RFR: JDK-8033917 Keep track of file paths in file streams and channels for instrumentation purposes

[Staffan Larsen]

On 7 feb 2014, at 16:24, Dmitry Samersoff <dmitry.samersoff at oracle.com> wrote:

> Staffan,
> 
> FileInputStream.java
> 
> 55:  It's better to initialize path with null.

I agree with Chris here. The value should be explicitly initialized by all constructors.

> 134: It's better to assign name at one of first lines, in this case we
> will be able to retrieve file name ever if open fails for some reason.

This is the constructor. If anything fails it will throw and exception, and there won’t be an object to look at.

> 171: It's not necessary

All constructors must initialize the value. 

Thanks,
/Staffan

> 
> (the same is applicable to other files)
> 
> I'm a bit scared changing signature of public methods of FileChannelImpl
> but if Alan says it's OK - lets go with it.
> 
> -Dmitry
> 
> 
> On 2014-02-07 16:07, Staffan Larsen wrote:
>> Instrumentation agents that want to instrument
>> FileInputStream/FileOutputStream to see which files are being
>> accessed do not currently have access to the file system path of the
>> stream. This is because the path is never stored in the stream class,
>> only the file descriptor is. (This is also true for RandomAccessFile
>> and FileChannel).
>> 
>> An agent could instrument the respective constructors to store the
>> path. The problem is where to store it. To add a field, the
>> instrumentation agent needs to change the schema of the class. This
>> is not possible at runtime but can be done at class-loading time.
>> However for a j.l.instrument agent these classes are already defined
>> when the agent is first called. For a native JVMTI agent the problem
>> becomes parsing and modifying byte codes in a native agent which is
>> error prone and requires a lot of code to maintain.
>> 
>> If instead the stream classes were modified to store a reference to
>> the path, it would be readily available for agents at a minimum of
>> cost to the libraries. This is what this patch does. FileInputStream,
>> FileOutputStream, RandomAccessFile and FileChannelImpl are modified
>> to record the path they operate on in a private field. There are no
>> accessors added to retrieve the path - it is purely stored for
>> instrumentation purposes. The path is intentionally not resolved to
>> be an absolute path since that would potentially add unwanted
>> overhead. If a stream is created from a file descriptor, no path will
>> be stored.
>> 
>> The overhead for this path will be keeping the path String alive for
>> a longer period of time. I hope this will not cause any problems.
>> 
>> A consumer of this feature will be Java Flight Recorder, but the
>> implementation is usable by other agents as well.
>> 
>> webrev: http://cr.openjdk.java.net/~sla/8033917/webrev.00/ bug:
>> https://bugs.openjdk.java.net/browse/JDK-8033917
>> 
>> Thanks, /Staffan
>> 
> 
> 
> -- 
> Dmitry Samersoff
> Oracle Java development team, Saint Petersburg, Russia
> * I would love to change the world, but they won't give me the sources.