RFR: JDK-8033917 Keep track of file paths in file streams and channels for instrumentation purposes
staffan.larsen at oracle.com
Fri Feb 7 12:07:04 UTC 2014
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.
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