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Learning Java, 4th Edition

Cover of Learning Java, 4th Edition by Daniel Leuck... Published by O'Reilly Media, Inc.
  1. Learning Java
  2. Preface
    1. Who Should Read This Book
    2. New Developments
      1. New in This Edition (Java 6 and 7)
    3. Using This Book
    4. Online Resources
    5. Conventions Used in This Book
    6. Using Code Examples
    7. Safari® Books Online
    8. How to Contact Us
    9. Acknowledgments
  3. 1. A Modern Language
    1. Enter Java
      1. Java’s Origins
      2. Growing Up
    2. A Virtual Machine
    3. Java Compared with Other Languages
    4. Safety of Design
      1. Simplify, Simplify, Simplify...
      2. Type Safety and Method Binding
      3. Incremental Development
      4. Dynamic Memory Management
      5. Error Handling
      6. Threads
      7. Scalability
    5. Safety of Implementation
      1. The Verifier
      2. Class Loaders
      3. Security Managers
    6. Application and User-Level Security
    7. A Java Road Map
      1. The Past: Java 1.0–Java 1.6
      2. The Present: Java 7
      3. The Future
      4. Availability
  4. 2. A First Application
    1. Java Tools and Environment
    2. Configuring Eclipse and Creating a Project
      1. Importing the Learning Java Examples
    3. HelloJava
      1. Classes
      2. The main() Method
      3. Classes and Objects
      4. Variables and Class Types
      5. HelloComponent
      6. Inheritance
      7. The JComponent Class
      8. Relationships and Finger Pointing
      9. Package and Imports
      10. The paintComponent() Method
    4. HelloJava2: The Sequel
      1. Instance Variables
      2. Constructors
      3. Events
      4. The repaint() Method
      5. Interfaces
    5. HelloJava3: The Button Strikes!
      1. Method Overloading
      2. Components
      3. Containers
      4. Layout
      5. Subclassing and Subtypes
      6. More Events and Interfaces
      7. Color Commentary
      8. Static Members
      9. Arrays
      10. Our Color Methods
    6. HelloJava4: Netscape’s Revenge
      1. Threads
      2. The Thread Class
      3. The Runnable Interface
      4. Starting the Thread
      5. Running Code in the Thread
      6. Exceptions
      7. Synchronization
  5. 3. Tools of the Trade
    1. JDK Environment
    2. The Java VM
    3. Running Java Applications
      1. System Properties
    4. The Classpath
      1. javap
    5. The Java Compiler
    6. JAR Files
      1. File Compression
      2. The jar Utility
      3. The pack200 Utility
    7. Policy Files
      1. The Default Security Manager
      2. The policytool Utility
      3. Using a Policy File with the Default Security Manager
  6. 4. The Java Language
    1. Text Encoding
      1. Javadoc Comments
    3. Types
      1. Primitive Types
      2. Reference Types
      3. A Word About Strings
    4. Statements and Expressions
      1. Statements
      2. Expressions
    5. Exceptions
      1. Exceptions and Error Classes
      2. Exception Handling
      3. Bubbling Up
      4. Stack Traces
      5. Checked and Unchecked Exceptions
      6. Throwing Exceptions
      7. try Creep
      8. The finally Clause
      9. Try with Resources
      10. Performance Issues
    6. Assertions
      1. Enabling and Disabling Assertions
      2. Using Assertions
    7. Arrays
      1. Array Types
      2. Array Creation and Initialization
      3. Using Arrays
      4. Anonymous Arrays
      5. Multidimensional Arrays
      6. Inside Arrays
  7. 5. Objects in Java
    1. Classes
      1. Accessing Fields and Methods
      2. Static Members
    2. Methods
      1. Local Variables
      2. Shadowing
      3. Static Methods
      4. Initializing Local Variables
      5. Argument Passing and References
      6. Wrappers for Primitive Types
      7. Autoboxing and Unboxing of Primitives
      8. Variable-Length Argument Lists
      9. Method Overloading
    3. Object Creation
      1. Constructors
      2. Working with Overloaded Constructors
      3. Static and Nonstatic Initializer Blocks
    4. Object Destruction
      1. Garbage Collection
      2. Finalization
      3. Weak and Soft References
