proposal to optimise the performance of the Jar utility

mike.skells at mike.skells at
Tue Apr 5 23:42:28 UTC 2011

Not sure if this is too late for Java 7 but I have made some optimisations for a 
client to improve the performance of the jar utility in their environment, and 
would like to promite then into the main code base

The optimisations that I have performed are

1. Allowing the Jar utility to have other compression levels (currently it 
allows default (5) only)
2. Multi-threading, and pipelining the the file information and access
3. multi-threading the compression and file writing

A little background
A part of the development process of where I work they regularly Jar the content 
of the working projects as part of the distribution to remote systems. This is a 
large and complex code base of 6 million LOC and growing. The Jar file ends up 
compressed to approx 100Mb, Uncompressed the jar size is approx 245mb, about 4-5 
times the size of rt.jar. 

I was looking at ways to improve the performance as this activity occurs several 
times a day for dozens of developers

In essence when compressing a new jar file the jar utility is single threaded 
and staged. Forgive me if this is an oversimplification

first  it works out all of the files that are specified, buffering the file 
names, (IO bound)
then it iterates through the files, and for each file, it load  the file 
information, and then the file content sending it to a JarOutputStream, (CPU 
bound or IO bound depending on the IO speed)

The JarOutputStream has a compression of 0 (just store) or 5 (the default), and 
the jar writing is single threaded by the design of the JarOutputStream

The process of creation of a Jar took about 20 seconds in windows with the help 
of an SSD, and considerable longer without one, and was CPU bound to one CPU 

The changes that I made were 
1. Allow deferent compression levels (for us a compression level of 1 increases 
the file size of the Jar to 110 Mb but reduces the CPU load in compression to 
approx 30% of what it was (rough estimate)
2. pipelining the file access 
2.1    one thread is started for each file root (-C on the Jar command line), 
which scans for files and places the file information into a blocking queue(Q1), 
which I set to abretrary size of 200 items
2.2    one thread pool of 10 threads reads the file information from the queue 
(Q1) and buffers the file content to a specified size (again I specified an 
arbetrary size limit of 25K for a file, and places the buffered content into a 
queue(q2) (again arbetrary size of 10 items
2.3    one thread takes the filecontent from Q2 and compresses it or checksums 
it and adds it the the  JarOutputStream. This process is single threaded due to 
the design of the JarOutputStream

some other minor performance gain occurred by increasing the buffer on the 
output stream to reduce the IO load

The end result is that the process takes about approx 5 seconds in the same 

The above is in use in production configuration for a few months now

As a home project I have completed some enhancements to the JarOutputStream, and 
produced a JarWriter that allows multiple threads to work concurrently deflating 
or calculating checksums, which seems to test OK for the test cases that Ihave 
generated,and successfully loads my quad core home dev machine on all cores. 
Each thread allocates a buffer, and the thread compresses a files into the 
buffer, only blocking other threads whenthe buffer is written to the output 
(which is after the compression is complete, unless the file is too large to 

This JarWriter is not API compatable with the JarOutputStream, it is not a 
stream. It allows the programmer to write a record based of the file information 
and an input stream, and is threadsafe. It is not a drop in replacement for 
I am not an expert in the ZIp file format, but much of the code from 
ZipOutputStream is unchanged, just restructured
I did think that there is some scope for improvement, that I have not looked at
a. thresholding of file size for compression (very small files dont compress 
b. some file types dont compress well (e.g. png, jpeg) as they have been 
compressed already)
c. using NIO to parallelise the loading of the file information or content
d. some pre-charging of the deflator dictionary (e.g. a class file contains the 
strings of the class name and packages), but this would make the format 
incompatable with zip, and require changes to the JVM to be useful, and is a 
long way from my comform zone, or skill set. This would reduce the file size

What is the view of the readers. Is this something, or at least some parts of 
this that could be incorperated into Java 7 or is this too late on the dev cycle



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