Class RELAXNGWriter

  • All Implemented Interfaces:
    GrammarWriter, Context

    public class RELAXNGWriter
    extends Object
    implements GrammarWriter, Context
    converts any Grammar into RELAX NG 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
    • Field Detail

      • grammar

        protected Grammar grammar
        Grammar object which we are writing.
      • exp2name

        protected Map exp2name
        map from ReferenceExp/ElementExp to its unique name. "unique name" is used to write/reference this ReferenceExp. ReferenceExps who are not in this list can be directly written into XML.
      • defaultNs

        protected String defaultNs
        namespace URI currently implied through "ns" attribute propagation.
      • patternWriter

        protected com.sun.msv.writer.relaxng.RELAXNGWriter.SmartPatternWriter patternWriter
    • Constructor Detail

      • RELAXNGWriter

        public RELAXNGWriter()
    • Method Detail

      • 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.
        SAXException
      • writeIsland

        protected void writeIsland​(Expression exp)
        writes a bunch of expression into one tree.
      • sniffDefaultNs

        protected String sniffDefaultNs​(Expression exp)
        sniffs namespace URI that can be used as default 'ns' attribute from expression. find an element or attribute, then use its namespace URI.