Interview Questions and Answers

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();

• Default bindings can be overridden
– at global scope
– on case-by-case basis
• Customizations include
– names of generated package, classes and methods
– method return types
– class property (field) types
– which elements are bound to classes, as opposed to being ignored
– class property to which each attribute and element declaration is bound

• Customizations can be specified in
– the XML Schema (our focus)
– a binding declarations XML document (not well supported by RI yet)
• The XML Schema must declare
the JAXB namespace and version
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema”
xmlns:jxb="http://java.sun.com/xml/ns/jaxb"
jxb:version="1.0">
• Customization elements are placed in annotation elements
<xsd:annotation>
<xsd:appinfo>
binding declarations
</xsd:appinfo>
</xsd:annotation>

Customizations can be made at four levels
– global
• defined at “top level” in a <jxb:globalBindings> element
• applies to all schema elements in the source schema
and in all included/imported schemas (recursively)
– schema
• defined at “top level” in a <jxb:schemaBindings> element
• applies to all schema elements in the target namespace of the source schema
– definition
• defined in a type or global declaration
• applies to all schema elements that reference the type or global declaration
– component
• defined in a particular schema element or attribute declaration
• applies only to it

• collectionType
– “indexed” (uses array and provides methods to get/set elements) or fully-qualified-java-class-name(must implement java.util.List)
– default is “java.util.ArrayList”
• enableFailFastCheck
– “true” or “false” (default)
– if true, invalid property values are reported as soon as they are set, instead of waiting until validation is requested
– not implemented yet in RI
• generateIsSetMethod
– “true” or “false” (default)
– if true, generates isSet and unSet methods for the property
• underscoreBinding
– “asCharInWord” or “asWordSeparator” (default)
– if “asWordSeparator” , underscores in XML names are removed and words are camel-cased to form Java name
– for example, “gear_shift_knob” goes to “gearShiftKnob”
• bindingStyle (was modelGroupAsClass)
– “modelGroupBinding” or “elementBinding” (default)
• choiceContentProperty
– “true” or “false” (default)
– allows objects to hold one of a number of property choices which may each have a different data type
• enableJavaNamingConventions
– “true” (default) or “false”
• fixedAttributeAsConstantProperty
– “true” or “false” (default)
– if true, “fixed” attributes will be represented as constants
• typesafeEnumBase
– “xsd:string” , “xsd:decimal” , “xsd:float” , “xsd:double” or “xsd:NCName” (default)
– defines field type used to represent enumerated values in generated typesafe enum class
• typesafeEnumMemberName
– “generateName” or “generateError” (default)
– specifies what to do if an enumerated value cannot be mapped to a valid Java identifier
– “generateName” generates names in the form VALUE_#
– “generateError” reports an error

• The syntax for the schemaBindings element is
<jxb:schemaBindings>
<jxb:package [name="package-name"]>
<jxb:javadoc> ... javadoc ... </jxb:javadoc>
</package>
<jxb:nameXmlTransform>
<jxb:typeName prefix="prefix" suffix="suffix"/>
<jxb:elementName prefix="prefix" suffix="suffix"/>
<jxb:modelGroupName prefix="prefix" suffix="suffix"/>
<jxb:anonymousTypeName prefix="prefix" suffix="suffix"/>
</jxb:nameXmlTransform>
</jxb:schemaBindings>
– every element and attribute within schemaBindings is optional