How to add an URL into the classpath?

[Yoshinori Kohyama]

Hello clojurians,

Adding an URL to the classpath dynamically in REPL succeeded.

  $ java -cp clojure-1.3.0.jar clojure.main

  Clojure 1.3.0

  user=> (def ccl (.getContextClassLoader (Thread/currentThread)))

  #'user/ccl

  user=> (class ccl)

  clojure.lang.DynamicClassLoader

  user=> (.addURL ccl (java.net.URL. "file:///some/path/"))

  nil

But the program below fails.

  $ cat addcp.clj 

  (def ccl (.getContextClassLoader (Thread/currentThread)))

  (println (class ccl))

  (.addURL ccl (java.net.URL. "file:///some/path/"))

  $ java -cp clojure-1.3.0.jar clojure.main addcp.clj 

  sun.misc.Launcher$AppClassLoader

  Exception in thread "main" java.lang.IllegalArgumentException: No matching method found: addURL for class sun.misc.Launcher$AppClassLoader

at clojure.lang.Reflector.invokeMatchingMethod(Reflector.java:52)

at clojure.lang.Reflector.invokeInstanceMethod(Reflector.java:30)

at user$eval3.invoke(addcp.clj:3)

at clojure.lang.Compiler.eval(Compiler.java:6465)

at clojure.lang.Compiler.load(Compiler.java:6902)

at clojure.lang.Compiler.loadFile(Compiler.java:6863)

at clojure.main$load_script.invoke(main.clj:282)

at clojure.main$script_opt.invoke(main.clj:342)

at clojure.main$main.doInvoke(main.clj:426)

at clojure.lang.RestFn.invoke(RestFn.java:408)

at clojure.lang.Var.invoke(Var.java:401)

at clojure.lang.AFn.applyToHelper(AFn.java:161)

at clojure.lang.Var.applyTo(Var.java:518)

at clojure.main.main(main.java:37)

While in REPL the loader gotten with .getContextClassLoader is an instance of clojure.lang.DynamicClassLoader,

the latter is an sun.misc.Launcher$AppClassLoader.

How can I get an clojure.lang.DynamicClassLoader?

Or am I wrong with something?

Please teach me any information about this.

Regards,

Yoshinori Kohyama

[Stuart Sierra]

Hello Yoshinori,

In general, you cannot modify the JVM classpath at runtime. Clojure uses its own classloader to do dynamic code generation, but you cannot rely on being able to control the classloader which is running the whole JVM.

-S

[Yoshinori Kohyama]

Hello programmers,

Hi, Sierra.  Thank you replying.

O.K. I can not always contol the classloader of the whole JVM.

Then, how can I get the dynamic classloader for my current code/thread?

Or can't I?

Regards,

Yoshinori Kohyama

[Stuart Sierra]

You can't. The dynamic classloader is an internal implementation detail of Clojure; you can't rely on it being available anywhere.

If you're interested in runtime control over the the Java classpath, look at https://github.com/cemerick/pomegranate

-S

[Yoshinori Kohyama]

Hi Sierra,

Thank you for your kind and quick answers to my questions.

I see.

I'll read the page you referred.

Regards,

Yoshinori Kohyama

[Yoshinori Kohyama]

Hello clojurians,

It seems not to be able to control the whole classpath of my runtime.

But my aim, compiling with a path given at runtime and execute it, has been achieved,

with

  (.setContextClassLoader (Thread/currentThread)

    (DynamicClassLoader. (.getContextClassLoader (Thread/currentThread)))

Details reported below.

Regards,

Yoshinori Kohyama

$ cat project.clj 

(defproject dce "0.0.1"

  :description "Dynamic Compling And Execution"

  :dependencies [[org.clojure/clojure "1.3.0"]]

  :main dce.core)

$ cat src/dce/core.clj 

(ns dce.core

  (:import java.net.URL clojure.lang.DynamicClassLoader)

  (:gen-class))

(defn -main [abs-path target-ns & args]

  (let [ccl (.getContextClassLoader (Thread/currentThread))

        dcl (if (instance? DynamicClassLoader ccl) ccl

                (let [l (DynamicClassLoader. ccl)]

                  (.setContextClassLoader (Thread/currentThread) l)

                  l))]

    (.addURL dcl (URL. (str "file://" abs-path "/src/")))

    (.addURL dcl (URL. (str "file://" abs-path "/classes/")))

    (binding [*compile-path* (str abs-path "/classes")]

      (compile (symbol target-ns)))

    (def f (future (apply (resolve (symbol (str target-ns "/-main"))) args)))

    (Thread/sleep 5000)

    (future-cancel f)))

$ tree samples

samples

├── classes

└── src

    └── foo

        └── core.clj

3 directories, 1 file

$ lein repl

REPL started; server listening on localhost port 5997

dce.core=> (-main (.getCanonicalPath (java.io.File. "samples")) "foo.core" "arg1" "arg2")

Foo:  arg1 arg2

Foo:  arg1 arg2

Foo:  arg1 arg2

Foo:  arg1 arg2

Foo:  arg1 arg2

Foo:  arg1 arg2

true

dce.core=> ^D

$ tree samplessamples

├── classes

│   └── foo

│       ├── core$_main.class

│       ├── core$loading__4505__auto__.class

│       └── core__init.class

└── src

    └── foo

        └── core.clj

4 directories, 4 files

$ lein uberjar

...

Created ... dce/dce-0.0.1-standalone.jar

$ rm -Rf samples/classes/*

$ tree samples

samples

├── classes

└── src

    └── foo

        └── core.clj

3 directories, 1 file

$ java -jar dce-0.0.1-standalone.jar `pwd`/samples foo.core arg1 arg2

Foo:  arg1 arg2

Foo:  arg1 arg2

Foo:  arg1 arg2

Foo:  arg1 arg2

Foo:  arg1 arg2

^C

$ tree samples

samples

├── classes

│   └── foo

│       ├── core$_main.class

│       ├── core$loading__4505__auto__.class

│       └── core__init.class

└── src

    └── foo

        └── core.clj

4 directories, 4 files

[Yoshinori Kohyama]

I forgot to show foo/core.clj

$ cat samples/src/foo/core.clj 

(ns foo.core)

(defn -main [& args]

  (loop []

    (println "Foo: " (apply str (interpose " " args)))

    (Thread/sleep 1000)

    (recur)))

[Vladimir Tsichevski]

Hi Yoshinori,

after (unsuccessfully) struggling with Clojure dynamic ClassLoaders in clojure-1.4, I ended up with the 'brute force' solution--just add URL entries to the Java system classloader. Here is a simple example on how to do it from clojure:

(defn add-system-classpath

  "Add an url path to the system class loader"

  [url-string]

  (let [field (aget (.getDeclaredFields java.net.URLClassLoader) 0)]

    (.setAccessible field true)

    (let [ucp (.get field (ClassLoader/getSystemClassLoader))]

      (.addURL ucp (java.net.URL. url-string)))))

(add-system-classpath "file:/some/clojure/root/")

(load "some/clojure/file") ;; load the some/clojure/file.clj from the /some/clojure/root/ catalog

(add-system-classpath "file:/some/java/classes/")

some.java.MyClass ;; use the some.java.MyClass in the /some/java/classes/ catalog

Regards,

Vladimir

[Alex Ott]

You can also use https://github.com/cemerick/pomegranate - it works fine...

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[Yoshinori Kohyama]

Hi Vladimir and Alex,

Thank you for very useful informations.

With regards,

Yoshinori Kohyama