    5. Enumerations
      1. Enum Values
      2. Customizing Enumerations
  8. 6. Relationships Among Classes
    1. Subclassing and Inheritance
      1. Shadowed Variables
      2. Overriding Methods
      3. Special References: this and super
      4. Casting
      5. Using Superclass Constructors
      6. Full Disclosure: Constructors and Initialization
      7. Abstract Methods and Classes
    2. Interfaces
      1. Interfaces as Callbacks
      2. Interface Variables
      3. Subinterfaces
    3. Packages and Compilation Units
      1. Compilation Units
      2. Package Names
      3. Class Visibility
      4. Importing Classes
    4. Visibility of Variables and Methods
      1. Basic Access Modifiers
      2. Subclasses and Visibility
      3. Interfaces and Visibility
    5. Arrays and the Class Hierarchy
      1. ArrayStoreException
    6. Inner Classes
      1. Inner Classes as Adapters
      2. Inner Classes Within Methods
  9. 7. Working with Objects and Classes
    1. The Object Class
      1. Equality and Equivalence
      2. Hashcodes
      3. Cloning Objects
    2. The Class Class
    3. Reflection
      1. Modifiers and Security
      2. Accessing Fields
      3. Accessing Methods
      4. Accessing Constructors
      5. What About Arrays?
      6. Accessing Generic Type Information
      7. Accessing Annotation Data
      8. Dynamic Interface Adapters
      9. What Is Reflection Good For?
    4. Annotations
      1. Using Annotations
      2. Standard Annotations
      3. The apt Tool
  10. 8. Generics
    1. Containers: Building a Better Mousetrap
      1. Can Containers Be Fixed?
    2. Enter Generics
      1. Talking About Types
    3. “There Is No Spoon”
      1. Erasure
      2. Raw Types
    4. Parameterized Type Relationships
      1. Why Isn’t a List<Date> a List<Object>?
    5. Casts
    6. Writing Generic Classes
      1. The Type Variable
      2. Subclassing Generics
      3. Exceptions and Generics
      4. Parameter Type Limitations
    7. Bounds
      1. Erasure and Bounds (Working with Legacy Code)
    8. Wildcards
      1. A Supertype of All Instantiations
      2. Bounded Wildcards
      3. Thinking Outside the Container
      4. Lower Bounds
      5. Reading, Writing, and Arithmetic
      6. <?>, <Object>, and the Raw Type
      7. Wildcard Type Relationships
    9. Generic Methods
      1. Generic Methods Introduced
      2. Type Inference from Arguments
      3. Type Inference from Assignment Context
      4. Explicit Type Invocation
      5. Wildcard Capture
      6. Wildcard Types Versus Generic Methods
    10. Arrays of Parameterized Types
      1. Using Array Types
      2. What Good Are Arrays of Generic Types?
      3. Wildcards in Array Types
    11. Case Study: The Enum Class
    12. Case Study: The sort() Method
    13. Conclusion
  11. 9. Threads
    1. Introducing Threads
      1. The Thread Class and the Runnable Interface
      2. Controlling Threads
      3. Death of a Thread
    2. Threading an Applet
      1. Issues Lurking
    3. Synchronization
      1. Serializing Access to Methods
      2. Accessing class and instance Variables from Multiple Threads
      3. The wait() and notify() Methods
      4. Passing Messages
      5. ThreadLocal Objects
    4. Scheduling and Priority
      1. Thread State
      2. Time-Slicing
      3. Priorities
      4. Yielding
    5. Thread Groups
      1. Working with ThreadGroups
      2. Uncaught Exceptions
    6. Thread Performance
      1. The Cost of Synchronization
      2. Thread Resource Consumption
    7. Concurrency Utilities
      1. Executors
      2. Locks
      3. Synchronization Constructs
      4. Atomic Operations
    8. Conclusion
  12. 10. Working with Text
    1. Text-Related APIs
    2. Strings
      1. Constructing Strings
      2. Strings from Things
      3. Comparing Strings
      4. Searching
      5. Editing
      6. String Method Summary
      7. StringBuilder and StringBuffer
    3. Internationalization
      1. The java.util.Locale Class
      2. Resource Bundles
    4. Parsing and Formatting Text
      1. Parsing Primitive Numbers
      2. Tokenizing Text
    5. Printf-Style Formatting
      1. Formatter
      2. The Format String
