What is 'Push Approach' and 'Pull Approach' to parsing?

Question

Under the push parsing approach, a push parser generates synchronous events as a document is parsed, and these events can be processed by an application using a callback handler model

This is the text given in the book Pro XML Development with Java about SAX 2.0.

As for StAX, the book says:

Under the pull approach, events are pulled from an XML document under the control of the application using the parser.

I want to ask, what is the meaning of the highlighted text ? An answer befitting a beginner is appreciated :)

Answer 1

Basically, a push is when the parser says to some handler, "I have a foo, do something with it." A pull is when the handler says to the parser, "give me the next foo."

Push:

if (myChar == '(')     handler.handleOpenParen(); // push the open paren to the handler 

Pull:

Token token = parser.next(); // pull the next token from the parser 

Answer 2

Push Parsers - Events are generated by the API in the form of callback functions like startDocument(), endDocument() and are beyond control of programmer. We as a programmer could handle the events but generation of events is beyond control.

Pull Parsers - Events are generated when we call some API. Example shown below. So we as programmer can decide when to generate events.

   int eventType = xmlr.getEventType(); while(xmlr.hasNext()){      eventType = xmlr.next();      //Get all "Book" elements as XMLEvent object      if(eventType == XMLStreamConstants.START_ELEMENT &&           xmlr.getLocalName().equals("Book")){         //get immutable XMLEvent         StartElement event = getXMLEvent(xmlr).asStartElement();         System.out.println("EVENT: " + event.toString());      } }  

, The client only gets (pulls) XML data when it explicitly asks for it.

With pull parsing, the client controls the application thread, and can call methods on the parser when needed. By contrast, with push processing, the parser controls the application thread, and the client can only accept invocations from the parser.