I'm afraid that still doesn't make any sense. I imagine many APIs were used to create Jira, some in the IDEs, some in the build tools, some in the test tools, and some were included within it. The application then has it's own APIs for various reasons.
What are you trying to do? And what API are you looking for?
The question "what API" is of no help - APIs, by their definition, are a way to talk to an existing system. You have to put your question in context so we know what system you are asking about.
From where? Just to emphasize what Nic said, you have to explain in detail what you are trying to do and and from where you want to create Jira issue.
If we give answers like the ones below, will it help you? (it's the same when you ask a questions without giving us the details)
So why didn't you say that instead of saying you wanted to develop on top of Jira?
As I said before, it's built on top of several libraries, but they're all pretty standard and you'll call them when you need them. It's pointless looking at Swing though - Jira has no gui in that sense, because it's a web-application.
Rather than trying to ask what it's built on, you really should concentrate on what you need it to do instead. If you're expanding/extending Jira, it's own API is far more useful because it's designed for the data within Jira. If you run into stuff that's in standard libraries, you'll find that Jira uses them in standard ways and if you need to extend them, you'll need to do it in the context in which you need to make the change. If you find Jira can't help you do something, then you'll need to add the most appropriate library for yourself.
Nic, relax, take two deep breaths :)
Hey Sofien, can you spend some time, write a detailed explaination of what you are trying to achieve? We can think about the way to achieve it later. To be precise, this exact line in Nic's comment "Rather than trying to ask what it's built on, you really should concentrate on what you need it to do instead."
We are all here to help you, provided you can get us a detailed writeup on what you are trying to achieve.
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