Is there are way to patch jar files?

Question

Suppose I sent a large jar or war file to someone. Could I later just change one small section and send that to him? Suppose I just changed one class file. I recompiled the java for into a class file. Other then exchanging the new class file for the old class file it there anything else I would have to do?

In java do you have to rebuild the entire jar/war file? Also, is there some open source package available for doing updates?

Solution 1

The person receiving the class file could simply add that file to the jar as long as they know which directory to put it in. Be weary of signed jars, as noted in the comments.

jar uf foo.jar foo.class

http://docs.oracle.com/javase/7/docs/technotes/tools/solaris/jar.html

Solution 2

There is JARDiff, designed for updating Java WebStart applications.

Solution 3

You can copy the modified .class file this way.

jar uf test.jar com\test\Test.class

If there is a logical way for you to separate out your components in individual jar files then I would create a jar file per component type. That way you will not have re-distribute everything back to the client. For example - take a look at how Spring 3 has the components separated out.

Comments

[Bryan Kyle]

yes, jar files are just zip files, however there is a jar utility that you can use for this. It's part of the standard JDK.

[GC_]

I mean does unjar/jar do anything special beyond unzip/zip?

[casablanca]

+1, this will usually work, however beware of modifying signed JARs in this way.

[Bryan Kyle]

I believe it will create a manifest for you if you don't already have one.

[GC_]

When you say simply zip file, I have some doubts. It is my understanding that jarfiles also have a manifest.

[Jeremy]

@Grae - Sure, but how does that change the fact that the compression is the same as zip? Either way, that piece of information is probably irrelevant to the question so I am going to remove it.

[GC_]

Even if I only had to change one jar file at a time, that would sill not help me in my particular case, because all of the jar files are in one war file.

[Paŭlo Ebermann]

@Grae: It depends what is in the manifest file. Often it contains nothing which would differ from one to next version of jar. If it contains signing information, you would have to change this, too. Simply create one jar with the old and new class as you would normally and compare the manifest files to see if this would matter for you.

[GC_]

jar umf foo.jar foo.class - I found this on the link. I think this is the best choice.

[Jeremy S]

FYI, the link in the answer no longer works. This seems to be the same article: docs.oracle.com/javase/7/docs/technotes/tools/solaris/jar.ht‌​ml