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.. _The_SAX_module:
**************
The SAX module
**************
Description
===========
Parsing XML streams can be done with two different methods. They each have
their pros and cons. Although the simplest and probably most usual way to
manipulate XML files is to represent them in a tree and manipulate it through
the DOM interface (see next chapter).
The **Simple API for XML** is another method that can be used for parsing. It
is based on a callbacks mechanism, and doesn't store any data in memory (unless
of course you choose to do so in your callbacks). It can thus be more efficient
to use SAX than DOM for some specialized algorithms. In fact, this whole Ada
XML library is based on such a SAX parser, then creates the DOM tree through
callbacks.
Note that this module supports the second release of SAX (SAX2), that fully
supports namespaces as defined in the XML standard.
SAX can also be used in cases where a tree would not be the most efficient
representation for your data. There is no point in building a tree with DOM,
then extracting the data and freeing the tree occupied by the tree. It is much
more efficient to directly store your data through SAX callbacks.
With SAX, you register a number of callback routines that the parser will call
them when certain conditions occur.
This documentation is in no way a full documentation on SAX. Instead, you
should refer to the standard itself, available at `http://sax.sourceforge.net
<http://sax.sourceforge.net>`_.
Some of the more useful callbacks are `Start_Document`, `End_Document`,
`Start_Element`, `End_Element`, `Get_Entity` and `Characters`. Most of these
are quite self explanatory. The `Characters` callback is called when characters
outside a tag are parsed.
.. highlight:: xml
Consider the following XML file::
<?xml version="1.0"?>
<body>
<h1>Title</h1>
</body>
The following events would then be generated when this file is parsed::
Start_Document Start parsing the file
Start_Prefix_Mapping (handling of namespaces for "xml")
Start_Prefix_Mapping Parameter is "xmlns"
Processing_Instruction Parameters are "xml" and "version="1.0""
Start_Element Parameter is "body"
Characters Parameter is ASCII.LF & " "
Start_Element Parameter is "h1"
Characters Parameter is "Title"
End_Element Parameter is "h1"
Characters Parameter is ASCII.LF & " "
End_Element Parameter is "body"
End_Prefix_Mapping Parameter is "xmlns"
End_Prefix_Mapping Parameter is "xml"
End_Document End of parsing
As you can see, there is a number of events even for a very small file.
However, you can easily choose to ignore the events you don't care
about, for instance the ones related to namespace handling.
Examples
========
There are several cases where using a SAX parser rather than a DOM
parser would make sense. Here are some examples, although obviously
this doesn't include all the possible cases. These examples are taken
from the documentation of libxml, a GPL C toolkit for manipulating XML files.
* Using XML files as a database
One of the common usage for XML files is to use them as a kind of a basic
database, They obviously provide a strongly structured format, and you could
for instance store a series of numbers with the following format::
<array> <value>1</value> <value>2</value> ....</array>
In this case, rather than reading this file into a tree, it would obviously
be easier to manipulate it through a SAX parser, that would directly create a
standard Ada array while reading the values.
This can be extended to much more complex cases that would map to Ada records
for instance.
* Large repetitive XML files
Sometimes we have XML files with many subtrees of the same format describing
different things. An example of this is an index file for a documentation
similar to this one. This contains a lot (maybe thousands) of similar
entries, each containing for instance the name of the symbol and a list of
locations.
If the user is looking for a specific entry, there is no point in loading the
whole file in memory and then traversing the resulting tree. The memory usage
increases very fast with the size of the file, and this might even be
infeasible for a 35 megabytes file.
* Simple XML files
Even for simple XML files, it might make sense to use a SAX parser. For
instance, if there are some known constraints in the input file, say there
are no attributes for elements, you can save quite a lot of memory, and maybe
time, by rebuilding your own tree rather than using the full DOM tree.
However, there are also a number of drawbacks to using SAX:
* SAX parsers generally require you to write a little bit more code than the
DOM interface.
* There is no easy way to write the XML data back to a file, unless you build
your own internal tree to save the XML. As a result, SAX is probably not the
best interface if you want to load, modify and dump back an XML file.
Note however that in this Ada implementation, the DOM tree is built through a
set of SAX callbacks anyway, so you do not lose any power or speed by using
DOM instead of SAX.
The SAX parser
==============
The basic type in the SAX module is the **SAX.Readers** package. It defines a
tagged type, called `Reader`, that represents the SAX parser itself.
