RFR: 8230857: Avoid reflection in sun.tools.common.ProcessHelper

[Magnus Ihse Bursie]

On 2019-09-17 01:01, David Holmes wrote:
> Hi Christoph,
>
> Sorry for the delay getting back you.
>
> cc'd build-dev to get some clarification on the below ...
>
> On 12/09/2019 7:30 pm, Langer, Christoph wrote:
>> Hi David,
>>
>>>> please review an enhancement which I've identified when working with
>>>> Processhelper for JDK-8230850.
>>>>
>>>> I noticed that ProcessHelper is an interface in common code with a
>>>> static method that would lookup the actual platform implementation via
>>>> reflection. This seems a little cumbersome since we can have a common
>>>> dummy for ProcessHelper and override it with the platform specific
>>>> implementation, leveraging the build system.
>>>
>>> I don't see you leveraging the build system. You have two source files
>>> that compile to the same destination class file. What is ensuring the
>>> platform specific version is compiled after the generic one?
>>>
>>> Service-provider patterns use reflection to instantiate the service
>>> implementation. I don't see any problem here that needs solving.
>>
>> TL;DR:
>> There are two source files, one in share/classes and one in 
>> linux/classes. The build system overrides the share/classes 
>> implementation with the linux/classes implementation in the linux 
>> build. This is not by coincidence and only one class is contained in 
>> the generated jdk.jcmd module. Then there won't be a need for having 
>> a service interface and a service implementation that is looked up 
>> via reflection (which is not a bad pattern by itself). I agree that 
>> it's not a big problem to be solved but still not "no problem".
>> Here is some longer elaboration how the build system prefers specific 
>> implementations of classes and filters generic duplicates:
>> The SetupJavaCompilation function from JavaCompilation.gmk [0] is 
>> used to compile the java sources for JDK modules. In its 
>> documentation, for argument SRC [1], it claims: "one or more 
>> directories to search for sources. The order of the source roots is 
>> significant. The first found file of a certain name has priority". In 
>> its implementation the found files are first ordered [3] and 
>> duplicates filtered out [4].
>> The potential source files are handed to SetupJavaCompilation in 
>> CompileJavaModules.gmk [5] and were collected by a call to 
>> FindModuleSrcDirs [6].  FindModuleSrcDirs iterates over all potential 
>> source dirs for Java classes in the module [7]. The evaluated subdirs 
>> are (in that order) $(OPENJDK_TARGET_OS)/classes, 
>> $(OPENJDK_TARGET_OS_TYPE)/classes and share/classes, as per [8].
>> Hope that explains what I'm trying to leverage here.
>
> I'm not 100% certain that what you describe actually ensures what you 
> want it to ensure. I can't reconcile "the first found file ... has 
> priority" with the fact found files are sorted and duplicates 
> eliminated. It is the sorting that concerns me as it suggests 
> linux/Foo.java might replace shared/Foo.java, but if we're on Windows 
> then we have a problem! That said there is also this comment:
>
> # Order src files according to the order of the src dirs. Correct 
> odering is
> # needed for correct overriding between different source roots.
>
> I'd need the build team to clarify what "correct overriding" is 
> actually defined as.
David,

Christoph is correct. linux/Foo.java will override share/Foo.java. I 
don't remember how the magic in JavaCompilation.gmk works anymore :-), 
but we have relied on this behavior in other places for a long time, so 
I'm pretty certain it is still working correctly. Presumably, the $(sort 
...) is there to remove (identical) duplicates, which is a side-effect 
of sort.

/Magnus

>
> Thanks,
> David
> -----
>
>> I've uploaded an updated webrev which contains some cleanup to the 
>> Test changes: http://cr.openjdk.java.net/~clanger/webrevs/8230857.1/
>>
>> Thanks
>> Christoph
>>
>> [0] 
>> http://hg.openjdk.java.net/jdk/jdk/file/ea93d6a9f720/make/common/JavaCompilation.gmk#l185
>> [1] 
>> http://hg.openjdk.java.net/jdk/jdk/file/ea93d6a9f720/make/common/JavaCompilation.gmk#l157
>> [3] 
>> http://hg.openjdk.java.net/jdk/jdk/file/ea93d6a9f720/make/common/JavaCompilation.gmk#l225
>> [4] 
>> http://hg.openjdk.java.net/jdk/jdk/file/ea93d6a9f720/make/common/JavaCompilation.gmk#l257
>> [5] 
>> http://hg.openjdk.java.net/jdk/jdk/file/ea93d6a9f720/make/CompileJavaModules.gmk#l603
>> [6] 
>> http://hg.openjdk.java.net/jdk/jdk/file/ea93d6a9f720/make/CompileJavaModules.gmk#l555
>> [7] 
>> http://hg.openjdk.java.net/jdk/jdk/file/ea93d6a9f720/make/common/Modules.gmk#l300
>> [8] 
>> http://hg.openjdk.java.net/jdk/jdk/file/ea93d6a9f720/make/common/Modules.gmk#l243
>>
>>