Using AXIOM/C As The XML Object Model

In Apache Axis2/C AXIOM is used as the basic object model to represent XML. AXIOM provide a DOM like API that allows to traverse and build the XML very easily.

Anyway in underneath, AXIOM is different from DOM, as it has used some techniques to optimize the parsing of the XML as suited specially for SOAP message processing in web services. For an example the SOAP processor can validate a SOAP message by reading only some parts of the SOAP header fields, and if it is not valid, they can completely skip processing the body part. And since AXIOM is designed to built from a stream of data retrieved from a transport, sometimes SOAP processors can validate the message without the need of reading the full stream.

Anyway there should be lot of application that needs this optimization in parsing XMLs. They can easily adapt AXIOM/C to their application. Here is an AXIOM/C tutorial that covers both parsing and building XMLs from AXIOM. In this post I’d like to mention a code that can be used to retrieve an AXIOM from a String (char buffer) which we call as deserialization.

    axiom_node_t* AXIS2_CALL
    deserialize_my_buffer (
        const axutil_env_t * env,
        char *buffer)
    {
        axiom_xml_reader_t *reader = NULL;
        axiom_stax_builder_t *builder = NULL;
        axiom_document_t *document = NULL;
        axiom_node_t *payload = NULL;

        reader = axiom_xml_reader_create_for_memory (env,
            buffer, axutil_strlen (buffer),
            AXIS2_UTF_8, AXIS2_XML_PARSER_TYPE_BUFFER);

        if (!reader)
        {
            return NULL;
        }

        builder = axiom_stax_builder_create (env, reader);

        if (!builder)
        {
            return NULL;
        }
        document = axiom_stax_builder_get_document (builder, env);
        if (!document)
        {
            AXIS2_LOG_ERROR (env->log, AXIS2_LOG_SI,
                    "Document is null for deserialization");
            return NULL;
        }

        payload = axiom_document_get_root_element (document, env);

        if (!payload)
        {
            AXIS2_LOG_ERROR (env->log, AXIS2_LOG_SI,
                    "Root element of the document is not found");
            return NULL;
        }
        axiom_document_build_all (document, env);

        axiom_stax_builder_free_self (builder, env);

        return payload;
    }

Regardless of the fact this piece of code is been used many time by Axis2 and application that uses Axis2, it has never been identified as a core AXIOM function. I think it is better we have this function as an alternative method to create an axiom.

axiom_node_t *AXIS2_CALL
axiom_node_create_from_buffer(const axutil_env_t *env, axis2_char_t *buffer);

I already suggested this in Axis2/C mailing list and hopefully it will be included from the next release.

Here when we create the axiom tree function from the character buffer, we used “axiom_xml_reader_create_for_memory” function. Anyway whenever transport read data stream from wire it always uses the “axiom_xml_reader_create_for_io” function.

    /**
     * This create an instance of axiom_xml_reader to
     * parse a xml document in a buffer. It takes a callback
     * function that takes a buffer and the size of the buffer
     * The user must implement a function that takes in buffer
     * and size and fill the buffer with specified size
     * with xml stream, parser will call this function to fill the
     * buffer on the fly while parsing.
     * @param env environment MUST NOT be NULL.
     * @param read_input_callback() callback function that fills
     * a char buffer.
     * @param close_input_callback() callback function that closes
     * the input stream.
     * @param ctx, context can be any data that needs to be passed
     * to the callback method.
     * @param encoding encoding scheme of the xml stream
     */
    AXIS2_EXTERN axiom_xml_reader_t *AXIS2_CALL
    axiom_xml_reader_create_for_io(
        const axutil_env_t * env,
        AXIS2_READ_INPUT_CALLBACK read_callback,
        AXIS2_CLOSE_INPUT_CALLBACK close_callback,
        void *ctx,
        const axis2_char_t * encoding);

As you may have noticed it requires us to implement a “read_callback” function. Here is an example function prototype to implement this callback.

    int AXIS2_CALL
    some_function(
            char *buffer,
            int size,
            void *ctx);

This function will be called by the parser as required to parse the XML read from some stream.

So if your application involves reading data from a stream you are always recommended to use this function (i.e. “axiom_xml_reader_create_for_io”) instead of “axiom_xml_read_create_for_buffer” to create the AXIOM model more effectively.