Standard HL7 Encodings

The bulk of the HL7 standard defines the abstract structure of individual messages (e.g. what segments and fields they have, and what they mean). In order to actually send a message over a network, it must be encoded as a sequence of bits. A small section of the HL7 standard is dedicated to defining how this is done. There are two standard encodings:

• The "traditional encoding". This is a method of serializing a message using special delimiter characters to distinguish various message elements. It looks like this: MSH|^~\&|foo|foo... * The "XML encoding". This method uses an XML syntax to identify parts of a message. It looks like this: <MSH><MSH.1>|<MSH.2>^~\&</MSH.2>... At the time of writing, the XML encoding has not been finalized. It is expected to become an official standard in fall 2002.

The Parser Classes

There are several Parser classes in HAPI, which work together to support both the traditional and XML encodings. The following diagram illustrates how they are related:

Programming with Parsers

The following code example demonstrates how to:

• Parse a "traditionally encoded" message string
• Change a value in the message
• Encode the message in XML form

  import ca.uhn.hl7v2.parser.*;
  import ca.uhn.hl7v2.model.Message;
  import ca.uhn.hl7v2.model.v24.message.ACK;
  
  public class ParserDemo {
      
      public static void main(String args[]) {
          //for demo purposes, we just declare a literal message string 
          String ackMessageString 
              = "MSH|^~\\&|foo|foo||foo|200108151718||ACK^A01^ACK|1|D|2.4|\rMSA|AA\r";
          
          //instantiate a PipeParser, which handles the "traditional encoding" 
          PipeParser pipeParser = new PipeParser();
          
          try {
              //parse the message string into a Message object 
              Message message = pipeParser.parse(ackMessageString);
              
              //if it is an ACK message (as we know it is),  cast it to an 
              // ACK object so that it is easier to work with, and change a value            
              if (message instanceof ACK) {
                  ACK ack = (ACK) message;
                  ack.getMSH().getProcessingID().getProcessingMode().setValue("P");
              }
              
              //instantiate an XML parser 
              XMLParser xmlParser = new DefaultXMLParser();
              
              //encode message in XML 
              String ackMessageInXML = xmlParser.encode(message);
              
              //print XML-encoded message to standard out
              System.out.println(ackMessageInXML);
          } catch (Exception e) {
              e.printStackTrace();
          }       
      }
  }

  Note that to run this demo you must have the Xerces XML Parser in your classpath.