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This adds basic support for adding methods and fields to already
loaded classes using redefinition. This 'structural class
redefinition' is currently limited to classes without any virtual
methods or instance fields. One cannot currently structurally redefine
multiple classes at once nor will structural redefinition trigger the
standard redefinition events.
After structural redefinition all references to the old class, and its
fields and methods are atomically updated. Any memory associated with
the static fields of the old class is zeroed. Offsets for field access
might change. If there are any active stack frames for methods from
the redefined class the original (obsolete method) code will continue
to execute. The identity hash code of the redefined class will not
change. Any locks being held, waited or blocked on by the old class
will be transferred to the new class.
To use this feature the process must be debuggable and running with
-Xopaque-jni-ids:true.
For device testing use a wrap.sh that adds the following flags:
'-Xopaque-jni-ids:true -Xcompiler-option --debuggable -XjdwpProvider:adbconnection'
Structural redefinition only available using the
"com.android.art.UNSAFE.class.structurally_redefine_class_direct"
extension. This will not trigger the normal class-redefinition events.
Only one class may be redefined at a time.
NB There are still some holes in this potentially allowing obsolete
methods/fields to be visible. Most notably during jni-id, MethodHandle
and VarHandle creation as well as potentially other places in the
runtime. These holes will be closed by later CLs. Until then the
extension to access structural class redefinition will remain tagged
as UNSAFE.
Test: ./test.py --host --all-compiler
Bug: 134162467
Change-Id: I825d3a4bdb9594c0147223ae69f433ce9bbfc307
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