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

Staffan Larsen staffan.larsen at oracle.com
Fri Feb 7 12:49:55 UTC 2014

On 7 feb 2014, at 13:27, Alan Bateman <Alan.Bateman at oracle.com> wrote:

> On 07/02/2014 12:07, 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
> I have reservations about doing this as hints of code making use of private fields which isn't good.
> For the comments in FileInputStream and other then it might be best to keep the line lengths consistent with the existing code if you can (it makes future side-by-side reviews a bit easier too).

I’ve updated the comments to have shorter lines and javadoc style comments.

> In WindowsChannelFactory then you've re-order and expand imports. The ordering of the import groups in this area has been Java SE, JDK-specific and then finally the JDK-internal. It's not a big deal of course. Personally I prefer the original static imports but I know some people don’t.

I’ve reverted to the original order, and only expanded the non-static imports. (The danger of IDEs).

Updated webrev here:  http://cr.openjdk.java.net/~sla/8033917/webrev.01/


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