JAXB Interview Questions and Answers

Ques 1. What Is XML Binding?

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: 

Ques 2. What is XML Binding Relationships?

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

Ques 3. Why Use XML Binding?

• It's not necessary. Everything that must be done with XML can be done with SAX and DOM.
• It's easier, don't have to write as much code, don't have to learn SAX and/or DOM.
• It's less error-prone, all the features of the schema are utilized, don't have to remember to manually implement them.
• It can allow customization of the XML structure unlike XMLEncoder and XMLDecoder in the java.beans package.

Ques 4. Please explain JAXB Use Cases.

• Create/Read/Modify XML using Java but without using SAX or DOM.
• Validate user input using rules described in XML Schemas.
• Use XML-based configuration files, access their values, write tools that creates and modifies these files.

Ques 5. What are the goals of JAXB?

• Easy to use require minimal XML knowledge, don't require SAX/DOM knowledge.
• Customizable can customize mapping of XML to Java.
• Portable can change JAXB implementation without changing source code.
• Deliver soon, deliver core functionality ASAP.
• Follow standard design and naming conventions in generated Java.
• Match schema, easy to identify generated Java components that correspond to schema features.
• Hide plumbing encapsulate implementation of unmarshalling, marshalling and validation.
• Validation, on demand validate objects without requiring marshalling.
• Preserve object equivalence (round tripping), marshalling objects to XML and unmarshalling back to objects results in equivalent objects.

Ques 6. What are the disadvantages/non-goals of JAXB?

• 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

Ques 7. Give me an example of XSD.

<?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>

Ques 8. Give me an example of XML document.

<?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>

Ques 9. How to generate Java from XML Schema. Please show the example.

• 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

Ques 10. How to unmarshall XML into Java objects? Convert from XML to Java objects.

• 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

Ques 11. Java code for marshalling Java objects into XML.

• 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

Ques 12. Java example/java program to set object for generating 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);

Ques 13. How to validate java objects?

• 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();