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).
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.
-Alan.
