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Java Jar Catalog


This essay does not describe an existing computer program, just one that should exist. This essay is about a suggested student project in Java programming. This essay gives a rough overview of how it might work. I have no source, object, specifications, file layouts or anything else useful to implementing this project. Everything I have prepared to help you is right here.

This project outline is not like the artificial, tidy little problems you are spoon-fed in school, when all the facts you need are included, nothing extraneous is mentioned, the answer is fully specified, along with hints to nudge you toward a single expected canonical solution. This project is much more like the real world of messy problems where it is up to you to fully the define the end point, or a series of ever more difficult versions of this project and research the information yourself to solve them.

Everything I have to say to help you with this project is written below. I am not prepared to help you implement it; or give you any additional materials. I have too many other projects of my own.

Though I am a programmer by profession, I don’t do people’s homework for them. That just robs them of an education.

You have my full permission to implement this project in any way you please and to keep all the profits from your endeavour.

Please do not email me about this project without reading the disclaimer above.


This project is similar to the NoClassDefFound Amanuensis. This student project searches your entire hard disk for *.jar files and makes a catalog of their *.class contents, sorted by path and classname.

You could then examine or search this file using high quality text editor such as SlickEdit.

Why would you want such a thing? When you get a NoClassDefFoundError you would then have a clue as to which jars need to put on the classpath. You could detect potential for getting a class from the wrong jar.

You could extend this to search *.zip and *.war files and look for *.java source, image resources (*.gif, *.jpg, *.png etc.) You might even include all files, not just container files.


As you include more types of container file and more types of contents, the need for more sophisticated searching and sorting presents itself. I suggest exporting the list to an SQL (Standard Query Language) database. You can then do ad-hoc queries from the command line, or cook up some simple app to do checkbox selected queries.

One type of query you could do is provide a classpath and then see which version of each class on that classpath would actually be selected if it were loaded. It might note duplicates that would be ignored, a potential source of trouble.

A tool you would want is a method to dump the classpath of a running program in a form that can be imported as for a query.

A report you would want is a list by package, listing which jars each package appears in and by jar, listing which packages are represented in each jar. This gives you a bird’s eye view of the entire jar system.

You might make your database available to the web, so that other without your enormous jar collection could look up where missing classes come from. For that you need to track the original source of each jar as well.


The basic tool is the class and its subclass java.util.jar.JarFile.

To make this easy, just scan the entire disk periodically and rebuild the catalog. Don’t try to monitor changes on the fly. You could, of course, avoid rescanning jars if their dates have not changed.

You could set his up with MySQL — a free database that you can also distribute freely.

Interface Finder: find classes on classpath implementing a given inteface
jar verifier Project
Path and Classpath tool project
Which and What Project: path and classpath analysis

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