      3. String Conversions
      4. Primitive and Numeric Conversions
      5. Flags
      6. Miscellaneous
    6. Formatting with the java.text Package
      1. MessageFormat
    7. Regular Expressions
      1. Regex Notation
      2. The java.util.regex API
  13. 11. Core Utilities
    1. Math Utilities
      1. The java.lang.Math Class
      2. Big/Precise Numbers
      3. Floating-Point Components
      4. Random Numbers
    2. Dates and Times
      1. Working with Calendars
      2. Time Zones
      3. Parsing and Formatting with DateFormat
      4. Printf-Style Date and Time Formatting
    3. Timers
    4. Collections
      1. The Collection Interface
      2. Iterator
      3. Collection Types
      4. The Map Interface
      5. Collection Implementations
      6. Hash Codes and Key Values
      7. Synchronized and Unsynchronized Collections
      8. Read-Only and Read-Mostly Collections
      9. WeakHashMap
      10. EnumSet and EnumMap
      11. Sorting Collections
      12. A Thrilling Example
    5. Properties
      1. Loading and Storing
      2. System Properties
    6. The Preferences API
      1. Preferences for Classes
      2. Preferences Storage
      3. Change Notification
    7. The Logging API
      1. Overview
      2. Logging Levels
      3. A Simple Example
      4. Logging Setup Properties
      5. The Logger
      6. Performance
    8. Observers and Observables
  14. 12. Input/Output Facilities
    1. Streams
      1. Basic I/O
      2. Character Streams
      3. Stream Wrappers
      4. Pipes
      5. Streams from Strings and Back
      6. Implementing a Filter Stream
    2. File I/O
      1. The Class
      2. File Streams
      3. RandomAccessFile
      4. Resource Paths
    3. The NIO File API
      1. FileSystem and Path
      2. NIO File Operations
      3. Directory Operations
      4. Watching Paths
    4. Serialization
      1. Initialization with readObject()
      2. SerialVersionUID
    5. Data Compression
      1. Archives and Compressed Data
      2. Decompressing Data
      3. Zip Archive As a Filesystem
    6. The NIO Package
      1. Asynchronous I/O
      2. Performance
      3. Mapped and Locked Files
      4. Channels
      5. Buffers
      6. Character Encoders and Decoders
      7. FileChannel
      8. Scalable I/O with NIO
  15. 13. Network Programming
    1. Sockets
      1. Clients and Servers
      2. author="pat” timestamp="20120926T110720-0500” comment="one of those sections I hate to get rid of but is less relevant in terms of the example... should probably find a more modern example...”The DateAtHost Client
      3. The TinyHttpd Server
      4. Socket Options
      5. Proxies and Firewalls
    2. Datagram Sockets
      1. author="pat” timestamp="20120926T141346-0500” comment="I actually rewrote this as a standalone client but then decided to leave it as an applet”The HeartBeat Applet
      2. InetAddress
    3. Simple Serialized Object Protocols
      1. A Simple Object-Based Server
    4. Remote Method Invocation
      1. Real-World Usage
      2. Remote and Nonremote Objects
      3. An RMI Example
      4. RMI and CORBA
    5. Scalable I/O with NIO
      1. Selectable Channels
      2. Using Select
      3. LargerHttpd
      4. Nonblocking Client-Side Operations
  16. 14. Programming for the Web
    1. Uniform Resource Locators (URLs)
    2. The URL Class
      1. Stream Data
      2. Getting the Content as an Object
      3. Managing Connections
      4. Handlers in Practice
      5. Useful Handler Frameworks
    3. Talking to Web Applications
      1. Using the GET Method
      2. Using the POST Method
      3. The HttpURLConnection
      4. SSL and Secure Web Communications
      5. URLs, URNs, and URIs
    4. Web Services
      1. XML-RPC
      2. WSDL
      3. The Tools
      4. The Weather Service Client
  17. 15. Web Applications and Web Services
    1. Web Application Technologies
      1. Page-Oriented Versus “Single Page” Applications
      2. JSPs
      3. XML and XSL
      4. Web Application Frameworks
      5. Google Web Toolkit
      6. HTML5, AJAX, and More...
    2. Java Web Applications
      1. The Servlet Lifecycle
      2. Servlets
