HTC J より送信
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送信者: "Alex Fowler" <alex.murat...@gmail.com>
宛先: <clojure@googlegroups.com>
件名: [BUG?] loading Clojure source files from various data sources (classloading)
日付: 木, 5月 21日, 2015年 20:25
Hello!
I am developing a Java application where Clojure is used as a library. The
application is required to load sources (including Clojure sources) from
arbitrary locations in filesystem, internet and maybe a dozen other places.
Hence I have developed corresponding classloaders for that, which properly
implement the required ClassLoader methods.
Loading Clojure sources is realized via this approach:
Compiler.LOADER.alterRoot(getMyClassLoader(), ...); // changing the
LOADER to my chain of classloaders
IFn require = Clojure.var("clojure.core", "require");
require.invoke(Clojure.read("example.namespace"));
where the contents of the "example.namespace" are:
(ns example.namespace
(:require [other.example.namespace :as other])))
I have figured that the loading happens with RT.load() which delegates to
RT.loadResourceScript(), so I just alter the Compiler.LOADER so that
RT.baseLoader() returns my classloaders. And in my classloaders I simply have
to override the getResourceAsStream and all works! The namespace
example.namespace starts loading ok. It successfully loads the whole
clojure.core and then begins to process the first require - the
other.example.namespace. And this is where something bad happens.
It appears to be that the (require) inside the ns form in the example.namespace
uses not the RT.load() method, but Compiler.load()where on line 7210 the
Compiler.LOADER, which by that moment points to the correct classloader created
by me, gets replaced by RT.makeClassLoader()where the current RT.baseLoader()
which is effectively Compiler.LOADER gets passed as the parent to the newly
created DynamicClassLoader.
So I get Compiler.LOADER which I have set to the correct classloader, replaced
by the DynamicClassLoader whose parent is my classloader. The problem is that
DynamicClassLoader inherits from URLClassLoader that inherits from ClassLoader
which, although accepts a parent classloader on its creation, does not delegate
getResourceAsStream() in case of its personal failure to its parent! That
results in that my classloader, although still present in the chain, never gets
called and the (require) in the ns form of example.namespace fails to locate
the required namespaces that *are* available at that data source.
The problem is that this behavior is hard-coded in Clojure and it looks like it
is impossible to change that. Clojure forces us to use a classloader that
inherits the getResourceAsStream() method from java.lang.ClassLoader which does
not allow delegation of this request to parent classloader. This effectively
prevents using Clojure as a library together with a plugin system because it
renders Clojure incapable of working with sources that are fetched from
elsewhere than the Java's URLClassLoader is capable of retrieving data from. It
is impossible to load sources from a database, or some internet data source or
an encrypted data source, or even another location in the filesystem. Wow!
We've went with Clojure and now are in the desperate need to resolve this
peculiar phenomenon.. What could be a proposed solution for this situation?
Thank you.
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