JAXB Interview Questions & Answers

  1. Question 1. What Is Xml Binding?

    Answer :

    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:

    • marshalling a tree of objects into an XML document
    • unmarshalling an XML document into a tree of objects

    includes validation of the XML against the schema:

    used to generate the classes of the objects validation of object trees against the schema used to generate their classes

    • some constraints are enforced while working with the objects
    • others are only enforced when validation is requested
  2. Question 2. What Is Xml Binding Relationships?

    Answer :

    The relationships between the components involved in XML binding (data binding) are shown below schema -> classes XML -> schema Schema generates classes.

    1. Objects are instanceof classes.
    2. Marshal from objects to XML
    3. Unmarshall from XML to objects
    4. XML validates and conforms to Schema
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  4. Question 3. Why Use Xml Binding?

    Answer :

    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
  5. Question 4. Explain Jaxb Use Cases?

    Answer :

    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
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  7. Question 5. What Are The Goals Of Jaxb?

    Answer :

    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

    Natural:

    •  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
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  9. Question 6. What Are The Disadvantages/non-goals Of Jaxb?

    Answer :

    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
  10. Question 7. Give Me An Example Of Xsd?

    Answer :

    xmlns:xs=”http://www.w3.org/2001/XMLSchema”

    xmlns=”http://www.withoutbook.com/cars”

    targetNamespace=”http://www.withoutbook.com/cars”>

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  12. Question 8. Give Me An Example Of Xml Document?

    Answer :

    xmlns:xsi=”http://www.w3.org/2001/XMLSchema-instance”

    xsi:schemaLocation=”http://www.withoutbook.com/cars cars.xsd”>

    BMW

    Z3

    yellow

    Honda

    Odyssey

    green

    Saturn

    SC2

    purple

  13. Question 9. How To Generate Java From Xml Schema. Please Show The Example?

    Answer :

    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:

     

    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 
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  15. Question 10. How To Unmarshall Xml Into Java Objects? Convert From Xml To Java Objects?

    Answer :

    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 UnmarshalExceptio
    • see discussion of ValidationEventCollector later

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  17. Question 11. Java Example/java Program To Set Object For Generating Xml?

    Answer :

    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);

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  19. Question 12. How To Validate Java Objects?

    Answer :

    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();

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  21. Question 13. Explain Customizing Type Bindings?

    Answer :

    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
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  23. Question 14. Explain The Syntax Of Customization?

    Answer :

    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

    xmlns:jxb=”http://java.sun.com/xml/ns/jaxb”

    jxb:version=”1.0″>

    Customization elements are placed in annotation elements

     

     binding declarations

  24. Question 15. Discuss Customization Levels?

    Answer :

    Customizations can be made at four levels

    global:

    • defined at “top level” in a element
    • applies to all schema elements in the source schema and in all included/imported schemas (recursively)

    schema:

    • defined at “top level” in a 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
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  26. Question 16. Discuss Global Bindings Attributes?

    Answer :

    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
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  28. Question 17. What Is The Syntax Of Schemabindings?

    Answer :

    The syntax for the schemaBindings element is

    … javadoc …

    – every element and attribute within schemaBindings is optional

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  30. Question 18. Java Code For Marshalling Java Objects Into Xml?

    Answer :

    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
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