RFR: 8257234 : Add gz option to SA jmap to write a gzipped heap dump [v9]

[Lin Zang]

On Tue, 26 Jan 2021 19:44:55 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>> Dear @plummercj,
>> I have made  investigation on extending GZIPOutputStream,  since it is not possible to override/overwrite private method writeHeader(), the only way I could figure out is to create a class named HProfGZIPOutputStream that extends DeflaterOutputStream,  copies most of GZIPOutputStream's code and rewrite a writeHeader() file. 
>> 
>> I am not sure whether it is nice since most of the code are same with GZIPOutputStream.  IMO, it may be nice if we can add another writeHeader() method to GZIPOutputStream that support adding file comments, but I am also not sure whether it is acceptable to add a new method in this general used class. 
>> 
>> BRs,
>> Lin
>
> I don't think we want to essentially clone GZIPOutputStream into a new HProfGZIPOutputStream in order to solve this problem. I think adding a GZIPOutputStream.writeHeader() method that supports adding a comment make sense, but probably not something that should be done with this CR. A new CR can be filed for it, and maybe take care of it sometime in the future, but for this CR we should focus on a solution that doesn't require the comment be added, and isn't too complicated. 
> 
> Looking back at my original suggestion to put the gzip reading support in Reader.getStack(), I'm not clear on the issue you ran into. You said that GzipRandomAccess requires the "HPROF BLOCKSIZE=" comment, but I'm not so sure you need to use GzipRandomAccess. You can use the same gzip detection mechanism as Reader.readFile() does to see if it can read the file without unzipping it, and if not just do what your test currently does now, and try to read it with GZIPInputStream. You don't need to use GzipRandomAccess here.

Dear Chris,
Thanks for your suggestion, I digressed from the test case and focus too much on the GzipRandomAccess. Thanks for pull me back.
And it is hard for me to read file without unzipping it, so I made a change to add unzipping in Reader.getStack(). 
Would you like to help reveiw again?

BRs,
Lin

-------------

PR: https://git.openjdk.java.net/jdk/pull/1712