      3. The HelloClient Servlet
      4. The Servlet Response
      5. Servlet Parameters
      6. The ShowParameters Servlet
      7. User Session Management
      8. The ShowSession Servlet
      9. The ShoppingCart Servlet
      10. Cookies
      11. The ServletContext API
      12. Asynchronous Servlets
    3. WAR Files and Deployment
      1. Configuration with web.xml and Annotations
      2. URL Pattern Mappings
      3. Deploying HelloClient
      4. Error and Index Pages
      5. Security and Authentication
      6. Protecting Resources with Roles
      7. Secure Data Transport
      8. Authenticating Users
      9. Procedural Authorization
    4. Servlet Filters
      1. A Simple Filter
      2. A Test Servlet
      3. Declaring and Mapping Filters
      4. Filtering the Servlet Request
      5. Filtering the Servlet Response
    5. Building WAR Files with Ant
      1. A Development-Oriented Directory Layout
      2. Deploying and Redeploying WARs with Ant
    6. Implementing Web Services
      1. Defining the Service
      2. Our Echo Service
      3. Using the Service
      4. Data Types
    7. Conclusion
  18. 16. Swing
    1. Components
      1. Peers and Look-and-Feel
      2. The MVC Framework
      3. Painting
      4. Enabling and Disabling Components
      5. Focus, Please
      6. Other Component Methods
      7. Layout Managers
      8. Insets
      9. Z-Ordering (Stacking Components)
      10. The revalidate() and doLayout() Methods
      11. Managing Components
      12. Listening for Components
      13. Windows, Frames and Splash Screens
      14. Other Methods for Controlling Frames
      15. Content Panes
      16. Desktop Integration
    2. Events
      1. Event Receivers and Listener Interfaces
      2. Event Sources
      3. Event Delivery
      4. Event Types
      5. The java.awt.event.InputEvent Class
      6. Mouse and Key Modifiers on InputEvents
      7. Focus Events
    3. Event Summary
      1. Adapter Classes
      2. Dummy Adapters
    4. The AWT Robot!
    5. Multithreading in Swing
  19. 17. Using Swing Components
    1. Buttons and Labels
      1. HTML Text in Buttons and Labels
    2. Checkboxes and Radio Buttons
    3. Lists and Combo Boxes
    4. The Spinner
    5. Borders
    6. Menus
    7. Pop-Up Menus
      1. Component-Managed Pop Ups
    8. The JScrollPane Class
    9. The JSplitPane Class
    10. The JTabbedPane Class
    11. Scrollbars and Sliders
    12. Dialogs
      1. File Selection Dialog
      2. The Color Chooser
  20. 18. More Swing Components
    1. Text Components
      1. The TextEntryBox Application
      2. Formatted Text
      3. Filtering Input
      4. Validating Data
      5. Say the Magic Word
      6. Sharing a Data Model
      7. HTML and RTF for Free
      8. Managing Text Yourself
    2. Focus Navigation
      1. Trees
      2. Nodes and Models
      3. Save a Tree
      4. Tree Events
      5. A Complete Example
    3. Tables
      1. A First Stab: Freeloading
      2. Round Two: Creating a Table Model
      3. Round Three: A Simple Spreadsheet
      4. Sorting and Filtering
      5. Printing JTables
    4. Desktops
    5. Pluggable Look-and-Feel
    6. Creating Custom Components
      1. Generating Events
      2. A Dial Component
      3. Model and View Separation
  21. 19. Layout Managers
    1. FlowLayout
    2. GridLayout
    3. BorderLayout
    4. BoxLayout
    5. CardLayout
    6. GridBagLayout
      1. The GridBagConstraints Class
      2. Grid Coordinates
      3. The fill Constraint
      4. Spanning Rows and Columns
      5. Weighting
      6. Anchoring
      7. Padding and Insets
      8. Relative Positioning
      9. Composite Layouts
    7. Other Layout Managers
    8. Absolute Positioning
  22. 20. Drawing with the 2D API
    1. The Big Picture
    2. The Rendering Pipeline
    3. A Quick Tour of Java 2D
      1. Filling Shapes
      2. Drawing Shape Outlines
      3. Convenience Methods
      4. Drawing Text
      5. Drawing Images
      6. The Whole Iguana
    4. Filling Shapes
      1. Solid Colors
      2. Color Gradients
      3. Textures
      4. Desktop Colors
    5. Stroking Shape Outlines
    6. Using Fonts
      1. Font Metrics
    7. Displaying Images
