Service Activators and the `.handle()` method

The .handle() EIP method’s goal is to invoke any MessageHandler implementation or any method on some POJO. Another option is to define an “activity” by using lambda expressions. Consequently, we introduced a generic GenericHandler<P> functional interface. Its handle method requires two arguments: P payload and MessageHeaders headers (starting with version 5.1). Having that, we can define a flow as follows:

@Bean
public IntegrationFlow myFlow() {
    return IntegrationFlow.from("flow3Input")
        .<Integer>handle((p, h) -> p * 2)
        .get();
}

The preceding example doubles any integer it receives.

However, one main goal of Spring Integration is loose coupling, through runtime type conversion from message payload to the target arguments of the message handler. Since Java does not support generic type resolution for lambda classes, we introduced a workaround with an additional payloadType argument for the most EIP methods and LambdaMessageProcessor. Doing so delegates the hard conversion work to Spring’s ConversionService, which uses the provided type and the requested message to target method arguments. The following example shows what the resulting IntegrationFlow might look like:

@Bean
public IntegrationFlow integerFlow() {
    return IntegrationFlow.from("input")
            .<byte[], String>transform(p - > new String(p, "UTF-8"))
            .handle(Integer.class, (p, h) -> p * 2)
            .get();
}

We also can register some BytesToIntegerConverter within ConversionService to get rid of that additional .transform():

@Bean
@IntegrationConverter
public BytesToIntegerConverter bytesToIntegerConverter() {
   return new BytesToIntegerConverter();
}

@Bean
public IntegrationFlow integerFlow() {
    return IntegrationFlow.from("input")
             .handle(Integer.class, (p, h) -> p * 2)
            .get();
}

Also see Lambdas And Message<?> Arguments.

Service Activators and the .handle() method

Service Activators and the `.handle()` method