com.sun.msv.writer.trex
Class TREXWriter

java.lang.Object
  |
  +--com.sun.msv.writer.trex.TREXWriter

All Implemented Interfaces:

GrammarWriter

public class TREXWriter

extends Object

implements GrammarWriter

converts any Grammar into TREX XML representation through SAX1 events.

How it works

Grammar object can be thought as a (possibly) cyclic graph made from Expression. For example, the following simple TREX pattern will be represented as following AGM.

 <grammar>
   <start name="X">
     <element name="foo">
       <choice>
         <string> abc </string>
         <ref name="Y" />
       </choice>
     </element>
   </start>
   <define name="Y">
     <element name="bar">
       <string> abc </string>
       <optional>
         <ref name="X" />
       </optional>
     </element>
   </define>
 </grammar>
 

Note that

• sub expressions are shared (see <string> expression).
• there is a cycle in the graph.
• several syntax elements are replaced by others (e.g., <optional>P</optional> -> <choice><empty/>P</choice>)

To write these expressions into TREX XML representation, we have to take care of cycles, since cyclic references cannot be written into XML without first cut it and use <ref>/<define> pair.

First, this algorithm splits the grammar into "islands". Island is a tree of expressions; it has a head expression and most importantly it doesn't contain any cycles in it. Member of an island can be always reached from its head.

TREXWriter will make every ElementExp and ReferenceExp a head of their own island. So each of them has their own island.

It is guaranteed that this split will always give islands without inner cycles. Several islands can form a cycle, but one island can never have a cycle in it. This is because there is always at least one ElementExp in any cycle.

Note that since expressions are shared, one expression can be a member of several islands (although this isn't depicted in the above figure.)

Then, this algorithm merges some islands. For example, island E is referenced only once (from island D). This means that there is no need to give a name to this pattern. Instead, island E can simply written as a subordinate of island D.

In other words, any island who is only referenced at most once is merged into its referer. This step makes the output more compact.

Next, TREXWriter assigns a name to each island. It tries to use the name of the head expression. If a head is anonymous ReferenceExp (ReferenceExp whose name field is null) or there is a name conflict, TREXWriter will add some suffix to make the name unique.

Finally, each island is written as one named pattern under <define> element. All inter-island references are replaced by <ref> element.

Why SAX1?

Due to the bug and insufficient supports for the serialization through SAX2, The decision is made to use SAX1. SAX1 allows us to control namespace prefix mappings better than SAX2.

Author:

Kohsuke KAWAGUCHI

Constructor Summary

TREXWriter()

Method Summary

void	setDocumentHandler(DocumentHandler handler) this DocumentHandler will receive XML representation of TREX pattern.
void	write(Grammar g) converts this grammar to the XML representation.
void	write(Grammar g, String defaultNs) generates SAX2 events of the specified grammar.

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

TREXWriter

public TREXWriter()

Method Detail

write

public void write(Grammar g)
           throws SAXException

Description copied from interface: GrammarWriter

converts this grammar to the XML representation.

Specified by:

write in interface GrammarWriter

Following copied from interface: com.sun.msv.writer.GrammarWriter

Throws:

UnsupportedOperationException - if this grammar cannot be serialized. this exception can be thrown on the half way of the conversion.

SAXException - DocumentHandler may throw a SAXException.

write

public void write(Grammar g,
                  String defaultNs)
           throws SAXException

generates SAX2 events of the specified grammar.

Parameters:

defaultNs - if specified, this namespace URI is used as "ns" attribute of grammar element. Can be null.

Throws:

IllegalArgumentException - If the given grammar is beyond the expressive power of TREX (e.g., some RELAX NG grammar), then this exception is thrown.

setDocumentHandler

public void setDocumentHandler(DocumentHandler handler)

this DocumentHandler will receive XML representation of TREX pattern.

Specified by:

setDocumentHandler in interface GrammarWriter