How to get a source file?

Question

I'm using Java 1.6 and extending AbstractProcessor to handle a custom annotation.
What I want to do is load the source file in which my annotation is found and extract a few lines of the code from around the annotation into a text file and save the text file.
I'm trying to use
compiler#getResource(...) to get a reference to a FileObject for the source file. I have the correct package and file name for the source but I have been unable to determine how I get the root of the source path from which all the source files originate. If I try the code with source root hardwired it all works i.e I prepend the source root to the package name and call
FileObject fObj = filer#getResource(StandardLocation.SOURCE_PATH, <src_root_string>+<package string>, fileName.java)
then the fObj correct, but this is not a practical solution. Without the <src_root_string> I just get a file not found exception.
So my question is how can I discover the source root folder from inside the AbstractProcessor and so not need to hardwire it in?
Alternatively is this the best approach? Should I try another approach?
Thanks

Answer 1

Mike,
You don't need a File to do what you want. Even if you could, it would be wrong to use a file.
It would be wrong, because the compiler might not by using a JavaFileManager that is actually backed by a filesystem. It could be backed by some form of repository such as a database, or it could be backed by a transient in memory file system ( An example of the latter is the [annotation processor test framework|https://hickory.dev.java.net/nonav/apidocs/index.html?net/java/dev/hickory/testing/package-summary.html] in hickory)
But probably of more interest to you, you don't need to because you can simply [get an InputStream|http://java.sun.com/javase/6/docs/api/javax/tools/FileObject.html#openInputStream()] from the FileObject returned from [filer.getResource(...)|http://java.sun.com/javase/6/docs/api/javax/annotation/processing/Filer.html#getResource(javax.tools.JavaFileManager.Location,%20java.lang.CharSequence,%20java.lang.CharSequence)] and read it.
You might also find the [Compiler Tree API|http://java.sun.com/javase/6/docs/jdk/api/javac/tree/index.html] useful. The Compiler Tree API is available to annotation processors running in Oracle's (Sun's) javac, but not in other compilers (possibly limiting portability).
With this API you can [convert |http://java.sun.com/javase/6/docs/jdk/api/javac/tree/com/sun/source/util/Trees.html#getPath(javax.lang.model.element.Element)] the Element representing the annotation to a Treepath then [get the leaf node|http://java.sun.com/javase/6/docs/jdk/api/javac/tree/com/sun/source/util/TreePath.html#getLeaf()] and from that tree [obtain its offsets|http://java.sun.com/javase/6/docs/jdk/api/javac/tree/com/sun/source/util/Trees.html#getSourcePositions()] within the source file.
Bruce