Many people consider the use of eval in "normal" code to be bad style. However, there are times when it is justified, such as genetic programming or dynamic code loading. In these cases Clojure falls down without the classloader trick.
On Wed, Mar 4, 2009 at 3:03 PM, Chas Emerick <cemer...@snowtide.com> wrote: > > Hendrik, > > I came across this issue with JNI libs some years ago, and also comes > up in the context of most app servers, which use a multitude of > classloaders to enable hot code reloading and such. The general > solution is to move your JNI lib into your boot classpath; in my > experience with Tomcat and Weblogic years ago, doing this caused the > boot classloader to link the JNI lib, making it available from all > child classloaders. See #3 here: > http://www.bostic.com/docs/bdb_current/ref/java/faq.html > > Creating a new classloader is really important to enable proper GC'ing > of any dynamically-generated classes later on, as others have said. > This also only happens when one loads code, which will generally only > happen at or near app startup outside of a development environment, so > performance isn't really a concern IMO. > > - Chas > > On Mar 4, 2009, at 10:21 AM, Hendrik wrote: > > > > > Hi, > > > > I got a question: Clojure seems to create poopillions of > > DynamicClassLoader instances. Why does it do that? Could I try > > patching it so that it creates less of them? I need this cause I ran > > into trouble working with JNI. > > > > I looked at the Clojure source rev 1323. I'm no expert, this is my > > first look at the sources, but the following struck me as hack-ish: > > (from Compiler.java, shortened slightly) > > > > public static Object eval(Object form) throws Exception{ > > if(true)//!LOADER.isBound()) > > makeClassLoader(); > > > > To me, that looks like for evaluating _every form_, a new classloader > > gets created. Isn't that a bit much? Also, it looks "temporary" that > > the if() has its condition commented out and replaced with > > true ... :-/ So Clojure's behavior seems to have been different at > > some point in the past. > > > > Thanks a bunch for your opinions. > > > > Cheers > > Hendrik > > > > > > > > Details: > > > > I'm asking because I was loading a native library through JNI. The > > rule here (apparently) is that you can load a library through only one > > classloader, not the same library through several loaders. > > > > For example, this program fails: > > > > (System/load "/home/hk/inotify-java-0.1/build/native/libinotify- > > java.so") > > (System/load "/home/hk/inotify-java-0.1/build/native/libinotify- > > java.so") > > > > I feel that the program should work. Apparently Clojure creates new > > classloaders between line 1 and 2. An equivalent Java program does > > work (the first System.load() loads the library, the second sees the > > library is already loaded and does nothing). > > > > This is the exact error message: > > > > $ cat t3.clj > > (System/load "/home/hk/inotify-java-0.1/build/native/libinotify- > > java.so") > > (System/load "/home/hk/inotify-java-0.1/build/native/libinotify- > > java.so") > > (println "42") > > > > $ java -jar /home/hk/clojure-svn-2/clojure.jar t3.clj > > java.lang.UnsatisfiedLinkError: Native Library /home/hk/inotify- > > java-0.1/build/native/libinotify-java.so already loaded in another > > classloader (t3.clj:0) > > at clojure.lang.Compiler.eval(Compiler.java:4533) > > at clojure.lang.Compiler.load(Compiler.java:4846) > > at clojure.lang.Compiler.loadFile(Compiler.java:4813) > > at clojure.main$load_script__5685.invoke(main.clj:206) > > at clojure.main$script_opt__5716.invoke(main.clj:258) > > at clojure.main$main__5740$fn__5742.invoke(main.clj:333) > > at clojure.main$main__5740.doInvoke(main.clj:328) > > at clojure.lang.RestFn.invoke(RestFn.java:413) > > at clojure.lang.Var.invoke(Var.java:346) > > at clojure.lang.AFn.applyToHelper(AFn.java:173) > > at clojure.lang.Var.applyTo(Var.java:463) > > at clojure.main.main(main.java:39) > > Caused by: java.lang.UnsatisfiedLinkError: Native Library /home/hk/ > > inotify-java-0.1/build/native/libinotify-java.so already loaded in > > another classloader > > at java.lang.ClassLoader.loadLibrary0(ClassLoader.java:1743) > > at java.lang.ClassLoader.loadLibrary(ClassLoader.java:1674) > > at java.lang.Runtime.load0(Runtime.java:770) > > at java.lang.System.load(System.java:1005) > > at user$eval__4.invoke(t3.clj:2) > > at clojure.lang.Compiler.eval(Compiler.java:4522) > > ... 11 more > > > > $ > > > > > > > > > > > > --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en -~----------~----~----~----~------~----~------~--~---