      1. The Image Class
      2. Image Observers
      3. Scaling and Size
    8. Drawing Techniques
      1. Double Buffering
      2. Limiting Drawing with Clipping
      3. Offscreen Drawing
    9. Printing
  23. 21. Working with Images and Other Media
    1. Loading Images
      1. ImageObserver
      2. MediaTracker
      3. ImageIcon
      4. ImageIO
    2. Producing Image Data
      1. Drawing Animations
      2. BufferedImage Anatomy
      3. Color Models
      4. Creating an Image
      5. Updating a BufferedImage
    3. Filtering Image Data
      1. How ImageProcessor Works
      2. Converting an Image to a BufferedImage
      3. Using the RescaleOp Class
      4. Using the AffineTransformOp Class
    4. Saving Image Data
    5. Simple Audio
    6. Java Media Framework
  24. 22. JavaBeans
    1. What’s a Bean?
      1. What Constitutes a Bean?
    2. The NetBeans IDE
      1. Installing and Running NetBeans
    3. Properties and Customizers
    4. Event Hookups and Adapters
      1. Taming the Juggler
      2. Molecular Motion
    5. Binding Properties
      1. Constraining Properties
    6. Building Beans
      1. The Dial Bean
      2. Design Patterns for Properties
    7. Limitations of Visual Design
    8. Serialization Versus Code Generation
    9. Customizing with BeanInfo
      1. Getting Properties Information
    10. Handcoding with Beans
      1. Bean Instantiation and Type Management
      2. Working with Serialized Beans
      3. Runtime Event Hookups with Reflection
    11. BeanContext and BeanContextServices
    12. The Java Activation Framework
    13. Enterprise JavaBeans and POJO-Based Enterprise Frameworks
  25. 23. Applets
    1. The Politics of Browser-Based Applications
    2. Applet Support and the Java Plug-in
    3. The JApplet Class
      1. Applet Lifecycle
      2. The Applet Security Sandbox
      3. Getting Applet Resources
      4. The <applet> Tag
      5. Attributes
      6. Parameters
      7. ¿Habla Applet?
      8. The Complete <applet> Tag
      9. Loading Class Files
      10. Packages
      11. appletviewer
    4. Java Web Start
    5. Conclusion
  26. 24. XML
    1. The Butler Did It
    2. A Bit of Background
      1. Text Versus Binary
      2. A Universal Parser
      3. The State of XML
      4. The XML APIs
      5. XML and Web Browsers
    3. XML Basics
      1. Attributes
      2. XML Documents
      3. Encoding
      4. Namespaces
      5. Validation
      6. HTML to XHTML
    4. SAX
      1. The SAX API
      2. Building a Model Using SAX
      3. XMLEncoder/Decoder
    5. DOM
      1. The DOM API
      2. Test-Driving DOM
      3. Generating XML with DOM
      4. JDOM
    6. XPath
      1. Nodes
      2. Predicates
      3. Functions
      4. The XPath API
      5. XMLGrep
    7. XInclude
      1. Enabling XInclude
    8. Validating Documents
      1. Using Document Validation
      2. DTDs
      3. XML Schema
      4. The Validation API
    9. JAXB Code Binding and Generation
      1. Annotating Our Model
      2. Generating a Java Model from an XML Schema
      3. Generating an XML Schema from a Java Model
    10. Transforming Documents with XSL/XSLT
      1. XSL Basics
      2. Transforming the Zoo Inventory
      3. XSLTransform
      4. XSL in the Browser
    11. Web Services
    12. The End of the Book
  27. A. The Eclipse IDE
    1. The IDE Wars
    2. Getting Started with Eclipse
      1. Importing the Learning Java Examples
    3. Using Eclipse
      1. Getting at the Source
      2. The Lay of the Land
      3. Running the Examples
      4. Building the Ant-Based Examples
      5. Loner Examples
    4. Eclipse Features
      1. Coding Shortcuts
      2. Autocorrection
      3. Refactoring
      4. Diffing Files
      5. Organizing Imports
      6. Formatting Source Code
    5. Conclusion
  28. B. BeanShell: Java Scripting
    1. Running BeanShell
    2. Java Statements and Expressions
      1. Imports
    3. BeanShell Commands
    4. Scripted Methods and Objects
      1. Scripting Interfaces and Adapters
    5. Changing the Classpath
    6. Learning More . . .
  29. Glossary
  30. Index
  31. About the Authors
  32. Colophon
  33. Copyright
O'Reilly logo

