Working with XML in RPGLE : XML-SAX

Working with XML in RPGLE : XML-SAX

Working with XML has never been easier before IBM's introduction of XML-INTO and XML-SAX compatibility to RPGLE.

My earlier post did contain detailed info about basics on XML with RPGLE and compatibility of XML-INTO with RPGLE along with its own advantages and disadvantages. If you haven't gone through them yet, Click on those links to have quick look.

XML-SAX:

Even though XML-INTO provides the greatest comfort to programmers by directly bringing the data to Respective variable/data structure. But, this doesn't help programmers when the XML elements/data has been inconsistent.

XML-SAX is the best way of handling such XMLs. This will read the XML character by character and calls Handling Procedure when the even gets triggered.

Syntax:

Xml-Sax(e) %Handler(XMLHandler_Procedure : CommsArea_DataStructure ) %XML(wXmlString: 'doc=string')

Below are some possible events that can occur while parsing XML. 

This has been split by the phase those events can occur along with its description to understand better.

Events discovered before the first XML element:

• *XML_START_DOCUMENT - Indicates that parsing has begun
• *XML_VERSION_INFO - The "version" value from the XML declaration
• *XML_ENCODING_DECL - The "encoding" value from the XML declaration
• *XML_STANDALONE_DECL - The "standalone" value from the XML declaration
• *XML_DOCTYPE_DECL - The value of the Document Type Declaration

Events related to XML elements

• *XML_START_ELEMENT- The name of the XML element that is starting
• *XML_CHARS - The value of the XML element
• *XML_PREDEF_REF - The value of a predefined reference
• *XML_UCS2_REF - The value of a UCS-2 reference
• *XML_UNKNOWN_REF - The name of an unknown entity reference
• *XML_END_ELEMENT - The name of the XML element that is ending

Events related to XML attributes

• *XML_ATTR_NAME - The name of the attribute
• *XML_ATTR_CHARS - The value of the attribute
• *XML_ATTR_PREDEF_REF - The value of a predefined reference
• *XML_ATTR_UCS2_REF - The value of a UCS-2 reference
• *XML_UNKNOWN_ATTR_REF - The name of an unknown entity reference
• *XML_END_ATTR - Indicates the end of the attribute

Events related to XML processing instructions

• *XML_PI_TARGET - The name of the target
• *XML_PI_DATA - The value of the data

Events related to XML CDATA sections

• *XML_START_CDATA - The beginning of the CDATA section
• *XML_CHARS - The value of the CDATA section
• *XML_END_CDATA - The end of the CDATA section

Other events

• *XML_COMMENT - The value of the XML comment
• *XML_EXCEPTION - Indicates that the parser discovered an error
• *XML_END_DOCUMENT - Indicates that parsing has ended

Below example will show on how to use some of the basic events that usually occur. 

E.g.:

H DftActGrp(*No)                                                           

 // Standalone Variables Declaration                                       

D wSampleXml      S            512A                                        

D wXmlOptions     S            512A   Varying                              

D SaveElement     S             50A                                        

                                                                           

 // Data Structure for Communication Area                                  

D DsCommunicationArea...                                                   

D                 DS                                                       

D  AttrName                     20A   Varying                              

D  HaveAttr                       N                                        

D  AttrValue                    20A   Varying                              

                                                                           

D DsCommArea      DS                  LikeDs(DsCommunicationArea)          

                                                                           

 // Handling Procedure Prototype                                           

D pHandlingProcedure...                                                    

D                 PR            10I 0                                      

D  pCommArea                          LikeDs(DsCommunicationArea)          

D  pEvent                       10I 0 Value                                

D  pString                        *   Value                                

D  pStringLen                   20I 0 Value                                

D  pExceptionId                 10I 0 Value                                

 /Free                                                                     

                                                                           

   wSampleXml  = '<Order>'                                                 

               + '<Type>Weborder</Type>'                                   

               + '<Number>123456789</Number>'                              

               + '<Customer>Mr. John</Customer>'                           

               + '<Status>Despatched</Status>'                             

               + '</Order>' ;                                              

                                                                           

   wSampleXml  = %Xlate(x'3f' : x'40' : wSampleXml) ;                      

                                                                           

   wXmlOptions = 'doc=string ';                                            

                                                                           

   Xml-Sax(E) %Handler(pHandlingProcedure : DsCommArea)                    

              %Xml(wSampleXml : wXmlOptions) ;                             

   If %Error() ;                                                           

      Dsply 'Error Occured While Parsing XML. Please Review.' ;            

   EndIf ;                                                                 

                                                                           

   *InLr = *On ;                                                           

