives in the SI process(String payload) method is:
LoggingHandler - GenericMessage [payload={"healthcheckType":"LOCAL","outcome":"PASS","dependencyType":"DB","endpoint":"NODE TABLE","description":"Read from DB","durationSecs":0.025}, headers={x-bn-service-name=TG10-CS2, x-bn-service-endpoint-name=TG Q10-CS2 Ready Check, jms_destination=topic://HEALTH_MONITOR, _type=com.healthcheck.response.Healthcheck, x-bn-heathcheck-timestamp=1558356538000, priority=4, jms_timestamp=1558356544244, x-bn-healthcheck-status=SEV0, jms_redelivered=false, x-bn-class=Healthcheck.class, httpStatus=200, jms_type=TextMessage, id=b29ffea7-7128-c543-9a14-8bab450f0ac6, jms_messageId=ID:39479-1558356520091-1:2:1:1:1, timestamp=1558356544409}]
I hadn't noticed the _type parameter in the jms_destination header before. But before I started screwing around with this (because it didn't work) that is the correct class name for what the other team provided.
I have not implemented a JMS message converter. But the supplied SimpleMessageConverter seems that it should do exactly what I want.

Answer 1

Your understanding is correct; works fine for me, so something else is going on...
#SpringBootApplication
public class So56169938Application {
public static void main(String[] args) {
SpringApplication.run(So56169938Application.class, args);
}
#Bean
public IntegrationFlow flow() {
return IntegrationFlows.from(() -> "{\"foo\":\"bar\"}", e -> e.poller(Pollers.fixedDelay(5000)))
.transform(Transformers.fromJson(Foo.class))
.handle("myBean", "method")
.get();
}
#Bean
public MyBean myBean() {
return new MyBean();
}
public static class MyBean {
public void method(Foo foo) {
System.out.println(foo);
}
}
public static class Foo {
private String foo;
String getFoo() {
return this.foo;
}
void setFoo(String foo) {
this.foo = foo;
}
#Override
public String toString() {
return "Foo [foo=" + this.foo + "]";
}
}
}
and
Foo [foo=bar]
Foo [foo=bar]
Foo [foo=bar]
Foo [foo=bar]
Foo [foo=bar]
Foo [foo=bar]

Answer 2

Well, Spring Integration is a Messaging framework. It transfers messages from endpoint to endpoint via channels in between. That's already the target endpoint responsibility to deal with consumed message the proper way. The framework doesn't care about the payload. It is really a business part of the target application. That's how we can make framework components as generic as possible leaving the room for target business types for end-users.
Anyway the Framework provides some mechanisms to interact with payloads. We call it POJO method invocation. So, you provide some business with arbitrary contract, however following some Spring Integration rules: https://docs.spring.io/spring-integration/docs/current/reference/html/#service-activator.
So, according your description it is really a surprise that it doesn't work for healtcheckActivator.process(Healthcheck healthcheck). Your transform(Transformers.fromJson(Healthcheck.class)) should really produce a Message with Healthcheck object as a payload. The framework consults a method signature and tries to map a payload and/or headers to the method invocation arguments, having the whole message as a container for data to delegate to the method call.
From here it would be great to see your healtcheckActivator.process() method to determine why the transform(Transformers.fromJson(Healthcheck.class)) result cannon be mapped to that method arguments.