what is need of java service while having flow service
There are some scenario, where the business logic may be very complex. If you go for java service you can do that in simple steps, but in flow service it may be too lengthy to implement the same business logic.
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I know I sound like a broken record but this is patently inaccurate. The need to implement “complex business logic” as a reason to use Java services is very ambiguous, misleading and ultimately wrong. IMO, Java services are best used for small, focused utilities. One should not have the majority of a given integration implemented in Java services.
As Mark points out, there are many, threads with long discussions on this topic.