                                                                           

 /End-Free                                                                 

 // Handling Procedure for XML-SAX                                         

P pHandlingProcedure...                                                    

P                 B                                                        

D pHandlingProcedure...                                                    

D                 PI            10I 0                                      

D  pCommArea                          LikeDs(DsCommunicationArea)          

D  pEvent                       10I 0 Value                                

D  pString                        *   Value                                

D  pStringLen                   20I 0 Value                                

D  pExceptionId                 10I 0 Value                                

                                                                           

 // Work Variables Declaration                                             

D wReturnValue    S             10I 0 Inz(0)                               

D Value           S          65535A   Based(pString)                       

                                                                           

 /Free                                                                     

                                                                           

    Select ;                                                               

    When pEvent = *XML_START_DOCUMENT ;                                    

      pCommArea.HaveAttr = *Off ;                                          

      Dsply 'Initialize Local Variables. If any' ;                         

    When pEvent = *XML_START_ELEMENT ;                                     

      SaveElement = %Subst(Value:1:pStringLen);                            

      Dsply SaveElement ;                                                  

    When pEvent = *XML_CHARS ;                                             

      SaveElement = %Subst(Value:1:pStringLen);                            

      Dsply SaveElement ;                                                  

    When pEvent = *XML_END_ELEMENT ;                                       

      SaveElement = %Subst(Value:1:pStringLen);                            

      Dsply SaveElement ;                                                  

    When pEvent = *XML_ATTR_NAME ;                                         

      pCommArea.AttrName = %Subst(Value:1:pStringLen);                     

      Dsply pCommArea.AttrName ;                                           

    When pEvent = *XML_ATTR_CHARS ;                                        

      pCommArea.AttrValue = %Subst(Value:1:pStringLen);                    

      Dsply pCommArea.AttrValue ;                                          

    When pEvent = *XML_END_ATTR ;                                          

      pCommArea.AttrName = %Subst(Value:1:pStringLen);                     

      Dsply pCommArea.AttrName ;                                           

    When pEvent = *XML_END_DOCUMENT ;                                      

      wReturnValue = -1 ;                                                  

      Dsply 'No Further Parsing is Needed' ;                               

    EndSl ;                                                                

    Return wReturnValue ;                                                  

                                                                           

 /End-Free                                                                 

P pHandlingProcedure...                                                    

P                 E                                                    

In this example, we are storing XML data into Variable and using Option 'doc=string'. 

If the XML has been stored on IFS path. Same program can be considered as an example by just simple change.

• Initialize the wSampleXml with IFS path.

D wSampleXml      S            512A   Inz('home/PReddy/SampleXml.xml')   

• Remove the XML Population into wSampleXml variable.
• Use 'doc=file' in XML options.

   wXmlOptions = 'doc=string ';        

You can try on by replacing 'Dsply' Statements with actual requirement.

Working with XML in RPGLE - Contd.

Working with XML in RPGLE

We have seen how to read XML in RPG by using XML-INTO in the previous post.

If you have not gone through this yet, Click Here to have quick look.

Before we get started with XML-SAX here are some other key points to be noted about XML.

Key Points to Remember:

The XML documents can be in a character or UCS-2 RPG variable, or they can be in an Integrated File System file.

• This would give freedom to the programmer to choose the best suited form. 

The parser is a SAX (Simple API for XML) parser. A SAX parser operates by reading the XML document character by character.

• Whenever it has located a fragment of the XML document, such as an element name, or an attribute value, it calls back to a handling procedure provided by the caller of the parser, passing it information about the fragment of XML that it has found.

It'd be easier to understand with an example.

E.g.:

When the parser has found an XML element name, it calls the handling procedure indicating that the "event" is a "start element" event and passing it the name of the element.
The handling procedure processes the information and returns to the parser which continues to read the XML document until it has enough information to call the handling procedure with another event. This process repeats until the entire XML document has been parsed, or until the handling procedure indicates that parsing should end.

Here you go with simple example to see how the events are defined.

<email type="text">
  <sendto>PReddy@email.com</sendto>
</email>

Lets break this data and see how the Parser will read the XML by Events & Event Data.

• If the Parsed Text is '<email' or '<sendto>', Event would be Start Element and Event Data would be 'email' or 'sendto' respectively.
• If the Parsed Text is 'type=', Event would be Attribute value and Event Data would be 'type'.
• If the Parsed Text is 'PReddy@email.com', Event would be Element Content and Event data would be 'PReddy@email.com'.
• If the Parsed Text is '</email>' or '</sendto>', Event would be End Element and Event Data would be 'email' or 'sendto' respectively.

