http://forums.java.net/jive/thread.jspa?threadID=63#8886 suggests that the javac compiler annotate generated .class files with a tag that lets subsequent tools determine which javac compiler produced the .class file.  The particular request is to know if one is mixing compilers, but a lunch table discussion came up with several other reasons why one might want to know which compiler was used: e.g., experimental versions of the compiler, upon discovering bugs in previous versions of the compiler, etc.

The tag doesn't have to be non-spoofable: it's just a hint for sane development scenarios.

Adding the annotation will increase the size of the .class file.  It might be good to have a way to aggregate the annotations when building a .jar file.
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