JAXB Code Binding and Generation

We’ve said that our ultimate goal in this chapter is automated binding of XML to Java classes. Now we’ll discuss the standard Java API for XML Binding, JAXB. (This should not be confused with JAXP, the parser API.) JAXB is a standard extension that is bundled with Java 6 and later. With JAXB, the developer does not need to create any fragile parsing code. An XML schema or Java code can be used as the starting point for transforming XML to Java and back. (“Schema first” and “code first” are both supported.) With JAXB, you can either mark up your Java classes with simple annotations that map (bind) them to XML or start with an XML schema and generate plain Java classes (POJOs) with the necessary annotations included. You can even derive an XML schema from your Java classes to use as a starting point or contract with non-Java systems.

At runtime, JAXB can read an XML document and parse it into the model that you have defined or you can go the other way, populating your object model and then writing it out to XML. In both cases, JAXB can validate the data to make sure it matches a schema. This may sound like the DOM interface, but in this case we’re not using generic classes—we’re using our own model. In this section, we’ll reuse the class model that we created for the SAX example with our zooinventory.xml file. We’ll use the familiar Inventory, Animal, and FoodRecipe classes directly, but this time you’ll see that we’ll be more focused on the schema and names and less on the parsing machinery.

Annotating Our Model

JAXB gives us a great deal of flexibility in mapping our Java classes to XML elements and there are a lot of special cases. But if we accept most of the default behavior for our model, we can get started with very little work. Let’s start by taking our zoo inventory classes and adding the necessary annotations to allow JAXB to bind it to XML:

public class Inventory {
       public List<Animal> animal = new ArrayList<>();

Well, that was easy! Yes, in fact as we hinted at the beginning of the chapter, adding just the @XmlRootElement annotation to the “top level” or root class of our model will yield nearly the same XML that we used before. To generate the XML, we’ll use the following test harness:

    import javax.xml.bind.JAXBContext;
    import javax.xml.bind.JAXBException;
    import javax.xml.bind.Marshaller;
    public class TestJAXBMarshall
        public static void main( String [] args ) throws JAXBException {
            Inventory inventory = new Inventory();
            FoodRecipe recipe = new FoodRecipe();
   = "Gorilla Chow";
            recipe.ingredient.addAll( Arrays.asList( "leaves", "insects",
                "fruit" ) );
            Animal animal = new Animal( Animal.AnimalClass.mammal, "Song Fang", 
                "Giant Panda", "China", "Bamboo", "Friendly", 45.0, recipe );
            inventory.animal.add( animal );
            marshall( inventory );
        public static void marshall( Object jaxbObject ) throws JAXBException {
            JAXBContext context = JAXBContext.newInstance(
                jaxbObject.getClass() );
            Marshaller marshaller = context.createMarshaller();
            marshaller.marshal(jaxbObject, System.out);

We’ve taken the liberty of adding some constructors to shorten the code for creating the model, but it doesn’t change the behavior here. It’s just the four lines of our marshall() method that actually use JAXB to write out the XML. We first create a JAXBContext, passing in the class type to be marshalled. We’ve made our marshall() method somewhat reusable by getting the class type from the object passed in. However, it’s sometimes necessary to pass in additional classes to the newInstance() method in order for JAXB to be aware of all of the bound classes that may be needed. In that case, we’d simpy pass more class types to the newInstance() method (it accepts a variable argument list with any number of arguments—of class types). We then create a Marshaller from the context and, for our purposes, set a flag indicating that we would like nice, human-readable output (the default output is one long line of XML). Finally, we tell the marshaller to send our object to System.out.

The output looks like this:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
        <name>Song Fang</name>
        <species>Giant Panda</species>
        <habitat>Ceneral Africa</habitat>
            <name>Gorilla Chow</name>

As we said, it’s almost identical to the XML we worked with earlier. Admittedly, we chose to create our XML using the same (common) conventions that JAXB uses, so it’s not entirely magic. The first thing to notice is that JAXB automatically mapped our class names to lowercase XML element names (e.g., class Animal to <animal>). If we had used JavaBeans-style getter methods instead of public fields, the same would be true; for example, a getSpecies() method would produce a default element name of species.

