Different behaviour for lambda and anonymous transformer - spring-integration

Question

When a lambda expression is used to define a transformer that accepts a source of type Message, a ClassCastException is raised. However if a semantically identical transformer is defined with an anonymous inner class, no errors occur.
#Bean
public IntegrationFlow lambda(MessageChannel outputChannel) {
return f -> f
.enrichHeaders(h -> h.header("add", 1))
.<Message<Integer>, Integer>transform(m -> m.getPayload() + m.getHeaders().get("add", Integer.class))
.channel(outputChannel);
}
#Bean
public IntegrationFlow anonymous(MessageChannel outputChannel) {
return f -> f
.enrichHeaders(h -> h.header("add", 1))
.transform(new GenericTransformer<Message<Integer>, Integer>() {
#Override
public Integer transform(Message<Integer> source) {
return source.getPayload() + source.getHeaders().get("add", Integer.class);
}
})
.channel(outputChannel);
}
The full stack trace is:
org.springframework.integration.transformer.MessageTransformationException: Failed to transform Message; nested exception is org.springframework.messaging.MessageHandlingException: nested exception is java.lang.ClassCastException: java.lang.Integer cannot be cast to org.springframework.messaging.Message, failedMessage=GenericMessage [payload=0, headers={add=1, id=ed1e39d9-a91b-0bdf-5083-48218ba4e26f, timestamp=1542652005152}], failedMessage=GenericMessage [payload=0, headers={add=1, id=ed1e39d9-a91b-0bdf-5083-48218ba4e26f, timestamp=1542652005152}]
at org.springframework.integration.transformer.MessageTransformingHandler.handleRequestMessage(MessageTransformingHandler.java:114)
at org.springframework.integration.handler.AbstractReplyProducingMessageHandler.handleMessageInternal(AbstractReplyProducingMessageHandler.java:123)
at org.springframework.integration.handler.AbstractMessageHandler.handleMessage(AbstractMessageHandler.java:169)
at org.springframework.integration.dispatcher.AbstractDispatcher.tryOptimizedDispatch(AbstractDispatcher.java:115)
at org.springframework.integration.dispatcher.UnicastingDispatcher.doDispatch(UnicastingDispatcher.java:133)
at org.springframework.integration.dispatcher.UnicastingDispatcher.dispatch(UnicastingDispatcher.java:106)
at org.springframework.integration.channel.AbstractSubscribableChannel.doSend(AbstractSubscribableChannel.java:73)
at org.springframework.integration.channel.AbstractMessageChannel.send(AbstractMessageChannel.java:459)
at org.springframework.integration.channel.AbstractMessageChannel.send(AbstractMessageChannel.java:400)
at org.springframework.messaging.core.GenericMessagingTemplate.doSend(GenericMessagingTemplate.java:187)
at org.springframework.messaging.core.GenericMessagingTemplate.doSend(GenericMessagingTemplate.java:166)
at org.springframework.messaging.core.GenericMessagingTemplate.doSend(GenericMessagingTemplate.java:47)
at org.springframework.messaging.core.AbstractMessageSendingTemplate.send(AbstractMessageSendingTemplate.java:109)
at org.springframework.integration.handler.AbstractMessageProducingHandler.sendOutput(AbstractMessageProducingHandler.java:415)
at org.springframework.integration.handler.AbstractMessageProducingHandler.produceOutput(AbstractMessageProducingHandler.java:334)
at org.springframework.integration.handler.AbstractMessageProducingHandler.sendOutputs(AbstractMessageProducingHandler.java:225)
at org.springframework.integration.handler.AbstractReplyProducingMessageHandler.handleMessageInternal(AbstractReplyProducingMessageHandler.java:129)
at org.springframework.integration.handler.AbstractMessageHandler.handleMessage(AbstractMessageHandler.java:169)
at org.springframework.integration.dispatcher.AbstractDispatcher.tryOptimizedDispatch(AbstractDispatcher.java:115)
at org.springframework.integration.dispatcher.UnicastingDispatcher.doDispatch(UnicastingDispatcher.java:133)
at org.springframework.integration.dispatcher.UnicastingDispatcher.dispatch(UnicastingDispatcher.java:106)
at org.springframework.integration.channel.AbstractSubscribableChannel.doSend(AbstractSubscribableChannel.java:73)
at org.springframework.integration.channel.AbstractMessageChannel.send(AbstractMessageChannel.java:459)
at org.springframework.integration.channel.AbstractMessageChannel.send(AbstractMessageChannel.java:400)
at org.example.LambdaVsAnonymousTest.lambda(LambdaVsAnonymousTest.java:39)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.springframework.test.context.junit4.statements.RunBeforeTestExecutionCallbacks.evaluate(RunBeforeTestExecutionCallbacks.java:74)
at org.springframework.test.context.junit4.statements.RunAfterTestExecutionCallbacks.evaluate(RunAfterTestExecutionCallbacks.java:84)
at org.springframework.test.context.junit4.statements.RunBeforeTestMethodCallbacks.evaluate(RunBeforeTestMethodCallbacks.java:75)
at org.springframework.test.context.junit4.statements.RunAfterTestMethodCallbacks.evaluate(RunAfterTestMethodCallbacks.java:86)
at org.springframework.test.context.junit4.statements.SpringRepeat.evaluate(SpringRepeat.java:84)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:251)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:97)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:70)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:190)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:538)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:760)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:460)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:206)
Caused by: org.springframework.messaging.MessageHandlingException: nested exception is java.lang.ClassCastException: java.lang.Integer cannot be cast to org.springframework.messaging.Message, failedMessage=GenericMessage [payload=0, headers={add=1, id=ed1e39d9-a91b-0bdf-5083-48218ba4e26f, timestamp=1542652005152}]
at org.springframework.integration.handler.LambdaMessageProcessor.processMessage(LambdaMessageProcessor.java:125)
at org.springframework.integration.transformer.AbstractMessageProcessingTransformer.transform(AbstractMessageProcessingTransformer.java:113)
at org.springframework.integration.transformer.MessageTransformingHandler.handleRequestMessage(MessageTransformingHandler.java:108)
... 55 more
Caused by: java.lang.ClassCastException: java.lang.Integer cannot be cast to org.springframework.messaging.Message
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.springframework.integration.handler.LambdaMessageProcessor.processMessage(LambdaMessageProcessor.java:122)
... 57 more
I am wondering which is the desired behaviour; should the transformer be able to accept a Message or should it fail because the payload type is expected? However I think that the behaviour should be identical for both styles.
A sample project can be found here. Please check org.example.LambdaVsAnonymous and org.example.LambdaVsAnonymousTest.

Answer 1

I believe you have to use the typed variant of the transform(..). For example .transform(Message.class, m -> m.getPayload() + m.getHeaders().get("add", Integer.class)).
That said I believe we should improve javadoc a bit since it talks about <P> the payload type - 'transform from' while it should simply talk about type of input transformer should accept. .

Answer 2

It's a limitation of Java Lambdas.
Method.getParameterTypes()
just returns Object when invoked on a Lambda.
The type information here...
.<Message<Integer>, Integer>transform(m -> m.getPayload() + m.getHeaders().get("add", Integer.class))
...is just syntactic sugar to allow you to reference the proper types in the lambda (similar to doing explicit casting within the lambda). It is not baked into the lambda.
There is just no way to infer that the lambda expects a Message<?>.
With the anonymous inner class, the type information is retained since the method has concrete types.