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From Stefano Bagnara <apa...@bago.org>
Subject Field, RawField, ParsedField and parsing methods
Date Mon, 28 Dec 2009 12:02:56 GMT
Hi all,

I've just reviewed some mime4j code about parsing and I find the
current behaviour not so intuitive.

1) We have a "Field" interface, a RawField and a ParsedField. Most
code deal with generic Fields but knows when it is a parsedfield or a
rawfield. Nowhere we check the Field implementation to understand if
it is already parsed or not, so it seems we always know when it is a
parsedfield and when it is a rawfield. Some code calling getName does
a trim and a lowercase, some other code simply lowercase without
trimming. Why don't we simply canonicalize things in getName and
publish a clear contract about what getName returns?

2) We have a FieldParser interface (that I just generified to
FieldParser<T extends ParsedField>) that create ParsedField instances
starting from String name, String body, ByteSequence raw. This seems
"weird" to me. name and body are already a parsing step beyond raw.
"FieldParser" is implemented by fields implementation and always call
the constructor for the field with the same 3 parameters. All of the
fields, in facts, simply parse "body" and bring around "raw" only as
convenience way to avoid recreating (reencoding, refolding) stuff when
writing to a mime stream.
We have 3 callers for FieldParser.parse method:
A. Fields.parse(name, body): this method generate a raw sequence
before parsing (ContentUtil.encode(MimeUtil.fold(fieldname+":
"+fieldbody))).
B. DefaultFieldParser.parse(ByteSequence raw, String rawStr): this
method, instead, generate name and body starting from raw (So, the
same functionality as RawField, but implemented differently).
C. DelegatingFieldParser.parse : this is simply a delegator, called in
the above 2 places (A&B).

The DefaultFieldParser.parse method is always called via
DefaultFieldParser.parse(ByteSequence) method, and this method is
called by MessageBuilder and SimpleContentHandler field methods (they
are our 2 implementations of ContentHandler). So, the
ContentHandler.field(Field) method is called by the
MimeStreamParser.parse(InputStream) method. Another weird thing: the
MimeStreamParser already do some parsing, because it checks for the
":" and build a RawField from the ByteArrayBuffer and the colon
position. So:
- AbstractEntity.parseField parse a ByteArrayBuffer to find the ":"
and build a RawField.
- MimeStreamParser calls the ContentHandler passing the RawField as a
generic "Field"
- Both ContentHandlers we distribute runs a
DefaultFieldParser.parse(field.getRaw) looking again for the ":" from
"raw" data and returning a ParsedField under the Field interface.

Isn't it weird that we work with Field interface, RawField
implementation but we simply "drop" any information but the
ByteSequence and parse the field again from scratch (getRaw) in order
to return "again" a Field (that this time is ParsedField but we don't
tell this to anyone)?

Also AbstractField tries to be optimized as it use final name/body/raw
fields. So AbstractFields is an immutable bean, but in practice, every
extension of AbstractField uses lazy parsing (exept the
ContentTransferEncodingField) so that it is functionally immutable but
not really immutable for the jvm. IMO we should make a choice and
embrace it. In order to alter the headers currently an user is
expected to parse it and then to create a new raw representation of
the new header and parse it to generate the new header to replace the
old one. Either we make the fields mutable so to make it easier to
alter them, or we make them immutable for real, so the JVM optimize
their use, don't you agree? (I mean that we could introduce an
interface for each field, and have 2 implementations for each of them:
one parsed and one raw, maybe referencing the underlying RawField. The
raw one could even be a simply proxy that encapsulate the RawField and
create an instance of the parsed field only when a method is
requested).

3) I fail to understand what is the contract between mime4j library
and its users wrt to fields and fieldsparsing.
Let say I have to parse a ContentType: am I expected to call
DefaultFieldParser.parse ? Fields.contentType and other methods?

As I fail to see the current "idea" maybe there is no idea and simply
this is the result of too many hands and refactorings done in the
years, so before being the next hand and applying the next refactoring
I'd like to collect some thought.

Do you think all I've written are foolish thoughts or do you think we
should try to sort this stuff out before releasing mime4j 1.0 ?

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