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 XML Data
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 utilize JAXBContext
'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 the JAXBContext
'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 an declaredType parameter use
JAXBContext
to unmarshal the root element of an XML data. The JAXBContext
instance is the one that was used to create this Unmarshaller. The JAXBContext
instance maintains a mapping of globally declared XML element and type definition names to
JAXB mapped classes. The unmarshal method checks if JAXBContext
has a mapping
from the root element's XML name and/or @xsi:type to 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 a JAXBElement
.
When the JAXBContext
object 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 a UnmarshalException
. This exception scenario can be worked around by using the unmarshal by
declaredType methods described in the next subsection.
Unmarshal by Declared Type
The unmarshal methods with a declaredType
parameter enable an
application to deserialize a root element of XML data, even when
there is no mapping in JAXBContext
of the root element's XML name.
The unmarshaller unmarshals the root element using the application provided
mapping specified as the declaredType parameter.
Note that even when the root element's element name is mapped by JAXBContext
,
the declaredType
parameter overrides that mapping for
deserializing the root element when using these unmarshal methods.
Additionally, when the root element of XML data has an xsi:type attribute and
that attribute's value references a type definition that is mapped
to a JAXB mapped class by JAXBContext
, that the root
element's xsi:type attribute takes
precedence over the unmarshal methods declaredType parameter.
These methods always return a JAXBElement<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 name |
value |
instanceof declaredType |
declaredType |
unmarshal method declaredType parameter |
scope |
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 the
unmarshal(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 is
DefaultValidationEventHandler
and 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 marshal operation after encountering a fatal error.
Supported Properties
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.
Unmarshal Event Callbacks
The Unmarshaller
provides 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:
// 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 class defined callback methods should be used when the callback method requires
access to non-public methods and/or fields of the class.
The external listener callback mechanism enables the registration of a Unmarshaller.Listener
instance with an Unmarshaller.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 proces 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 Unmarshaller.Listener.beforeUnmarshal(Object, Object)
and Unmarshaller.Listener.afterUnmarshal(Object, Object)
.
An event callback method throwing an exception terminates the current unmarshal process.