Re: reading binary data - C like bit field idiom

Mark Space <>
Mon, 04 May 2009 20:58:42 -0700
Joshua Cranmer wrote:

Some other notes is that the field ordering is significant--which is
probably not a terribly big deal, but it can open up incompatibility
between JVMs, if the order of fields in reflective calls is modified.
It's also rather error-intolerant, relying on the fact that all fields
are part of the structure.

Ken's idea does have merit. Anything to save the harried programmer
time! However, I noticed the same thing. The Java docs say that the
fields returned are "in no particular order." It would be impossible to
rely on them to match some binary format.

However, Ken does mention on his blog "parsing" the annotations... I
wonder if this idea could be extended literally? Fields and annotations
aren't ordered, but Strings sure are.

     private static final String binaryFormat =
             " @parm( LITTLE_ENDIAN );"
             + " int : header1; "
             + " int : header2; "
             + " short : length; "
             + " short : bitFields; ";

One could create a string, like above, that was literally read by a
library object which in turn inserts the vales, read in the order
specified in this string, into the classes fields. I'm not sure how
this would work with security (I don't normally go poking around in
other classes internals) but there might be some way around it.

I checked out the Serializable interface and noticed a little used API
for implementing your own protocols: it's grafted on top of
Serializable and works through the same APIs, I think (I haven't
actually tried it yet).

Something like this:

public class BinSerial implements Externalizable

     private static final String binaryFormat =
             " @parm( LITTLE_ENDIAN );"
             + " int : header1; "
             + " int : header2; "
             + " short : length; "
             + " short : bitFields; ";

     private int header1;
     private int header2;
     private short length;
     short bitFields;

     public void writeExternal(ObjectOutput out)
             throws IOException
         // do something snazzy here...

     public void readExternal(ObjectInput in)
             throws IOException, ClassNotFoundException
         // ditto

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