A Clojure abstraction for modifiable/readable class loaders.


dynapath Build Status

dynapath provides a protocol and util functions for class loaders that make their effective classpaths readable and/or modifiable.


Clojure uses a clojure.lang.DynamicClassLoader by default (an extension of java.net.URLClassLoader), which provides .getURLs for reading the effective classpath and .addURL for modifying it. It's common for projects that need to read or modify the effective classpath to assume that a URLClassLoader is always available. But in some environments, the available class loader may not be a URLClassLoader, and may not be readable or modifiable.

Some projects (notably pomegranate) handle this by providing a protocol that can be implemented for other class loaders that may provide similar functionality.

dynapath provides a protocol that is based on an extraction of pomegranate's protocol, and is intended to be a standard way for accessing or modifying the effective classpath. Using dynapath in your library instead of assuming a class loader or implementing your own protocol provides the following benefits:

  • Your library can work with any modifiable/readable class loader without any changes
  • Any project that has already implemented DynamicClasspath for whatever esoteric class loader they are using will not need any other changes to use your library as well


Add it as a dependency:

For a Leiningen/Boot project:

[org.tcrawley/dynapath "0.2.5"]

For a maven project:


If you need to access or modify the effective classpath:

(require '[dynapath.util :as dp])

;; returns a seq of the urls for the classloader. Takes any classloader
;; (whether it implements DynamicClasspath or not) and does the right thing
(dp/classpath-urls a-classloader)
;; returns a seq of all the urls available from the classloader and its 
;; parentage chain
(dp/all-classpath-urls a-classloader)

;; adds a url to the given classloader if it is addable
(dp/add-classpath-url a-classloader a-url)

Loading the dynapath.defaults namespace will automatically implement classpath-urls and add-classpath-url for URLClassLoader and DynamicClassLoader.

If you need to implement DynamicClasspath:

(require '[dynapath.dynamic-classpath :as dc])

(extend-type AReadableButNotModifiableClassLoader
  (can-read? [_] true)
  (can-add? [_] false)
  (classpath-urls [cl] (seq ...)))

(extend AReadableAndModifiableClassLoader
  (assoc dc/base-readable-addable-classpath ;; implements can-read? and can-add?
         :classpath-urls (fn [cl] ...)
         :add-classpath-url (fn [cl url] ...)))

Note on Java 9

If you are using Java 9, you'll have to use dynapath 0.2.5. And under Java 9, URLClassLoader instances won't be modifiable via dynapath by default, since it uses reflection to access the protected method addURL, and security changes prevent calling .setAccessible on it. You can work around that by passing --add-opens java.base/java.net=ALL-UNNAMED to java.

If you are building dynapath under Java 9, you'll have to provide that options for the tests to pass:

JVM_OPTS="--add-opens java.base/java.net=ALL-UNNAMED" lein test-all

Who's using it?

Are you using it? If so, add yourself to this list and send me a PR.


Copyright © 2012-2016 Tobias Crawley

Distributed under the Eclipse Public License.