Building jar targeting multiple Java versions, including 9

[Alan Bateman]

On 26/08/2016 09:52, Oliver Gondža wrote:

> I am about to stretch support of my project from java 6, 7 and 8 to 
> version 9 as well. It does not work there out of the box as it 
> accesses sun.tools.attach.HotSpotVirtualMachine:
>
> ```
> class MyClass (in unnamed module @0x4d14b6c2) cannot access class 
> sun.tools.attach.HotSpotVirtualMachine (in module jdk.attach) because 
> module jdk.attach does not export sun.tools.attach to unnamed module
> @0x4d14b6c2.
> ```
>
> Before I had a chance to experiment with introducing modules to my 
> application and declaring dependency on jdk.attach, my project refuses 
> to compile on java 9 as soon as I add module-info.java as I instruct 
> javac to produce java 6 bytecode (-target 1.6 -source 1.6):
>
> ```
> modules are not supported in -source 1.6; use -source 9 or higher to 
> enable modules

You can invoke javac twice, as Uwe mentions. One starting point is:

javac -release 6 -d classes src/p/MyClass.java
javac -d classes src/module-info.java

The resulting classes should work with JDK 6+ on the class path, or as a 
module on the module path with JDK 9. The important thing with the 
second compilation is that you specify the same output directory as the 
compiler access to the source or class files when compiling the module 
declaration.

I see multi release JARs have been mentioned. This is also something to 
look at, assuming you end up with classes (beyond module-info) that are 
Java release specific.

In your mail then the class is "MyClass", I'm guessing this isn't really 
your actual class name. If it is then keep in mind that named modules 
require the classes to be in packages, you can't have types in the 
unnamed package in a named module.

On the attach API then the supported/documented API is 
com.sun.tools.attach. It's never been documented to make direct use of 
types in sun.tools.attach. Are you casting the VirtualMachine to a 
HotSpotVirtualMachine and hacking into implementation? It might be best 
to explain what you are doing. You can of temporarily break 
encapsulation to allow the above to compile/run with `--add-exports 
jdk.attach/sun.tools.attach=<target>", where <target> is your module 
name or ALL-UNNAMED if your library is on the class path.

-Alan