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From Steve Poole <spoole...@googlemail.com>
Subject Re: My experience with DTFJ
Date Wed, 22 Apr 2009 13:20:48 GMT
Thanks Andrew - appreciate the input.  You've listed many of the weaknesses
of the API plus some additional requirements.  I'm going to use your note as
a resource and turn the items listed into various discussion items - either
on the kato wiki or the Jira project.



On Tue, Apr 21, 2009 at 3:20 PM, Andrew Johnson
<andrew_johnson@uk.ibm.com>wrote:

> I've used DTFJ to write the DTFJ Adapter for the Eclipse Memory Analyzer
> Tool.
> http://www.eclipse.org/mat
>
> Here are some comments I have about DTFJ API which I think also apply to
> Kato.
>
> Predictable order from iterators
> - I want the same results with same dump, but with the same or different
> VM versions to run Kato/my program. This is needed for ease of debugging
> etc.
> - No iterators anywhere from straight HashMaps
> - This should be documented as a requirement of the API
>
> Generics for iterators
> - Specifies the return type from getNext()
> - Would reduce coding errors
> - CorruptData as a return type might cause a problem, but it is an
> interface
> - need CorruptData objects of different types for errors. e.g.
> MyCorruptJavaObject implements JavaObject, CorruptData
> - <? extends JavaClass> etc. in case the implementation is using a
> subclass collection
> - getAddress() - clash in method names between CorruptData and JavaObject
> etc. - but does this matter if the object is corrupt? Should a CorruptData
> object which also implements another interface throw CorruptDataException
> whenever possible from the additional methods?
>
> In general iterator methods don't themselves return exceptions - except
> ImageProcess:
>    Iterator getLibraries() throws DataUnavailable, CorruptDataException;
> Why?
>
> Equality between objects from different dumps
> If you get an object twice from a dump and it is meant to represent the
> same thing then the hash code must match and they must compare equal. It's
> okay for for the implementation to dynamically build objects however.
> If you open the same dump twice should objects compare equal?
> If you open two dumps at different times from the same run should
> identical objects compare equal (JavaRuntime, JavaClass)? How do we cope
> with JavaObjects which get moved by GC? Even a persistentHashCode and
> address match doesn't guarantee that its the same object. The field values
> might have changed by then anyway.
>
> ImageProcess.getID() - string or int
> for process - should be convertible to int or long if possible using
> Integer.decode() or Long.decode()
>
> ImageAddressSpace.getID() for address space - this is needed
>
> Image.close() - this would be useful to tidy up temporary files etc., but
> how should this work. Should all API calls that need the image then throw
> an exception. If so, what?
>
> Class names with slash or dot as separator?
> java/lang/Object or java.lang.Object ?
>
> Iterators over heap - should JavaObjects representing classes be found
> when walking over the heaps?
> c.f. DTFJ
> Java 1.4.2 - in separate heap
> Java 5.0 - missing
> Java 6 interspersed, but JavaClass at different address to JavaObject
>
> Thread roots
> Global roots
> JavaReference.getSource()
> - if the location of a root is known to be connected with a thread but
> cannot be connected to a particular JavaStackFrame can a JavaThread be a
> source?
>
> JavaReference.getSource() getTarget()
> These just return an Object. Is there a more type-safe way of doing this?
> Perhaps not - the choice is a runtime ClassCastException or some other
> sort of exception from a getSourceJavaObject() etc. calls.
>
> JavaObject.getReferences
> If there are hidden references (i.e. from a JavaObject of a
> JavaClassLoader to the defined classes) should these be returned as well
> as the fields and the type. I.E should getRoots and getReferences be
> sufficient to traverse the graph of objects and locate every live object?
>
> JavaObject/JavaClass - any commonality - should they have a common
> superclass?
>
> Should the API have one-way or two-way links between objects. Two-way
> makes combining data from two different dump readers a bit harder. E.g. if
> the first reader delegates to the secondary reader, but the data from the
> secondary reader returns an ImagePointer (even from CorruptData) then you
> can get the ImageAddressSpace from the secondary reader, which won't match
> the primary reader.
>
> JavaClass - would a getInstanceSize() method be useful? This wouldn't make
> sense for arrays unless it was for a zero length array.
>
> ImagePointer - getPointerAt() knows the size as 32-bit or 64-bit, but
> pointer size is obtained from ImageProcess, so conceivably there could be
> one address space with several processes with different pointer sizes! How
> does getPointerAt know the size then?
>
> ImageSection.isShared() - with other VMs? Does this include VMs in the
> same address space or process? Is shared a address space, process or
> runtime concept? Does it depend on the type of the section?
>
> ImageStackFrame.getBasePointer() - do we have a hint whether the frame
> goes up or down from this address and what the size of the frame is?
>
> JavaStackFrame.getBasePointer() - do we have a hint whether the frame goes
> up or down from this address and what the size of the frame is?
>
> hashCode() and equals()
> Why are these listed as methods in some of the interfaces? Is an
> implementation required to override the java.lang.Object version?
> Why does JavaStackFrame have hashCode() listed in JavaDoc?
>
> ManagedRuntime.getVersion() - Why CorruptDataException if no
> understandable version data (does that include no version data stored in
> the dump?)
>
> JavaClass.getConstantPoolReferences() - this seems to just return
> JavaObjects. If the JavaObject represents a class, is there an easy way to
> get to the JavaClass? The reverse step is easy with JavaClass.getObject().
> Should this API return JavaClass and JavaObjects? That would be harder to
> make it use a generics for the iterator.
>
>
> Andrew Johnson
>
>
>
>
>
>
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>
>
>
>
>
>

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