most likely the compiler is creating a new DynamicClassLoader, it uses a var clojure.lang.Compiler/LOADER and pushes and pops class loaders from there.
On Thu, Sep 22, 2011 at 1:52 PM, Brent Millare <brent.mill...@gmail.com> wrote: > Hi, > > An update to this question. While Chouser gave a good explanation > about the details behind proxy, reify, and gen-class, I feel that the > explanation behind deftype is incomplete. It's not clear how clojure > is able to reload deftype defined classes. Calling deftype eventually > leads to a call to clojure.lang.DynamicClassLoader/defineClass. > Calling deftype again leads to another call to defineClass, but doing > this manually results in a Linkage Error. What is happening underneath > here that allows clojure to do this with deftypes? > > -Brent > > On Sep 19, 9:07 am, Brent Millare <brent.mill...@gmail.com> wrote: >> (Note: I've copied my question from stackoverflow to get more looks in >> case there are people in here that are not on >> stack.http://stackoverflow.com/questions/7471316/how-does-clojure-class-rel... >> I will sync the good answers) >> >> I've been reading code and documentation to try to understand how >> class reloading works in clojure. According to many websites, such >> ashttp://tutorials.jenkov.com/java-reflection/dynamic-class-loading-rel... >> , whenever you load a class essentially you obtain the bytecode (via >> any data mechanism), convert the bytecode into an instance of class >> Class (via defineClass), and then resolve (link) the class via >> resolveClass. (Does defineClass implicitly call resolveClass?). Any >> given classloader is only allowed to link a class once. If it attempts >> to link an existing class, it does nothing. This creates a problem >> since you cannot link a newly instantiated class, therefore you have >> to create a new instance of a classloader everytime you reload a >> class. >> >> Going back to clojure, I tried examining the paths to load classes. >> >> In clojure, you can define new classes in multiple ways depending on >> what you want: >> >> Anonymous Class: reify proxy >> >> Named Class: deftype defrecord (which uses deftype under the hood) gen- >> class >> >> Ultimately, those codes point to clojure/src/jvm/clojure/lang/ >> DynamicClassLoader.java >> >> where DynamicClassLoader/defineClass creates an instance with super's >> defineClass and then caches the instance. When you want to retrieve >> the class, clojure load with a call to forName which calls the >> classloader and DynamicClassLoader/findClass, which first looks in the >> cache before delegating to the super class (which is contrary to the >> way most normal classloaders work, where they delegate first, than try >> it themselves.) *********The important point of confusion is the >> following: forName is documented to link the class before it returns >> but this would imply you can not reload a class from the existing >> DynamicClassLoader and instead need to create a new >> DynamicClassLoader, however I don't see this in the code.******** I >> understand that proxy and reify define anonymous classes, so their >> names are different thus can be treated as if its a different class. >> However, for the named classes, this breaks down. In real clojure >> code, you can have references to the old version of the classes and >> references to the new version of the classes simultaneously, but >> attempts to create new class instances will be of the new version. >> >> Please explain how clojure is able to reload classes without creating >> new instances of DynamicClassLoader, if I can understand the mechanism >> to reload classes, I would like to extend this reloading functionality >> to java's .class files I may create using javac. >> >> Notes: This question refers to class RELOADING, not simply dynamic >> loading. Reloading means that I have already interned a class but want >> to intern a new updated version of that instance. > > -- > 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 > Note that posts from new members are moderated - please be patient with your > first post. > 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 -- And what is good, Phaedrus, And what is not good— Need we ask anyone to tell us these things? -- 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 Note that posts from new members are moderated - please be patient with your first post. 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
