I need to add some jars from JRE7 library to my Android project. But for example rt.jar is in conflict with android.jar from Adroid 2.2 SDK, so I get this error:
Ill-advised or mistaken usage of a core class (java.* or javax.*) when not building a core library.
This is often due to inadvertently including a core library file in your application's project, when using an IDE (such as Eclipse). If you are sure you're not intentionally defining a core class, then this is the most likely explanation of what's going on.
However, you might actually be trying to define a class in a core namespace, the source of which you may have taken, for example, from a non-Android virtual machine project. This will most assuredly not work. At a minimum, it jeopardizes the compatibility of your app with future versions of the platform. It is also often of questionable legality.
If you really intend to build a core library -- which is only appropriate as part of creating a full virtual machine distribution, as opposed to compiling an application -- then use the "--core-library" option to suppress this error message.
If you go ahead and use "--core-library" but are in fact building an application, then be forewarned that your application will still fail to build or run, at some point. Please be prepared for angry customers who find, for example, that your application ceases to function once they upgrade their operating system. You will be to blame for this problem.
If you are legitimately using some code that happens to be in a core package, then the easiest safe alternative you have is to repackage that code. That is, move the classes in question into your own package namespace. This means that they will never be in conflict with core system classes. JarJar is a tool that may help you in this endeavor. If you find that you cannot do this, then that is an indication that the path you are on will ultimately lead to pain, suffering, grief, and lamentation.
I know there have been several threads about it and things like JarJar, OneJar or FatJar might be good for me. But I don't know how to make any of them work and documentation doesn't really make it clear (for me). I guess they use Ant commands, but I have always used Eclipse built-in builder and now I have no idea how to use neither Ant nor any of mentioned above.
So my question is: how can I repack this rt.jar so I could compile it in my Android project?
Thank you!
EDIT:
Ok, so what I want to achieve is to create a .jar, which can be used during developing Android application (simplifies some functionalities, doesn't really matter). But I would also like to be able to add the very same .jar to standard Java project in order to use some functions there as well. It would look like this: Whoever writes an application adds this .jar to his Java project -> it enables him to generate certain files (internet is needed to do it) -> these generated files are then added to Android project -> later on, when somebody uses this Android app, these files provide certain functionalities without using internet (off-line).
It would be ill-advised to do this in any project at all, even if it were possible. You would be opening yourself to a wealth of class incompatibility and loading problems. But in any case it doesn't even matter because the core Java libraries are loaded way before your archives are even touched, making any such attempt at overriding them moot.
Not to even talk about the fact that Android is using its own JVM implementation which is not fully compatible with JDK 6 (forget JDK 7). Also note that it may be a copyright violation to package the core Java libraries with your code and could change your licensing options (IANAL).
You need to find another way to resolve whatever issue you are having (which you failed to mention in your question).