If we wanted to map our class names and property names to completely different XML names, we could easily accomplish that using the name attribute of the @XmlRootElement and @XmlElement annotations. For example, we can call our Animal “creature” and rename temperament to “personality” like so:

public class Animal 
    public String temperament;

The real difference between our generated XML and our earlier sample is that our animalClass attribute is not acting like an attribute. By default, is has been mapped to an element, like the other properties of Animal. We can rectify that with another annotation, @XmlAttribute:

public class Animal 
    public AnimalClass animalClass;    

// Produces...

    <animal animalClass="mammal">
        <name>Song Fang</name>

Also note that JAXB has shown the food element in the first animal and the foodRecipe in the second. JAXB will ignore a field or property that is null (as is the case here) unless you specify that the property is “nillable” using @XmlElement(nillable=true). That behavior automatically supported the alternation between our two properties.

There are many additional annotations that provide support for mapping Java classes, fields, and properties to other features of XML. Table 24-6 attempts to provide a concise description of what each annotation is used for. Some of the usages get a little complex,so you may want to refer to the Javadoc for more details.

Table 24-6. JAXB Annotations

@XmlAccessorOrderUsed on a package or class to set alphabetic ordering of marshalled fields and properties. (The default ordering is undefined.) See @XmlType to specify the ordering yourself. As a reminder: package-level annotations in Java are placed on a (lonely) package statement in a special file named within the corresponding package structure. (See Annotations.)
@XmlAccessorTypeUsed on a package or class to specify whether fields and properties are marshalled by default. You can choose: only fields, only properties (getters/setters), none (only those annotated by the user), or all public fields and properties. See @XmlTransient to exclude items.
@XmlAnyAttributeDesignates a Java Map object to receive any unbound XML attribute name-value pairs for an entity (i.e., the Map will collect any leftover attributes for which no corresponding property or field can be found).
@XmlAnyElementDesignates a Java List or Array object to receive any unbound XML elements for an entity (i.e., the List will accumulate any leftover elements for which no corresponding property or field can be found).
@XmlAttachmentRefDesignates a java.activation.DataHandler object to handle an XML MIME attachment.
@XmlAttributeBinds a Java field or property to an XML attribute. The name attribute can be used to specify an XML attribute name that is different from the name of the field or property. Use the required attribute to specify whether the attribute is required.
@XmlElementBinds a a Java field or property to an XML element. The name attribute can be used to specify an XML element name different from the name of the field or property. Use the required attribute to specify whether the element is required.
@XmlElementsUsed on a Java collection to specify distinct element names for contained items based on their Java type. Holds a list of @XmlElement annotations with name and type attributes that explicitly map Java types in the collection to XML element names (e.g., in our example, inventory contains animal elements because our List property is named “animal”). If we chose to have subclasses of Animal in our inventory collection, we could map them to XML element names such as gorilla and lemur. See @XmlElementRef.
@XmlElementRefSimilar to @XmlElements, used to generate individualized names for Java types in a collection. However, instead of the names for each type being specified directly, they are determined at runtime by the individual types’ Java type bindings (e.g., in our example, inventory contains animal elements because our List is named “animal”). Using @XmlElementRef, we could subclass Animal and have our inventory contain elements like gorilla and lemur, with the names determined by @XmlRootElement annotations on the respective subclasses. See important class binding info in @XmlElementRefs.
@XmlElementRefsUsed on a Java collection to provide a list of @XmlElementRef annotations with type attributes that explicitly specify the Java types that may appear in the collection. The effect is the same as using a simple @XmlElementRef on the collection, but we actively tell JAXB the class names that have bindings. If not supplied in this way, we have to provide the full list of bound classes to the JAXBContextnewInstance() method in order for them to be recognized.
@XmlElementWrapperUsed on a Java collection to cause the sequence of XML elements to be wrapped in the specified element instead of appearing directly inline in the XML (e.g., our animal elements appear directly in inventory). Using this annotation, we could nest them all within a new animals element.
@XmlEnumBinds a Java Enum to XML and allows @XmlEnumValues annotations to be used to map the enum values for XML if required.
@XmlEnumValueBinds an individual Java Enum value to a string to be used in the XML (e.g., our mammal enum value could be mapped to “mammalia”).
@XmlIDSupports referential integrity by designating a Java property or field of a class as being the XML ID attribute (a unique key) for the XML element within the document.
@XmlIDREFSupports referential integrity by designating a Java property or field as an idref attribute pointing to an element with an @XmlID. The annotated property or field must contain an instance of a Java type containing an @XmlID annotation. When marshalled, the attribute name will be the property name and the value will be the contained XML ID value.
@XmlInlineBinaryDataBind a Java byte array to receive base64 binary data encoded in the XML.
@XmlListUsed on a Java collection to map items to a single simple content element with a whitespace-separated list of values instead of a series of elements.
@XmlMimeTypeUsed with a Java Image or Source type to specify a MIME type for XML base64-encoded binary data bound to it.
@XmlMixedBinds a Java object collection to XML “mixed content” (i.e., XML containing both text and element tags within it). Text will be added to the collection as String objects interleaved with the usual Java types representing the other elements.
@XmlRootElementBind a Java class to an XML element optionally provide a name. This is the minimum annotation required on your class to make it possible to marshal it to XML and back.
@XmlElementDeclUsed in binding XML schema elements to methods in Java object factories created in some code generation scenarios.
@XmlRegistryUsed with @XmlElementDecl in designating Java object factories used in some code generation scenarios.
@XmlSchemaBinds a Java package to a default XML namespace.
@XmlNsUsed with @XmlSchema to bind a Java package to one or more XML namespace prefixes.
@XmlSchemaTypeUsed on a Java property, field, or package. Specifies a Java type to be used for a standard XML schema built-in types, such as date or a numeric type.
@XmlSchemaTypesUsed on a Java package. Holds a list of @XmlSchemaType annotations mapping Java types to built-in XML schema types.
@XmlTransientDesignates that a Java property or field should not be marshaled to the XML. This can be used in conjunction with defaults that marshal all properties or fields to exclude individual items. See @XmlAccessorType.
@XmlTypeBinds a Java class to an XML schema type. Additionally, the propOrder attribute may be used to explicitly list the order in which elements are marshalled to XML.
@XmlValueDesignates that a Java property or field contains the “simple” XML content for the Java type; that is, instead of marshalling the class as an XML element containing a nested element for the property, the value of the annotated property will appear directly as the content. The Java type may have only one property designated as @XmlValue.

