At the moment, I have a completely functional Clojure library which is called from Java.
The way I do this : I have a file that uses gen-class to wrap the entire API as static methods of a single class and passes data in and out in the form of IPersistentVector and IPersistentMap.
Now, however, I'm refactoring the library and putting the functionality behind various Protocols.
I have four protocols, lets call them A, B, C and D. And two defrecords, X and Y. X and Y both implement protocols A, B and C. While Y also implements D.
What do I need to do to make these available to Java? Are these automatically available as Interfaces and Classes? Or do I still have to do the equivalent of the gen-class to make them public?
If not, what is the equivalent of the gen-class :methods clause, where I define the Java types for the arguments to the methods?
Does anyone have a simple example of making Protocols and records available to Java?
defprotocol
Every Clojure protocol is also a Java interface with the same name and methods. If I take an example from ibm developerworks, we see that :
(ns com.amalgamated)
(defprotocol Fulfillment
(invoice [this] "Returns an invoice")
(manifest [this] "Returns a shipping manifest"))
Is equivalent to :
package com.amalgamated;
public interface Fulfillment {
public Object invoice();
public Object manifest();
}
Clojure.org also has some (rather terse) information on this.
A Java client looking to participate in the protocol can do so most efficiently by implementing the protocol-generated interface. External implementations of the protocol (which are needed when you want a class or type not in your control to participate in the protocol) can be provided using the extend construct:
(extend AType AProtocol {:foo an-existing-fn :bar (fn [a b] ...) :baz (fn ([a]...) ([a b] ...)...)} BProtocol {...} ...)
definterface
If you are aiming at performance, you could consider using definterface
, which use is similar to the protocols. This SO post also has details about how to use it :
(definterface Foo
[^int foo [x ^String y]]
[^void bar [^ints is]])
definterface
seem to be faster than protocols.
defrecord
Similarly, record
s (as well as deftype
and definterface
) will generate Java Classes.
Again, Clojure.org/datatypes has useful information (emphasis mine) :
deftype and defrecord dynamically generate compiled bytecode for a named class with a set of given fields, and, optionally, methods for one or more protocols and/or interfaces. They are suitable for dynamic and interactive development, need not be AOT compiled, and can be re-evaluated in the course of a single session. They are similar to defstruct in generating data structures with named fields, but differ from defstruct in that: [...]
So yes if will be available from Java. Just be careful with naming.
As a side note, you may want to have a look at calling Clojure from Java.
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