java类库详细讲解

				
				
				
				Getting the Declared Entities in a DOM Document
				(Java Developers Almanac Example)
				
				
				
				
				
				
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				The Java Developers Almanac 1.4
				
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				    e529.  
				    Getting the Declared Entities in a DOM Document
				
				The entities declared in the DTD of an XML document are available in
				the DOM document. Unfortunately, with J2SE 1.4's default parser, only
				the names, not the values, are available. In order to obtain the
				values, you must parse the file without expanding entity references
				and then scan the DOM document for the unexpanded entity
				references. The unexpanded entity references contain the values.
				
				 Note: By default, a parser expands entity references while
				constructing the DOM tree. See e516 Preventing Expansion of Entity References While Parsing an XML File
				to prevent expansion. The default parser in J2SE 1.4
				expands entity references in attribute values. There is no way to
				prevent this.
				
				
				
				    // Obtain a document; this method is implemented in
				    // e516 Preventing Expansion of Entity References While Parsing an XML File
				    Document doc = parseXmlFileNoExpandER("infilename.xml", true);
				    
				    // Scan the document for entity references and get their values.
				    // The values are stored in the map using the entity name as the key.
				    Map entityValues = new HashMap();
				    getEntityValues(doc, entityValues);
				    
				    // Get list of declared entities
				    NamedNodeMap entities = doc.getDoctype().getEntities();
				    for (int i=0; i<entities.getLength(); i++) {
				        Entity entity = (Entity)entities.item(i);
				        String entityName = entity.getNodeName();
				        String entityPublicId = entity.getPublicId();
				        String entitySystemId = entity.getSystemId();
				    
				        // Get the value of the entity, which is its set of child nodes
				        Node entityValue = (Node)entityValues.get(entityName);
				    }
				    
				    // This method walks the document looking for entity references.
				    // When one is found, this method adds the entity reference node
				    // to `map' using the name as the key.
				    public static void getEntityValues(Node node, Map map) {
				        if (node instanceof EntityReference) {
				            map.put(node.getNodeName(), node);
				        }
				    
				        // Visit the children
				        NodeList list = node.getChildNodes();
				        for (int i=0; i<list.getLength(); i++) {
				            getEntityValues(list.item(i), map);
				        }
				    }
				
				This is the sample input for the example:
				
				
				    <?xml version="1.0" encoding="UTF-8"?>
				    <!DOCTYPE root [
				        <!ENTITY entity1 "an internal entity">
				    
				        <!-- This is a parameter entity; it will not appear in the DOM document -->
				        <!ENTITY % entity2 "a | b">
				    
				        <!-- This is an external text entity -->
				        <!ENTITY entity3 SYSTEM "External.xml">
				    
				        <!-- This is an external parameter entity; it includes DTD from
				             another file. It will not appear in the DOM document. However,
				             any non-parameter entities declared in the file will be included. -->
				        <!ENTITY % entity4 SYSTEM "More.dtd">
				        %entity4;
				    
				        <!-- entity5 is an unparsed entity; it can only appear as an attribute value -->
				        <!ENTITY entity5 SYSTEM "pic.jpg" NDATA NOTA1>
				        <!NOTATION NOTA1 SYSTEM "jpgviewer.exe">
				        <!ELEMENT elem2 EMPTY>
				        <!ATTLIST elem2 attr ENTITY #REQUIRED>
				    ]>
				    <root a="&entity1;">
				        &entity1;
				        &entity3;
				        &ent1;
				        &ent2;
				        <elem2 attr="entity5"/>
				    </root>
				
				External.xml:
				
				
				    <!-- a file with XML markup -->
				    <i>external</i> text
				
				More.dtd:
				
				
				    <!-- a file with more DTD declarations -->
				    <!ENTITY ent1 "xx">
				    <!ENTITY ent2 "yy">
				    <!ELEMENT elem1 (%entity2;)>
				
				The following lists the entities that would appear in the DOM document.
				Their values are also listed. Notice the parameter entities entity2 and
				entity4 do not appear in the list.
				
				
				
				    entity1=an internal entity
				    entity3=<i>external</i> text
				    ent1=xx
				    ent2=yy
				    ent5=null
				
				If the input file were parsed with entity expansion, 
				the resulting XML would be:
				
				
				    <?xml version="1.0" encoding="UTF-8"?>
				    <root a="an internal entity">
				        an internal entity
				        <!-- a file with XML markup -->
				    <i>external</i> text
				    
				        xx
				        yy
				        <elem2 attr="entity5"/>
				    </root>
				
				Note: The J2SE 1.4 DOM writing routines don't appear to write
				entity references properly. In particular, only text nodes
				that are descendants of the entity reference 
				are written; all other node types are simply not printed.
				
				
				
				
				             Related Examples
				        
				
				
				
				
				e1071. 
				    Getting the Root Element in a DOM Document
				
				
				
				e527. 
				    Getting a Node Relative to Another Node in a DOM Document
				
				
				
				e528. 
				    Getting the Notations in a DOM Document
				
				
				
				e530. 
				    Getting the Value of an Entity Reference in a DOM Document
				
				
				
				
				
				
				
				
				        See also: 
				
				    Adding and Removing Nodes
				  
				
				
				    Element Attributes
				  
				
				
				    Elements
				  
				
				
				    Text Nodes
				  
				
				
				    XPath
				  
				
				
				
				
				
				
				
				
				
				© 2002 Addison-Wesley.