Clojure Q&A - Recent questions tagged gen-class

Why gen-class does not generate default interface methods?

Mon, 21 Aug 2023 17:30:20 +0000

When using gen-class, the generated .class doesn't have the default methods from java interfaces.

I created this repro, you can confirm a exception is throw when trying to call the method from the generated class from gen-class:

clj -T:build jar to generate the classes at target/classes
java -cp clj -Spath client/main/Main.java

A exception is throw:

Exception in thread "main" java.lang.UnsupportedOperationException: otherthing (clojure-sample.foo/-otherthing not defined?)
    at clojure_sample.Foo.otherthing(Unknown Source)
    at client.main.Main.main(Main.java:9)

The not ideal workaround is to add the default method in the namespace implementing what is done in the default method inside the java interface.

Fails to load gen-classed class with pf4j from uberjar

Tue, 26 Jan 2021 16:23:57 +0000

The problem here might be more related to the pf4j default classloader but still:

I have a clojure plugin generating a class

(gen-class
 :name myplugin.SomeExtension
 :implements [plugin.SomeExtensionPoint]
 :prefix "-"
 :impl-ns my-plugin.some-extension)

The plugin jar is loaded in my clojure application using pf4j. When it is compiled with lein jar the class loads (but then I naturally have to put any plugin dependencies in the apps project.clj). When compiled with lein uberjar it fails with:

 Execution error (IllegalStateException) at myplugin.SomeExtension/<clinit> (REPL:-1).
; Attempting to call unbound fn: #'clojure.core/refer

with top of call stack

jdk.internal.reflect.NativeConstructorAccessorImpl/newInstance0 (NativeConstructorAccessorImpl.java:-2)
jdk.internal.reflect.NativeConstructorAccessorImpl/newInstance (NativeConstructorAccessorImpl.java:62)
jdk.internal.reflect.DelegatingConstructorAccessorImpl/newInstance (DelegatingConstructorAccessorImpl.java:45)
java.lang.reflect.Constructor/newInstance (Constructor.java:490)
java.lang.Class/newInstance (Class.java:584)
org.pf4j.DefaultExtensionFactory/create (DefaultExtensionFactory.java:38)
org.pf4j.ExtensionWrapper/getExtension (ExtensionWrapper.java:37)
org.pf4j.AbstractPluginManager/getExtensions (AbstractPluginManager.java:971)

I've struggled a bit but I am unsure about exactly what happens here and how to solve it?
Any ideas are most welcome!

(and the uberjar profile uses {:aot :all :omit-source true})

How to invoke native C-functions in Clojure via JNR-FFI?

Fri, 04 Dec 2020 13:57:56 +0000

Hi Everyone,

I want to call native functions in a C-Library from Clojure via JNR-FFI without having to write Java code.

(defproject mypkg
	:dependencies [[org.clojure/clojure "1.10.1"]
				   [com.github.jnr/jnr-ffi "2.2.0"]])

Based on the jnr-ffi examples here and here a Java wrapper might look like

package blahblah;

public class BlahNative {
	private static int[] intDummy;
	private static double[] doubleDummy;

	public BlahNative() {
	}

	public static native void native_a(char var0, char var1, int var2, int var3, int var4, double[] var7, int var8, int var9);

	public static native void native_b(char var0, char var1, int var2, int var3, int var4, double[] var7, int var8, int var9);

	static {
		lib = LibraryLoader
            .create(LibSodium.class)
            .search("/usr/local/lib")
            .search("/opt/local/lib")
            .search("/usr/lib")
            .search("/lib")
            .load(LIBRARY_NAME);
        initializeLibrary(lib);
    }
}

I presume my Clojure namespace should use the gen-class feature for implementing Java classes.

(ns mypkg.native
	(:import [jnr.ffi LibraryLoader])
	(:import [jnr.ffi.annotations IgnoreError])
	(:import [jnr.ffi.provider FFIProvider])
    (:gen-class))

Then,

How do I write a native method in Clojure? In other words, do I even need to specify the method type (as native) when using Clojure's Java interop features?
After loading the native library I do not know how to access the C-functions in the library. How do I do it?
To avoid reloading the library for every Clojure wrapper function of each C-functions, can I write a method like

(defn lib (. LibraryLoader load LIBRARY_NAME))

and then access the native functions in it?

I am neither a C nor a Java expert so any guidance will be much appreciated.

Thanks.

Is it possible to gen-class with a private final field?

Fri, 26 Jul 2019 01:07:51 +0000

How to create a Java class with a private final field in Clojure?

ClojureDocs for gen-class says that state field will be public

:state name - if supplied, a public final instance field with the given name will be
created.

So, in other words, do we have a way to create a class and instantiate a java object with the encapsulated state?