https://issues.apache.org/bugzilla/show_bug.cgi?id=49236
Summary: JAR indexing does not have any benefits and can cause
some harm
Product: Tomcat 7
Version: trunk
Platform: PC
OS/Version: Windows XP
Status: NEW
Severity: normal
Priority: P2
Component: Packaging
AssignedTo: dev@tomcat.apache.org
ReportedBy: knst.koli...@gmail.com
In TC 7, 6, 5.5 build scripts in several places we use <jar index="true"/>,
which creates META-INF/INDEX.LIST file in the JAR.
I should say that
a) The JAR Index feature is designed to speed up location and download of other
*.jar files. It is not intended to be used on a single file.
See specification [1], blog article that describes main problems [2], ANT
documentation on JAR task and especially the nested "indexjars" element as
described there [3].
One should use <indexjars> element inside the <jar> task to exploit this
feature properly.
b) It does not speed up processing of a single JAR.
First, a JAR file already has an index, "Central Directory" - see picture in
Wikipedia article [4], or official specification from PKWARE [5].
AFAIK, in Sun JRE this central directory is always read and loaded into memory
before any random-access query on the file is performed, such as
ZipFile#getEntry(String).
Second, an URLClassLoader has to consider JARs mentioned in the Index file. It
incurs some overhead, though I have not measured how much it is.
c) Using Index file can conflict with "Class-Path" entry of the manifest. See
[2] for explanation.
Also the text of the index file starts with the name of the current JAR file. I
suppose that it can have some consequences if the file is renamed.
Based on the above, we have to use index="false" on all our <jar> tasks.
Links:
[1] JAR Index specification - JDK 1.5
http://java.sun.com/j2se/1.5.0/docs/guide/jar/jar.html#JAR%20Index
[2] To Use or not to Use Jar Index - A blog article
http://javahowto.blogspot.com/2006/08/to-use-or-not-to-use-jar-index.html
[3] Jar Task - Apache ANT Documentation
http://ant.apache.org/manual/CoreTasks/jar.html
[4] ZIP (file format) - Wikipedia
http://en.wikipedia.org/wiki/ZIP_%28file_format%29#Technical_information
[5] ZIP APPNOTE - PKWARE
http://www.pkware.com/support/zip-application-note
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