Unmarshalling from XML

Creating our object model from XML just requires a few lines to create an Unmarshaller from our JAXBContext and a cast to the Java type of our root element:

JAXBContext context = JAXBContext.newInstance( Inventory.class );
Unmarshaller unmarshaller = context.createUnmarshaller();
Inventory inventory = (Inventory)unmarshaller.unmarshal(
    new File("zooinventory.xml") );

The Unmarshaller class has a setValidating() method like the SAXParser, but it is deprecated. Instead, we could use the setSchema() method to set an XML Schema representation if we want validation as part of the parsing process. Alternately, we could just validate the schema separately. See XML Schema.

Generating a Java Model from an XML Schema

If you are starting with an XML Schema (xsd file), you can generate annotated Java classes from the schema using the JAXB xjc command-line tool that comes with the JDK.

xjc zooinventory.xsd

// Output
parsing a schema...
compiling a schema...

By default, the output is placed in the default package in a directory named generated. You can control the package name with the -p switch and the directory with -d. See the xjc documentation for more options.

Studying the generated classes will give you some hints as to how many annotations are used, although xjc is a little more verbose than it has to be. Also note that xjc produces a class called ObjectFactory that contains factory methods for each type, such as createInventory() and createAnimal(). If you look at these methods, you’ll see that they really just call new on the plain Java objects and they seem superfluous. The ObjectFactory is mainly there for legacy reasons. In ealier versions of JAXB, before annotations, the generated classes were not as simple to construct. Additionally, the ObjectFactory contains a helper method to create a JAXBElement type, which may be useful in special situations. For the most part, you can ignore these.

Generating an XML Schema from a Java Model

You can also generate an XML Schema directly from your annotated Java classes using the JAXB XML Schema binding generator: schemagen. The schemagen command-line tool comes with the JDK. It can generate a schema starting with Java source or class files. Use the -classpath argument to specify the location of the classes or source files and then provide the name of the root class in your hierarchy:

schemagen -classpath . Inventory

Having worked our way through the options for bridging XML to Java, we’ll now turn our attention to transformations on XML itself with XSL, the styling language for XML.

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