All about JAXP

http://www-128.ibm.com/developerworks/java/library/x-jaxp

Brett McLaughlin

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Document Object Model (DOM)

Simple API for XML (SAX)

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JDK1.4

javax.xml.parsers
javax.xml.transform
javax.xml.transform.dom
javax.xml.transform.sax
javax.xml.transform.stream

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JDK5.0

javax.xml
javax.xml.datatype
javax.xml.namespace
javax.xml.parsers
javax.xml.transform
javax.xml.transform.dom
javax.xml.transform.sax
javax.xml.transform.stream
javax.xml.validation
javax.xml.xpath

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嚴格來說， JAXP 并沒有提供新的解析 XML 的方法，但是它使得我們更容易地使用 DOM 或者 SAX 來進行解析任務(wù)，更能以一種 vendor-neutral 的方式來使用 DOM 和 SAX 。

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JAXP 與 SAX, DOM, JDOM 和 dom4j （這四個都可以解析 XML ）沒有可比較性，它并沒有提供一個新的解析 XML 的方法。

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JAXP API, located in the javax.xml.parsers package. All of these classes sit on top of an existing parser. 其中的 6 個類都是建立在已有的解析上面的。

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JDOM 與 dom4j 都提供不同的模型來接受來自 SAX/DOM 的數(shù)據(jù)，他們從內(nèi)部來講都是使用了 SAX ，不過是做了些修改。

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此外， Java 1.5 中 Xerces 的包 org.apache.xerces 被放到了 com.sun.org.apache.xerces.internal

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First, com.sun.xml.tree.XMLDocument class is not part of JAXP. It is part of Sun's Crimson parser, packaged in earlier versions of JAXP.

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Second, a major purpose of JAXP is to provide vendor independence when dealing with parsers. With JAXP, you can use the same code with Sun's XML parser, Apache's Xerces XML parser, and Oracle's XML parser.

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先從 SAX 說起，我們只用繼承 DefaultHandler 類 (org.xml.sax.helpers 包中 ) 就能獲得所有的 callbacks ，只用在需要的方法中加入實現(xiàn)的代碼。

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Here's the typical SAX routine:

1 ． Create a SAXParser instance using a specific vendor's parser implementation.

2 ． Register callback implementations (by using a class that extends DefaultHandler, for example).

3 ． Start parsing and sit back as your callback implementations are fired off.

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SAX 必須 指定 XML 驅(qū)動（如 org.apache.xerces.parsers.SAXParser ），而 JAXP 提供了更好的選擇，只要我們提供什么 XML 驅(qū)動（在 classpath 中配置），它就調(diào)用什么驅(qū)動，不需要改動代碼。

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new SAXParserFactory.newSAXParser() 就返回 JAXP SAXParser 類，這個類包裝了 SAX parser (an instance of the SAX class org.xml.sax.XMLReader ) 。

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In Listing 1, you can see that two JAXP-specific problems can occur in using the factory: the inability to obtain or configure a SAX factory, and the inability to configure a SAX parser.

The first of these problems, represented by a FactoryConfigurationError, usually occurs when the parser specified in a JAXP implementation or system property cannot be obtained.

The second problem, represented by a ParserConfigurationException, occurs when a requested feature is not available in the parser being used. Both are easy to deal with and shouldn't pose any difficulty when using JAXP.

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SAXParser 的 parse 方法可以接受 SAX InputSource ， Java InputStream 或者 URL in String form

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可以通過 SAXParser 的 getXMLReader() 方法來獲得底層的 SAX parser （ org.xml.sax.XMLReader 的實例），這樣就可以使用各個 SAXParser 方法。 [ 參照 Listing2]

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使用 DOM??

The only difference between DOM and SAX in this respect is that with DOM you substitute DocumentBuilderFactory for SAXParserFactory, and DocumentBuilder for SAXParser.

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The major difference is that variations of the parse() method do not take an instance of the SAX DefaultHandler class. Instead they return a DOM Document instance representing the XML document that was parsed. The only other difference is that two methods are provided for SAX-like functionality:

• setErrorHandler(), which takes a SAX ErrorHandler implementation to handle problems that might arise in parsing
• setEntityResolver(), which takes a SAX EntityResolver implementation to handle entity resolution

JAXP 的使用

1. Source for input

The javax.xml.transform.Source interface is the basis for all input into JAXP and the transformation API. This interface defines only two methods -- getSystemId() and setSystemId(String systemId) .

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JAXP 提供了三個實現(xiàn) Source 接口的類：

• javax.xml.transform.dom.DOMSource passes a DOM Node (and its children) into JAXP.
• javax.xml.transform.sax.SAXSource passes the results of SAX callbacks (from an XMLReader ) into JAXP.
• javax.xml.transform.stream.StreamSource passes XML wrapped in a File , InputStream , or Reader into JAXP.? ?

2. Result for output

? javax.xml.transform.Result 也有兩個方法： getSystemId() 和 setSystemId(String systemId) 。 同樣有三個實現(xiàn)類：

• javax.xml.transform.dom.DOMResult passes transformed content into a DOM Node .
• javax.xml.transform.sax.SAXResult passes the results of a transformation to a SAX ContentHandler .
• javax.xml.transform.stream.StreamResult passes the transformed *ML into a File , OutputStream , or Writer .

3. Performing transformations with JAXP

1)Getting a Factory? ???

2)Creating a Transformer? ?
3)Performing the transformation?? ?
4)Caching XSL stylesheets

JAXP this way has two significant limitations:

• The Transformer object processes the XSL stylesheet each and every time transform() is executed.
• Instances of Transformer are not thread-safe. You can't use the same instances across multiple threads.

Transformer 實例不是線程安全的，不能通過多線程去使用同一個 Transformer 實例。

5)Loading a Template

javax.xml.transform.Templates .

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The Templates interface is thread-safe (addressing the second limitation) and represents a compiled stylesheet (addressing the first limitation).

Templates 實例是線程安全的，可以處理一堆 XSL ，解決了上述兩個限制。

6)From Transformer to Templates

如果只要對一個 stylesheet 進行一個 transformation ，那么用 Transformer 比較快，沒有必要選擇 Templates 對象。但是考慮到線程安全問題，還是推薦使用 Templates 。

7)Changing the XSL processor

JAXP 默認使用 Xalan-J ，如果要使用其它 parser ，可以通過 javax.xml.transform.TransformerFactory 修改。


java -D javax.xml.transform.TransformerFactory=[transformer.impl.class] TestTransformations? simple.xml simple.xsl

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