Locating a class file from a class loader by name

Alan Bateman Alan.Bateman at oracle.com
Fri Nov 13 16:00:17 UTC 2015

On 13/11/2015 12:04, Rafael Winterhalter wrote:
> Hello,
> I recently started testing several byte code manipulation frameworks
> against a recent build of Jigsaw and found a common problem of all of these
> libraries. I am myself the developer of Byte Buddy (
> https://github.com/raphw/byte-buddy) and I know most tools for code
> manipulation fairly well. I hope that this mailing list can provide me some
> me advice on patching my library and providing help to other maintainers.
> Especially in the context of using the instrumentation API, it is often not
> possible to do reflection on load classes. Rather, code manipulation tools
> read Java class files and parse these files for exposing classes using a
> similar API but without the possibility to invoke a method or to read a
> field.
> Typically these tools read Java class files by querying a class loader for
> the class file, e.g.:
> ClassLoader.getSystemClassLoader("java/lang/Object.class");
> This does not longer work using Jigsaw and I wonder how one is now supposed
> to locate a class file. Do I need to find all runtime images and query each
> single one? How would the path be for such a class file?
Java agents that do byte code instrumentation with java.lang.instrument 
should continue to work, this goes for both load-time and dynamic 
instrumentation and also where the code is instrumented to calling into 
supporting classes on the class path. There is still work to do for the 
scenario where agents modify the classes with references to types in 
other modules but that will come in time.

As regards ClassLoader.getResourceXXX (which I think is what you mean) 
then it continues to search the class path as before. It doesn't however 
locate resources in named modules because they are encapsulated. The 
same named methods in java.lang.Class to get resources allow code to get 
at resources in its own module but not other modules. As to how to get 
at resources in named modules then there are a few ways but the simplest 
for now is to look at java.lang.reflect.Module getResourceAsStream.


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