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From "Rory O'Donnell" <>
Subject JDK 9 b148 including a refresh of the module system is available on
Date Fri, 09 Dec 2016 10:16:21 GMT

Hi Stefan,

JDK 9 build b148 <> includes an important 
Refresh of the module system [1] , summary of changes are listed here 

*This refresh includes a disruptive change that is important to understand.

*For those that have been trying out modules with regular JDK 9 builds 
then be aware that `requires public` changes to `requires transitive`. 
In addition, the binary representation of the module declaration 
(module-info.class) has changed so that you need to recompile any 
modules that were compiled with previous JDK 9 builds.

As things stand today in JDK 9 then you use setAccessible to break into 
non-public elements of any type in exported packages. However, it cannot 
be used to break into any type in non-exported package. The current 
specified behavior was a compromise for the initial integration of the 
module system. It is of course not very satisfactory, hence the 
#AwkwardStrongEncapsulation issue [2] on the JSR 376 issues list. With 
the updated proposal in the JSR, this refresh changes setAccessible 
further so that it cannot be used to break into non-public types, or 
non-public elements of public types, in exported packages. Code that 
uses setAccessible to hack into the private constructor of 
java.lang.invoke.MethodHandles.Lookup will be disappointed for example.

This change will expose hacks in many existing libraries and tools. As a 
workaround then a new command line option `--add-opens` can be used to 
open specific packages for "deep reflection". For example, a really 
popular build tool fails with this refresh because it uses setAccessible 
+ core reflection to hack into a private field of an unmodifiable 
collection so that it can mutate it, facepalm! This code will continue 
to work as before when run with `--add-opens 
java.base/java.util=ALL-UNNAMED` to open the package java.util in module 
java.base to "all unnamed modules" (think class path).

*Any help reporting issues to popular tools and libraries would be 
appreciated. *

A debugging aid that is useful to identify issues is to run with 
-Dsun.reflect.debugModuleAccessChecks=true to get a stack trace when 
setAccessible fails, this is particularly useful when code swallows 
exceptions without any logging.



Rgds,Rory O'Donnell
Quality Engineering Manager
Oracle EMEA , Dublin, Ireland

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