Jar File - Prevent Access to Source Code

Question

I want to hand over a small Java app as a runnable jar but I do not want anybody to have access to my source code. Am I right in presuming that there is no source code (.java files) included with a jar file?

User269799

Answer 1

You are right, there is no source code in the jar (unless you configure your build system to specifically put it in there). But you are always at the risk you code gets decompiled from the bytecode. An obfuscater might help here.

Answer 2

Assuming you don't put the java files in the jar file, they're not going to magically appear :) You can include anything you like in the jar file of course. You can list the contents to check:

jar tvf foo.jar

Note that Java can be decompiled pretty easily though - so while any recipients wouldn't have access to your actual source code with comments etc, they could see your logic pretty clearly. You may want to use an obfuscator to help protect your IP. Personally I try to avoid obfuscators - given how hard most of us find to maintain code when we do have the real source with commments and tests, imagine how hard it is when you don't have those things :) It's your call though. Just make sure you test obfuscated code thoroughly - there can be subtle issues, particularly if you use reflection.

Answer 3

Yes. Usually, jars contain only byte-compiled .class files. That said, they can contain source code as well—it depends on what you (or your tools, respectively) put into them.

Note, however, that decompilation works pretty well on .class files, so don't make anything security-related rely on code obfuscation techniques such as this one.

Answer 4

If a computer can run it, a human can reverse engineer it, and it is not particularly hard for Java.

So technical protection simply won't work. You need legal protection in form of a binding contract or similar. You may even put your works under the GPL except for those paying you, which is sufficient for most businesses to avoid stealing your work.

What situation exactly do you want to avoid?

Answer 5

Jar files usually only include .class files, which are java bytecode files, as well as resources. However, to be a little more secure about your code, you'll want to turn off debugging information and if you really want to be secure, run it through an obfuscator.

Edit: berry120's comment is right - they can contain source files, but usually they do not. I just want to clarify for any future readers of this. It depends on the settings of the tool you use to generate the jar.

Answer 6

Normally there isn't but you can use the jar -tvf <filename> command to check it.

However I have to warn you that it's extremely easy to decompile most .class files into reasonably readable java source code.

To avoid this, you'll have to use an obfuscator, but that needs some extra effort on your behalf. (E.g. RetroGuard.)

Having said that, ask yourself the question: "Is my code really that valuable or special that I need to do all this?" Usually the answer is no, most of the code we write is nothing special.

Answer 7

You are are correct, however the .class files can easily be disassembled to java code, and its pretty accurate in most cases.

If you really need it to be properly secure then you'll need to obfuscate.

Answer 8

It will depend on the way you generated that .jar, Eclipse does have an option to include .java files on the .jar but it is disabled by default and you have to activate it if wanted.

Answer 9

Jar files might contain the source (you can choose whether to include it or not) so not including the source specifically isn't an issue. What you need to be aware of though is people potentially reverse engineering the class files that will be in the jar file.

You can get around this usng an obfuscator such as yGuard which easily hooks in as an ant task, but as others have said, is your code really that important that no-one else sees it?

Answer 10

The .jar file does not include source code, only the bytecode (.class). But as the byte code is machine independent, it can be decompiled very easily. There is no way to prevent others to access your source code.