Several features are defined in the SAX standard for the parsers. They indicate
what behavior can be expected from the parser. The package `SAX.Readers`
defines a number of constant strings for each of these features. Some of these
features are read-only, whereas others can be modified by the user to adapt the
parser. See the `Set_Feature` and `Get_Feature` subprograms for how to
manipulate them.
The main primitive operation for the parser is `Parse`. It takes an input
stream for argument, associated with some XML data, and then parses it and
calls the appropriate callbacks. It returns once there are no more characters
left in the stream.
Several other primitive subprograms are defined for the parser, that are called
the **callbacks**. They get called automatically by the `Parse` procedure when
some events are seen.
As a result, you should always override at least some of these subprograms to
get something done. The default implementation for these is to do nothing,
except for the error handler that raises Ada exceptions appropriately.
An example of such an implementation of a SAX parser is available in the DOM
module, and it creates a tree in memory. As you will see if you look at the
code, the callbacks are actually very short.
Note that internally, all the strings are encoded with a unique character
encoding scheme, that is defined in the file :file:`sax-encodings.ads`. The
input stream is converted on the fly to this internal encoding, and all the
subprograms from then on will receive and pass parameters with this new
encoding. You can of course freely change the encoding defined in the file
:file:`sax-encodings.ads`.
The encoding used for the input stream is either automatically detected by the
stream itself (:ref:`The_Input_module`), or by parsing the::
<?xml version='1.0' encoding='UTF-8' ?>
processing instruction at the beginning of the document. The list of supported
encodings is the same as for the Unicode module (:ref:`The_Unicode_module`).
The SAX handlers
================
We do not intend to document the whole set of possible callbacks associated
with a SAX parser. These are all fully documented in the file
:file:`sax-readers.ads`.
here is a list of the most frequently used callbacks, that you will probably
need to override in most of your applications.
*Start_Document*
This callback, that doesn't receive any parameter, is called once, just
before parsing the document. It should generally be used to initialize
internal data needed later on. It is also guaranteed to be called only once
per input stream.
*End_Document*
This one is the reverse of the previous one, and will also be called only
once per input stream. It should be used to release the memory you have
allocated in Start_Document.
*Start_Element*
This callback is called every time the parser encounters the start of an
element in the XML file. It is passed the name of the element, as well as
the relevant namespace information. The attributes defined in this element
are also passed as a list. Thus, you get all the required information for
this element in a single function call.
*End_Element*
This is the opposite of the previous callback, and will be called once per
element. Calls to `Start_Element` and `End_Element` are guaranteed
to be properly nested (ie you can't see the end of an element before seeing
the end of all its nested children.
*Characters and Ignore_Whitespace*
This procedure will be called every time some character not part of an
element declaration is encountered. The characters themselves are passed as an
argument to the callback. Note that the white spaces (and tabulations) are
reported separately in the Ignorable_Spaces callback in case the XML
attribute `xml:space` was set to something else than `preserve` for this
element.
You should compile and run the :file:`testsax` executable found in this
module to visualize the SAX events that are generated for a given XML file.
.. _Using_SAX:
Using SAX
=========
This section will guide you through the creation of a small SAX application.
This application will read an XML file, assumed to be a configuration file, and
setup some preferences according to the contents of the file.
The XML file is the following::
<?xml version="1.0" ?>
<preferences>
<pref name="pref1">Value1</pref>
<pref name="pref2">Value2</pref>
</preferences>
This is a very simple example which doesn't use namespaces, and has a very
limited nesting of nodes. However, that should help demonstrate the basics of
using SAX.
.. highlight:: ada
:linenothreshold: 2
Parsing the file
----------------
The first thing to do is to declare a parser, and parse the file. No callback
is put in place in this first version, and as a result nothing happens.
The main program is the following:
.. literalinclude:: sax/saxexample_main.adb
:language: ada
:linenos:
A separate package is provided that contain our implementation of an XML
parser::
with Sax.Readers;
package SaxExample is
type Reader is new Sax.Readers.Reader with null record;
end SaxExample;
There are two steps in setting up an XML parser:
* Create an input stream
This input stream is in charge of providing the XML input to the parser.
Several input streams are provided by XML/Ada, including the one we use in
this example to read the XML data from a file on the disk. The file is called
:file:`pref.xml`.
