The Unmarshaller class governs the process of deserializing XML
data into newly created Java content trees, optionally validating the XML
data as it is unmarshalled. It provides an overloading of unmarshal methods
for many different input kinds.
Unmarshalling from a File:
JAXBContext jc = JAXBContext.newInstance( "com.acme.foo" );
Unmarshaller u = jc.createUnmarshaller();
Object o = u.unmarshal( new File( "nosferatu.xml" ) );
Unmarshalling from an InputStream:
InputStream is = new FileInputStream( "nosferatu.xml" );
JAXBContext jc = JAXBContext.newInstance( "com.acme.foo" );
Unmarshaller u = jc.createUnmarshaller();
Object o = u.unmarshal( is );
Unmarshalling from a URL:
JAXBContext jc = JAXBContext.newInstance( "com.acme.foo" );
Unmarshaller u = jc.createUnmarshaller();
URL url = new URL( "http://beaker.east/nosferatu.xml" );
Object o = u.unmarshal( url );
Unmarshalling from a StringBuffer using a
javax.xml.transform.stream.StreamSource:
JAXBContext jc = JAXBContext.newInstance( "com.acme.foo" ); Unmarshaller u = jc.createUnmarshaller(); StringBuffer xmlStr = new StringBuffer( "<?xml version="1.0"?>..." ); Object o = u.unmarshal( new StreamSource( new StringReader( xmlStr.toString() ) ) );
Unmarshalling from a org.w3c.dom.Node:
JAXBContext jc = JAXBContext.newInstance( "com.acme.foo" );
Unmarshaller u = jc.createUnmarshaller();
DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
dbf.setNamespaceAware(true);
DocumentBuilder db = dbf.newDocumentBuilder();
Document doc = db.parse(new File( "nosferatu.xml"));
Object o = u.unmarshal( doc );
Unmarshalling from a javax.xml.transform.sax.SAXSource using a
client specified validating SAX2.0 parser:
// configure a validating SAX2.0 parser (Xerces2)
static final String JAXP_SCHEMA_LANGUAGE =
"http://java.sun.com/xml/jaxp/properties/schemaLanguage";
static final String JAXP_SCHEMA_LOCATION =
"http://java.sun.com/xml/jaxp/properties/schemaSource";
static final String W3C_XML_SCHEMA =
"http://www.w3.org/2001/XMLSchema";
System.setProperty( "javax.xml.parsers.SAXParserFactory",
"org.apache.xerces.jaxp.SAXParserFactoryImpl" );
SAXParserFactory spf = SAXParserFactory.newInstance();
spf.setNamespaceAware(true);
spf.setValidating(true);
SAXParser saxParser = spf.newSAXParser();
try {
saxParser.setProperty(JAXP_SCHEMA_LANGUAGE, W3C_XML_SCHEMA);
saxParser.setProperty(JAXP_SCHEMA_LOCATION, "http://....");
} catch (SAXNotRecognizedException x) {
// exception handling omitted
}
XMLReader xmlReader = saxParser.getXMLReader();
SAXSource source =
new SAXSource( xmlReader, new InputSource( "http://..." ) );
// Setup JAXB to unmarshal
JAXBContext jc = JAXBContext.newInstance( "com.acme.foo" );
Unmarshaller u = jc.createUnmarshaller();
ValidationEventCollector vec = new ValidationEventCollector();
u.setEventHandler( vec );
// turn off the JAXB provider's default validation mechanism to
// avoid duplicate validation
u.setValidating( false )
// unmarshal
Object o = u.unmarshal( source );
// check for events
if( vec.hasEvents() ) {
// iterate over events
}
Unmarshalling from a StAX XMLStreamReader:
JAXBContext jc = JAXBContext.newInstance( "com.acme.foo" );
Unmarshaller u = jc.createUnmarshaller();
javax.xml.stream.XMLStreamReader xmlStreamReader =
javax.xml.stream.XMLInputFactory().newInstance().createXMLStreamReader( ... );
Object o = u.unmarshal( xmlStreamReader );
Unmarshalling from a StAX XMLEventReader:
JAXBContext jc = JAXBContext.newInstance( "com.acme.foo" );
Unmarshaller u = jc.createUnmarshaller();
javax.xml.stream.XMLEventReader xmlEventReader =
javax.xml.stream.XMLInputFactory().newInstance().createXMLEventReader( ... );
Object o = u.unmarshal( xmlEventReader );
Unmarshalling can deserialize XML data that represents either an entire XML document or a subtree of an XML document. Typically, it is sufficient to use the unmarshalling methods described by Unmarshal root element that is declared globally. These unmarshal methods utilizeJAXBContext's mapping of global XML element declarations and type definitions to JAXB mapped classes to initiate the unmarshalling of the root element of XML data. When theJAXBContext's mappings are not sufficient to unmarshal the root element of XML data, the application can assist the unmarshalling process by using the unmarshal by declaredType methods. These methods are useful for unmarshalling XML data where the root element corresponds to a local element declaration in the schema.
An unmarshal method never returns null. If the unmarshal process is unable to unmarshal the root of XML content to a JAXB mapped object, a fatal error is reported that terminates processing by throwing JAXBException.