The XML-SAX and XML-INTO operation codes allow you to use the XML parser. Some more points to make a note about these Opcodes.

• The XML-SAX operation allows you to specify an event handling procedure to handle every event that the parser generates. This is useful if you do not know in advance what an XML document may contain (XML-INTO will be useful when you know what XML will contain exactly).

E.g.:

• If you know that an XML document will contain an XML attribute with the name type, and you want to know the value of this attribute, your handling procedure can wait for the "attribute name" event to have a value of "type". Then the next time the handler is called, it should be an "attribute value" event, with the required data ("text" in the example above).

• The XML-INTO operation allows you to read the contents of an XML document directly into an RPG variable. This is useful if you know the format of the XML document and you know that the names of the XML elements in the document will be the same as the names you have given to your RPG variables.

E.g.:

• If you know that the XML document will always have the form of the document above, you can define an RPG data structure with the name "email", and with subfields "type" and "sendto".
• Then you can use the XML-INTO operation to read the XML document directly into the data structure. When the operation is complete, the "type" subfield would have the value "text" and the "sendto" subfield would have the value "PReddy@email.com".

• The XML-INTO operation also allows you to obtain the values of an unknown number of repeated XML elements. You provide a handling procedure that receives the values of a fixed number of elements each time the handling procedure is called. This is useful if you know that the XML document will contain a series of identical XML elements, but you don't know in advance how many there will be.

• The XML data is always returned by the parser in text form. If the data is known to represent other datatypes such as numeric data, or date data, the XML-SAX handling procedure must use conversion functions such as %INT or %DATE to convert the data.

We'll see more about XML-SAX & %Handler along with an example in next post. Click Here to go through.

Working with XML in RPGLE : XML-INTO

Working with XML in RPGLE

XML has been one of the best format to pass a communication message between different Applications/Servers. This can be easily readable by both Humans & Machines.

With the introduction of XML-INTO Opcode in RPGLE, Reading XML has become easier in RPG.

XML-INTO:

XML-INTO is an Opcode introduced for RPG programmers to retrieve XML data directly to RPG Variable. This Opcode need to be used in conjuction with %XML & %HANDLER.

This can be used in both Free & Fixed Formats. Yet, There will be limitations for using in fixed format.

1. One of the Main Reasons being the its limitation on Number of digits.

XML-INTO parses an XML document extracting a single element directly into a variable (Or an array). Usually, This will be used to extract multiple elements and values will be populated to Data Structure.

But, This approach would be better suited if number of items retrieved is known.

The basic syntax of the basic variant of XML-INTO is:

XML-INTO { (E H} } variable %XML( xmlDoc { : options } );

E - will set on %Error BIF if there is any occured while parsing xml.
H - will be used if we require any numeric to be rounding during charecter - numeric conversion.

But, These are Optional. And, cannot be using in Fixed format.

variable - This parameter can be a simple variable or an array or Data Structure based on XML.
The parser will be looking to match the names, nesting levels of the fields in the receiver with element and attribute names as per XML.

Here is a Small example to show how the data will be parsed into RPG variable.

E.g.:

XML:

<Order Type = "Weborder">
   <Number>123456789</Number>
   <Customer>Mr. John</Customer>
   <Status>Despatched</Status>
</Order>

Data Structure to recieve Data:

D Order           DS              
D  Type                         10A
D  Customer                     30A
D  Status                       10A

Statement:

XML-INTO Order %XML('/home/PReddy/Orders.xml' : 'doc=file')

This Statement will retrieve the xml data directly into Data Structure "Order".

Here is one more example on how the data can be retrieved if there are multiple layers of data.
This can be done using Nested Data Structures.

E.g.:

XML:

<Order Type = "Weborder">
   <Number>123456789</Number>
   <Customer Name = "Mr. John">
      <Address1>West Street</Address1>
      <Address2>xxxxxxxxxxx</Address2>
      <Country>United States</Country>
   <Status>Despatched</Status>
</Order>

Data Structure:

D Order           DS              
D  Type                         10A
D  Customer                          LikeDs(wCustomer)
D  Status                       10A

D wCustomer       DS
D  Name                         30A
D  Address1                     30A
D  Address2                     30A
D  Country                      25A

Statement:

XML-INTO Order %XML('/home/PReddy/Orders.xml' : 'doc=file')

Below are the few additional Options that can be used along with %XML BIF.

1. path option
2. doc option
3. ccsid option
4. case option
5. trim option
6. allow missing option
7. allow extra option
8. data subfield
9. count prefix

Signing off with the basics. Click Here to see more details about XML.