It has two properties, that should generally be set: the public id will be
used by XML/Ada in its error message to reference locations in that file; the
system id should be the location of the file on the system. It is used to
resolve relative paths found in the XML document.
* Setup the parser
The behavior of an XML parser can be changed in several ways by activating or
deactivating some features. In the example above, we have specified that the
XML document doesn't contain namespaces, and that we do not intend to
validate the XML file against a grammar.
Once the two steps above are done, we can simply call the procedure `Parse` to
perform the actual parsing. Since we are using SAX, XML/Ada will call the
primitive operations of `My_Reader`, which, so far, are inherited form the
default ones provided by XML, and do nothing.
Reacting to events
------------------
We are now going to enhance the example a little, and make it react to the
contents of the XML file.
We are only interested in two particular type of events, which are the opening
and closing of an XML tag, and finding the value of each preference.
The way to react to these events is to override some of the primitive
subprograms in the package :file:`saxexample.ads` as follows:
.. literalinclude:: sax/saxexample.ads
:language: ada
:linenos:
The primitive operations will be called automatically when the corresponding
events are detected in the XML file.
The implementation for these subprograms is detailed below.
Start of XML tags
^^^^^^^^^^^^^^^^^
When an XML tag is started, we need to check whether it corresponds to the
definition of a preference value. If that is the case, we get the value of the
`name` attribute, which specifies the name of a preference::
with Unicode.CES; use Unicode.CES;
with Sax.Attributes; use Sax.Attributes;
with Ada.Text_IO; use Ada.Text_IO;
package body SaxExample is
procedure Start_Element
(Handler : in out Reader;
Namespace_URI : Unicode.CES.Byte_Sequence := "";
Local_Name : Unicode.CES.Byte_Sequence := "";
Qname : Unicode.CES.Byte_Sequence := "";
Atts : Sax.Attributes.Attributes'Class)
is
begin
Handler.Current_Pref := Null_Unbounded_String;
Handler.Current_Value := Null_Unbounded_String;
if Local_Name = "pref" then
Handler.Current_Pref :=
To_Unbounded_String (Get_Value (Atts, "name"));
end if;
end Start_Element;
Characters
^^^^^^^^^^
XML/Ada will report the textual contents of an XML tag through one or more
calls to the `Characters` primitive operation. An XML parser is free to
divide the contents into as many calls to `Characters` as it needs, and
we must be prepared to handle this properly. Therefore, we concatenate the
characters with the current value::
procedure Characters
(Handler : in out Reader;
Ch : Unicode.CES.Byte_Sequence) is
begin
if Handler.Current_Pref /= Null_Unbounded_String then
Handler.Current_Value := Handler.Current_Value & Ch;
end if;
end Characters;
End of tag
^^^^^^^^^^
Once we meet the end of a tag, we know there will be no more addition to
the value, and we can now set the value of the preference. In this example,
we simply display the value on the standard output::
procedure End_Element
(Handler : in out Reader;
Namespace_URI : Unicode.CES.Byte_Sequence := "";
Local_Name : Unicode.CES.Byte_Sequence := "";
Qname : Unicode.CES.Byte_Sequence := "")
is
begin
if Local_Name = "pref" then
Put_Line ("Value for """ & To_String (Handler.Current_Pref)
& """ is " & To_String (Handler.Current_Value));
end if;
end End_Element;
In a real application, we would need to handle error cases in the XML file.
Thankfully, most of the work is already done by XML/Ada, and the errors will be
reported as calls to the primitive operation `Fatal_Error`, which by default
raises an exception.
Understanding SAX error messages
================================
XML/Ada error messages try to be as explicit as possible. They are not,
however, meant to be understood by someone who doesn't know XML.
In addition to the location of the error (line and column in the file),
they might contain one of the following abbreviations:
* `[WF]`
.. index:: WF
This abbreviation indicates that the error message is related to a
well-formedness issue, as defined in the XML standard. Basically, the
structure of the XML document is invalid, for instance because an
open tag has never been closed. Some of the error messages also indicate
a more precise section in the XML standard.
* `[VC]`
.. index:: VC
.. index:: DTD
This abbreviation indicates that the error message is related to an
unsatisfied validity-constraint, as defined in the XML standard. The XML
document is well-formed, although it doesn't match the semantic rules
that the grammar defines. For instance, if you are trying to validate an
XML document against a DTD, the document must contain a DTD that defines the
name of the root element.
|
