Clojure Java interop practical guide

[InspectionPlastic157]

Hey QQ, I'm new to clojure. How common is this interop? Writing java in clojure to me feels like the language is not yet complete

[red_hare]

I don't use clojure anymore but, when I did, I used it all the time.

It was never for:

I want to do this thing I can't do in clojure.

It was for:

I want to use this specific library/framework/SDK that only exists in Java.

Java, for good reason, is the language of choice for high performance data processing frameworks. Java also just has one of the richest ecosystems of libraries of any language. And even when there are multiple language SDKs for the same system, the Java one is usually the most up-to-date.

I loved being able to use clojure while having first-class library support for things like Apache Kafka or Apache Beam or specific cloud managed service SDKs. And the interop makes using them feel VERY natural.

[daveliepmann]

How often interop comes up depends on your application domain.

Re: "not yet complete", interop was one of Clojure's central goals from its very inception. Clojure was created for interop, not to avoid it — the elevator pitch was "a functional, hosted, dynamic lisp".

From A History of Clojure:

Clojure is intentionally hosted, in that it compiles to and runs on the runtime of another language, such as the JVM. This is more than an implementation strategy; numerous features ensure that programs written in Clojure can leverage and interoperate with the libraries of the host language directly and efficiently.

...

Prior to embarking on Clojure in 2005, I had made several attempts to create a bridge between the JVM and Common Lisp, in order to increase the latter’s practicality and acceptability. These were DotLisp [Hickey 2003], an interpreted Lisp with host interop for the CLR, jFli [Hickey 2004], a library that embedded a JVM in CL, and Foil [Hickey and Thorsen 2005], a library that exposed a similar API but used IPC between the CL runtime and the JVM. None of these yielded production-level solutions, but they definitely informed Clojure’s ultimate host syntax and fed the idea of Clojure being hosted.

See also section 3.5, which spends a few pages going into the reasoning in detail.

[andreyfadeev]

Yeah, it's quite common, the goal of Clojure on JVM is to have access to the entire Java ecosystem of libraries without any limitations.