面试题与答案

Maps XML to in-memory objects according to a schema. Generates classes to represent XML elements so developers don't have to write them the binding compiler does this the classes follow JavaBeans property access conventions. Supports three primary operations: 

The relationships between the components involved in XML binding (data binding) are shown below:

� Standardize generated Java
� classes generated by different
JAXB implementations may not be
compatible with each other
� Preserve XML equivalence
� unmarshalling XML to objects and
marshalling back to XML may not
result in equivalent XML
� Bind existing JavaBeans
to schemas
� can only marshal and unmarshal
classes generated by JAXB
� may be added later
� Schema evolution support
� can� t modify previously generated
code to support schema changes
� must generated new code
� Allow generated Java to access
XML elements/attributes not
described in initial schema
� Partial binding
� unmarshalling only a subset
of an XML document
breaks round tripping
� Implement every feature of the
schema language
� it� s tough to implement
all of XML Schema!
� Support DTDs
� focusing on XML Schema
� DTDs were supported in an earlier
version, but won� t be anymore
� tools for converting DTDs to
XML Schemas exist

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema elementFormDefault="qualified"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns="http://www.withoutbook.com/cars"
targetNamespace="http://www.withoutbook.com/cars">
<xs:complexType name="car">
<xs:sequence>
<xs:element name="make" type="xs:string"/>
<xs:element name="model" type="xs:string"/>
<xs:element name="color" type="xs:string"/>
</xs:sequence>
<xs:attribute name="year" type="xs:positiveInteger" use="required"/>
</xs:complexType>
<xs:element name="cars">
<xs:complexType>
<xs:sequence>
<xs:element name="car" type="car" maxOccurs="unbounded"/>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:schema>

<?xml version="1.0" encoding="UTF-8"?>
<cars xmlns="http://www.withoutbook.com/cars"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.withoutbook.com/cars cars.xsd">
<car year="2001">
<make>BMW</make>
<model>Z3</model>
<color>yellow</color>
</car>
<car year="2001">
<make>Honda</make>
<model>Odyssey</model>
<color>green</color>
</car>
<car year="1997">
<make>Saturn</make>
<model>SC2</model>
<color>purple</color>
</car>
</cars>

� From command-line
� Windows: %JAXB_HOME%binxjc cars.xsd
� UNIX: %JAXB_HOME%/bin/xjc.sh cars.xsd
� these write generated files to current directory
� From Ant
<java jar="${env.JAXB_HOME}/lib/jaxb-xjc.jar" fork="yes">
<arg line="-d ${gen.src.dir} cars.xsd"/>
</java>

Generated Files:

� com/withoutbook/cars directory
� Car.java
� interface representing the �car� complex type
� only describes get and set methods for car properties
� Cars.java
� interface representing �cars� global element
� extends CarsType and javax.xml.bind.Element (just a marker interface)
� describes no additional methods
� CarsType.java
� interface representing anonymous complex type
defined inside the �cars� global element
� provides method to get collection of Car objects (as a java.util.List)
� ObjectFactory.java
� class used to create objects of the above interface types
� extends DefaultJAXBContextImpl which extends JAXBContext
� bgm.ser
� a serialized object of type com.sun.msv.grammar.trex.TREXGrammar
� can�t find any documentation on this - don�t know its purpose
� jaxb.properties
� sets a property that defines the class used to create JAXBContext objects

� com/withoutbook/cars/impl directory
� CarImpl.java
� class that implements Car
� corresponds to the �car� XML Schema complexType
� CarsTypeImpl.java
� class that implements CarType
� corresponds to the XML Schema anonymous type inside the �cars� element
� CarsImpl.java
� class that extends CarsTypeImpl and implements Cars
� corresponds to the �cars� XML Schema element

� Example
ObjectFactory factory = new ObjectFactory();
Unmarshaller u = factory.createUnmarshaller();
Cars cars = (Cars) u.unmarshal(new FileInputStream("cars.xml"));
� unmarshal method accepts
� java.io.File
� java.io.InputStream
� java.net.URL
� javax.xml.transform.Source
� related to XSLT
� org.w3c.dom.Node
� related to DOM
� org.xml.sax.InputSource
� related to SAX

� Other Unmarshaller methods
� void setValidating(boolean validating)
� true to enable validation during unmarshalling; false to disable (the default)
� boolean setEventHandler(ValidationEventHandler handler)
� handleEvent method of ValidationEventHandler is called
if validation errors are encountered during unmarshalling
� default handler terminates marshalling after first error
� return true to continue unmarshalling
� return false to terminate with UnmarshalException
� see discussion of ValidationEventCollector later

� Example
Marshaller m = factory.createMarshaller();
m.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, Boolean.TRUE);
Writer fw = new FileWriter("newCars.xml");
m.marshal(cars, fw);
� marshal method accepts
� java.io.OutputStream
� java.io.Writer
� javax.xml.transform.Result
� related to XSLT
� org.w3c.dom.Node
� related to DOM
� org.xml.sax.ContentHandler
� related to SAX

� Other Marshaller methods
� boolean setEventHandler(ValidationEventHandler handler)
� same as use with Unmarshaller, but validation events
are delivered during marshalling
� void setProperty(String name, Object value)
� supported properties are
� jaxb.encoding - value is a String
� the encoding to use when marshalling; defaults to �UTF-8�
� jaxb.formatted.output - value is a Boolean
� true to output line breaks and indentation; false to omit (the default)
� jaxb.schemaLocation - value is a String
� to specify xsi:schemaLocation attribute in generated XML
� jaxb.noNamespaceSchemaLocation - value is a String
� to specify xsi:noNamespaceSchemaLocation attribute in generated XML

Cars cars = factory.createCars();
Car car = factory.createCar();
car.setColor("blue");
car.setMake("Mazda");
car.setModel("Miata");
car.setYear(BigInteger.valueOf(2012));
cars.getCar().add(car);
car = factory.createCar();
car.setColor("red");
car.setMake("Ford");
car.setModel("Mustang II");
car.setYear(BigInteger.valueOf(2011));
cars.getCar().add(car);

� The graph of Java objects can contain invalid data
� could occur when objects created by unmarshalling are modified
� could occur when objects are created from scratch
� Use a Validator to validate the objects
� Example
Validator v = factory.createValidator();
try {
v.validateRoot(cars);
v.validate(car);
} catch (ValidationException e) {
// Handle the validation error described by e.getMessage().
}

� Other Validator methods
� boolean setEventHandler(ValidationEventHandler handler)
� handleEvent method of ValidationEventHandler is called
if validation errors are encountered
� default handler terminates marshalling after first error
� return true to continue validating
� return false to terminate with ValidationException

Pass an instance of javax.xml.bind.util.ValidationEventCollector
(in jaxb-api.jar) to setEventHandler to collect validation errors and
query them later instead of handling them during validation.
ValidationEventCollector vec =
new ValidationEventCollector();
v.setEventHandler(vec);
v.validate(cars);
ValidationEvent[] events = vec.getEvents();