Thursday, May 31, 2012

Eclipse ADT: Conditional Breakpoint Error

When I tried to understand the way Android's RelativeLayout measures and re-sizes the views it contains, I had do debug the Android SDK sources. Eclipse makes that quiet easy. All you have to do is attach the sources of the SDK you are using to the «android.jar» in your project's preferences.
Stepping through the source and setting breakpoints worked flawlessly. But when I wanted to set a conditional breakpoint (because I wanted to stop execution when RelativeLayout.onMeasure measured a specific sub-view), Eclipse failed with an error:

Conditional Breakpoint Error

It reads: «Conditional Breakpoint Error. Conditional breakpoint has compilation error(s). Reason: Unable to compile conditional breakpoint - missing Java project context.» Also, evaluating and displaying Java expressions would not work and gave the following error:

Error Evaluating

It reads: «Error Evaluating. Unable to evaluate the selected expression: To perform an evaluation, an expression must be compiled in the context of a Java project's build path. The current execution context is not associated with a Java project in the workspace.» Evaluated expressions come in handy, for instance, when picking the MeasureSpec of a View apart, which is a bit-field composed of a length and a mode.

Searching the internet for a solution, I only unearthed the rather vague suggestion to—surprise!—create a Java project for the Android library sources. I was not really fond of that idea, subdued by the size of the library. Not without reason, as I saw later. So I asked for a solution over at, but to no avail. I suppose the problem is to specific. When nothing came out of it, I had to swallow the pill and create a project for the sources of the Android library. And guess what? It fixed the problem!

Step by step do the following. First create a new Java project. A wizard dialog will pop up. Name the project as you like, «Android library sources» seems appropriate. On the next page, link an additional source and point it to the appropriate source directory in the Android SDK. Eclipse will now ask for «Inclusion and Exclusion Patterns». Exclude everything by adding the asterisk «*» to the exclusion pattern. This both surprising and important. Surprising because Eclipse does not need to do anything with the sources (like compiling) for conditional breakpoints and the like to work. And it is important, because otherwise your log will overflow with warnings, and error and info messages. Finish the dialog.
Now you need to attach the source code to the «android.jar» library of your project. Open your project's properties. You will find the library under «Java Build Path» on tab «Libraries» as show by the following screen shot:

Attaching the source code to the Android library

I my original configuration I had attached the source from an «External Folder» instead of a workspace, which only worked partway.

And that's it!

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