Question

I noticed that when I convert an existing Android Application project to an Android Library project by checking the “is Library” checkbox, nothing changes in the project’s source code or XML files.

So what really happens when the “is Library” checkbox is checked? What changes internally, in the package and/or project files? Where can I learn more about this?

To better explain my question:

1. What I am interested to know (mainly
   for troubleshooting purposes) is
   what differentiates an Application package from a Library
   package “under the cover”?
2. Is the fact that a project is
   “Library Project” marked
   somewhere? If so, where does it
   mark it (obviously not in the source
   files and not even in the res XML
   files)

Answer 1

From the Managing Projects from the Command Line page:

The create lib-project command creates
a standard project structure that
includes preset property that
indicates to the build system that the
project is a library. It does this by
adding this line to the project’s
default.properties file:

android.library=true

In other words, it’s a property that is utilized by the build system and not by the operating system.

UPDATE: I’m on my iPhone so I won’t type out a whole paraphrase, but here’s a pretty good blog article discussing the differences between a Java JAR and an Android Library Project, including how the dex tools add the resources and dex code to the .apk:

http://devmaze.wordpress.com/2011/05/22/android-application-android-libraries-and-jar-libraries/