25 October 2011

Changes to Library Projects in Android SDK Tools, r14

Last week, we released the SDK for Android 4.0 and a new set of developer tools, now in revision 14. The updated tools include a lot of build changes, many that improve build performance. Also included is an under-the-hood change in how libraries are used by main projects — a first step in improving library support and code reusability. While the change should have little impact on existing projects, some developers have had issues when migrating to the updated tools. Please read below for more information about the change to library projects and how to solve migration issues.

Previously, library projects were handled as extra resource and source code folders to be used when compiling the resources and the application’s source respectively. While this worked fine for most cases, there were two issues.

1. Developers asked us for the ability to distribute a library as a single jar file that included both compiled code and resources. The nature of Android resources, with their compiled IDs prevented this.

2. The implementation of the library projects was extremely fragile in Eclipse. Adding extra source folders outside of the project folders is non-trivial when it needs to be handled automatically, in a way that doesn’t expose a user’s local installation path (which is required for people working in teams through a source control system such as SVN or git).

For r14, we decided to fix both issues at once, by moving to a compiled-code based library mechanism. This solves the implementation fragility in Eclipse and will allow us to, later, enable distribution of libraries as a single jar file.

As you might have seen in the release notes, moving to this new mechanism can affect existing projects in some cases, but there are simple fixes.

The first impact of this change is that the new library project requires the resource IDs generated by libraries to be non final. This prevents the Java compiler from inlining the values in the library code, and therefore prevents usage of the switch statement in the library code. To address such occurrences in your code, Eclipse provides a refactoring action to convert from switch statements to if/else (see here).

Second, some projects may not have been properly migrated to the new mechanism, resulting in projects that fail to compile, with errors such as duplicated classes being added in the dex build step. ADT 14 should have migrated older projects to the new mechanism but the fragility of the old mechanism may have prevented it from happening. This makes projects reference the libraries twice, using both the old and new mechanisms, which then triggers the libraries classes being packaged twice. If you see this in your projects, look in the Package Explorer for extraneous source folders named with the pattern <libraryname>_src. The screenshot to the right shows an example of this.

To fix the project, you must remove the extraneous source folders with the following steps:

  • Right click source folder and choose Build Path > Remove from Build path
  • A dialog will pop up. In it, make sure to check “Also unlink the folder from the project” to completely remove the folder.

With this change to library projects, we pave the way to better support for reusable components. We will continue working to make components easier to create, work with, and manage. Our goal is to make it easy for developers to create apps with great user experiences that easily adapt to all form factors.

Some developers have also told us that they only use library projects internally, that they don’t need to distribute binary versions and would prefer to continue with a source-based mechanism. We are investigating how we could support this alongside the new mechanism.

Finally, I wanted to point out that we are tracking a few known issues (and workaround for them) in the current r14 tools at this page: http://tools.android.com/knownissues.

We are working on a tools update that will include fixes for most of these. We are hoping to have it out shortly.