Unmarshal a root element that is globally declared
The unmarshal methods that do not have andeclaredTypeparameter useJAXBContextto unmarshal the root element of an XML data. TheJAXBContextinstance is the one that was used to create thisUnmarshaller. TheJAXBContextinstance maintains a mapping of globally declared XML element and type definition names to JAXB mapped classes. The unmarshal method checks ifJAXBContexthas a mapping from the root element's XML name and/or@xsi:typeto a JAXB mapped class. If it does, it umarshalls the XML data using the appropriate JAXB mapped class. Note that when the root element name is unknown and the root element has an@xsi:type, the XML data is unmarshalled using that JAXB mapped class as the value of aJAXBElement. When theJAXBContextobject does not have a mapping for the root element's name nor its@xsi:type, if it exists, then the unmarshal operation will abort immediately by throwing aUnmarshalException. This exception scenario can be worked around by using the unmarshal by declaredType methods described in the next subsection.
The unmarshal methods with adeclaredTypeparameter enable an application to deserialize a root element of XML data, even when there is no mapping inJAXBContextof the root element's XML name. The unmarshaller unmarshals the root element using the application provided mapping specified as thedeclaredTypeparameter. Note that even when the root element's element name is mapped byJAXBContext, thedeclaredTypeparameter overrides that mapping for deserializing the root element when using these unmarshal methods. Additionally, when the root element of XML data has anxsi:typeattribute and that attribute's value references a type definition that is mapped to a JAXB mapped class byJAXBContext, that the root element'sxsi:typeattribute takes precedence over the unmarshal methodsdeclaredTypeparameter. These methods always return aJAXBElement<declaredType>instance. The table below shows how the properties of the returned JAXBElement instance are set.
Unmarshal By Declared Type returned JAXBElement JAXBElement Property Value name xml element namevalue instanceof declaredTypedeclaredType unmarshal method declaredTypeparameterscope null(actual scope is unknown)
The following is an example of unmarshal by declaredType method.
Unmarshal by declaredType from a org.w3c.dom.Node:
Schema fragment for example <xs:schema> <xs:complexType name="FooType">...<\xs:complexType> <!-- global element declaration "PurchaseOrder" --> <xs:element name="PurchaseOrder"> <xs:complexType> <xs:sequence> <!-- local element declaration "foo" --> <xs:element name="foo" type="FooType"/> ... </xs:sequence> </xs:complexType> </xs:element> </xs:schema> JAXBContext jc = JAXBContext.newInstance( "com.acme.foo" ); Unmarshaller u = jc.createUnmarshaller(); DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance(); dbf.setNamespaceAware(true); DocumentBuilder db = dbf.newDocumentBuilder(); Document doc = db.parse(new File( "nosferatu.xml")); Element fooSubtree = ...; // traverse DOM till reach xml element foo, constrained by a // local element declaration in schema. // FooType is the JAXB mapping of the type of local element declaration foo. JAXBElement<FooType> foo = u.unmarshal( fooSubtree, FooType.class);
Support for SAX2.0 Compliant Parsers
A client application has the ability to select the SAX2.0 compliant parser
of their choice. If a SAX parser is not selected, then the JAXB Provider's
default parser will be used. Even though the JAXB Provider's default parser
is not required to be SAX2.0 compliant, all providers are required to allow
a client application to specify their own SAX2.0 parser. Some providers may
require the client application to specify the SAX2.0 parser at schema compile
time. See unmarshal(Source)
for more detail.
Validation and Well-Formedness
A client application can enable or disable JAXP 1.3 validation mechanism via the
setSchema(javax.xml.validation.Schema)API. Sophisticated clients can specify their own validating SAX 2.0 compliant parser and bypass the JAXP 1.3 validation mechanism using theunmarshal(Source)API.Since unmarshalling invalid XML content is defined in JAXB 2.0, the Unmarshaller default validation event handler was made more lenient than in JAXB 1.0. When schema-derived code generated by JAXB 1.0 binding compiler is registered with
JAXBContext, the default unmarshal validation handler isjavax.xml.bind.helpers.DefaultValidationEventHandlerand it terminates the marshal operation after encountering either a fatal error or an error. For a JAXB 2.0 client application, there is no explicitly defined default validation handler and the default event handling only terminates the unmarshal operation after encountering a fatal error.
There currently are not any properties required to be supported by all JAXB Providers on Unmarshaller. However, some providers may support their own set of provider specific properties.
TheUnmarshallerprovides two styles of callback mechanisms that allow application specific processing during key points in the unmarshalling process. In 'class defined' event callbacks, application specific code placed in JAXB mapped classes is triggered during unmarshalling. 'External listeners' allow for centralized processing of unmarshal events in one callback method rather than by type event callbacks.'Class defined' event callback methods allow any JAXB mapped class to specify its own specific callback methods by defining methods with the following method signature:
The class defined callback methods should be used when the callback method requires access to non-public methods and/or fields of the class.// This method is called immediately after the object is created and before the unmarshalling of this // object begins. The callback provides an opportunity to initialize JavaBean properties prior to unmarshalling. void beforeUnmarshal(Unmarshaller, Object parent); //This method is called after all the properties (except IDREF) are unmarshalled for this object, //but before this object is set to the parent object. void afterUnmarshal(Unmarshaller, Object parent);The external listener callback mechanism enables the registration of a
Listenerinstance with anUnmarshaller.setListener(Listener). The external listener receives all callback events, allowing for more centralized processing than per class defined callback methods. The external listener receives events when unmarshalling process is marshalling to a JAXB element or to JAXB mapped class.The 'class defined' and external listener event callback methods are independent of each other, both can be called for one event. The invocation ordering when both listener callback methods exist is defined in
Listener.beforeUnmarshal(Object, Object)andListener.afterUnmarshal(Object, Object).An event callback method throwing an exception terminates the current unmarshal process.
JAXBContext, Marshaller, Validator