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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
    2. Comments
      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 java.io.File 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

Datagram Sockets

TinyHttpd used a Socket to create a connection to the client using the TCP protocol. In that example, the TCP protocol took care of data integrity; we didn’t have to worry about data arriving out of order or incorrect. Now, let’s take a walk on the wild side, building an applet that uses a java.net.DatagramSocket, which uses the UDP protocol. A datagram is sort of like a letter sent via the postal service: it’s a discrete chunk of data transmitted in one packet. Unlike the previous example, where we could get a convenient OutputStream from our Socket and write the data as if writing to a file with a DatagramSocket, we have to work one datagram at a time. (Of course, the TCP protocol was taking our OutputStream and slicing the data into packets, too, but we didn’t have to worry about those details.)

UDP doesn’t guarantee that the data is received. If the data packets are received, they may not arrive in the order in which they were sent; it’s even possible for duplicate datagrams to arrive (under rare circumstances). Using UDP is something like cutting the pages out of the encyclopedia, putting them into separate envelopes, and mailing them to your friend. If your friend wants to read the encyclopedia, it’s his or her job to put the pages in order. If some pages get lost in the mail, your friend has to send you a letter asking for replacements.

Obviously, you wouldn’t use UDP to send a huge amount of data without error correction. However, it’s significantly more efficient than TCP, particularly if you don’t care about the order in which messages arrive or whether 100 percent of their arrival is guaranteed. For example, in a simple periodic database lookup, the client can send a query; the server’s response itself constitutes an acknowledgment. If the response doesn’t arrive within a certain time, the client can just send another query. It shouldn’t be hard for the client to match responses to its original queries. Some important applications that use UDP are the Domain Name System (DNS) and Sun’s Network File System (NFS).

The HeartBeat Applet

In this section, we build a simple applet, HeartBeat, that runs in a web browser and sends a datagram to its server each time it’s started and stopped. We also build a simple standalone server application, Pulse, that receives these datagrams and prints them. Tracking the output could give you a crude measure of who is currently looking at your web page at any given time (assuming that firewalls do not block the UDP packets). This is the kind of thing UDP is good for: we don’t want the overhead of a TCP socket, and if the datagrams get lost, it’s no big deal.

First, the HeartBeat applet:

    //file: HeartBeat.java
    import java.net.*;
    import java.io.*;
      
    public class HeartBeat extends java.applet.Applet {
        String myHost;
        int myPort;
      
        public void init() {
            myHost = getCodeBase().getHost();
            myPort = Integer.parseInt( getParameter("myPort") );
        }
      
        private void sendMessage( String message ) {
            try {
                byte [] data = message.getBytes("UTF-8");
                InetAddress addr = InetAddress.getByName( myHost );
                DatagramPacket packet =
                  new DatagramPacket( data, data.length, addr, myPort );
                DatagramSocket ds = new DatagramSocket();
                ds.send( packet );
                ds.close();
            } catch ( IOException e ) {
                System.out.println( e );  // Error creating socket
            }
        }
      
        public void start() {
            sendMessage("Arrived");
        }
        public void stop() {
            sendMessage("Departed");
        }
    }

Compile the applet and include it in an HTML document with an <applet> tag:

<html><body>
    <h1>Heartbeat!</h1>
    <applet height="1 " width="1 " code="HeartBeat ">
        <param name="myPort" value="1234">
    </applet>
    </body></html>

Make sure to place the compiled HeartBeat.class file in the same directory as the HTML document (which we’ll refer to as heartbeat.html). We talk more about embedding applets in HTML documents in Chapter 23.

The myPort parameter should specify the port number on which our server application listens for data (“1234” as just shown).

Next, the server-side application, Pulse:

    //file: Pulse.java
    import java.net.*;
    import java.io.*;
      
    public class Pulse {
        public static void main( String [] argv ) throws IOException {
            DatagramSocket s =
              new DatagramSocket( Integer.parseInt(argv[0]) );
      
            while ( true ) {
                DatagramPacket packet =
                  new DatagramPacket( new byte [1024], 1024 );
                s.receive( packet );
                String message = new String(
                    packet.getData(), 0, packet.getLength(),"UTF-8" );
                System.out.println( "Heartbeat from: "
                  + packet.getAddress().getHostName()
                  + " - " + message );
            }
        }
    }

Compile Pulse and run it on your web server, specifying the port number as an argument:

    % java Pulse 1234

The port number should be the same as the one you used in the myPort parameter of the <applet> tag for HeartBeat.

Now to run the example we’re going to need a web server. Opening the file directly in your browser will not work here because, as we mentioned earlier, applets are only allowed to talk to the host that served them and when no web server is involved, the security manager doesn’t allow any network communications at all. Fortunately, we wrote a satisfactory web server, TinyHttpd, earlier in this chapter! Just run TinyHttpd in the directory with your heartbeat.html file—being careful to specify a different port number than our HeartBeat client and Pulse server—and use it to serve up the page. We tested this in Safari on a Mac at the time of this writing and it worked as expected. But if you have issues you can try a “real” web server and we won’t be offended.

Now, pull up the web page in your browser. You won’t see much there (a better application might do something visual as well), but you should get a blip from the Pulse application. Leave the page and return to it a few times. Each time the applet is started or stopped, it sends a message that Pulse reports:

    Heartbeat from: foo.bar.com - Arrived
    Heartbeat from: foo.bar.com - Departed
    Heartbeat from: foo.bar.com - Arrived
    Heartbeat from: foo.bar.com - Departed
    ...

Cool, eh? Just remember that the datagrams are not guaranteed to arrive (although it’s highly unlikely you’ll ever see them fail on a normal network), and it’s possible that you could miss an arrival or a departure. Now let’s look at the code.

The HeartBeat applet code

HeartBeat overrides the init(), start(), and stop() methods of the Applet class and implements one private method of its own, sendMessage(), which sends a datagram. (We haven’t covered applets yet, so if you want more details, refer to Chapter 23.) HeartBeat begins its life in init(), where it determines the destination for its messages. It uses the Applet getCodeBase() and getHost() methods to find the name of its originating host and fetches the correct port number from the myPort parameter of the <applet> tag. After init() has finished, the start() and stop() methods are called whenever the applet is started or stopped. These methods merely call sendMessage() with the appropriate message.

sendMessage() is responsible for sending a String message to the server as a datagram. It takes the text as an argument, constructs a datagram packet containing the message, and then sends the datagram. All the datagram information is packed into a java.net.DatagramPacket object, including the destination and port number. The DatagramPacket is like an addressed envelope, stuffed with our bytes. After the DatagramPacket is created, sendMessage() simply has to open a DatagramSocket and send it.

The first five lines of sendMessage() build the DatagramPacket:

    try {
        byte [] data = message.getBytes("UTF-8");
        InetAddress addr = InetAddress.getByName( myHost );
        DatagramPacket pack =
          new DatagramPacket(data, data.length, addr, myPort );

First, the contents of message are placed into an array of bytes called data. Next a java.net.InetAddress object is created by looking up the hostname myHost. An InetAddress holds the network IP address for a host. This class also provides the static getByName() method for looking up an IP address by hostname using the system’s name service. (We’ll say more about InetAddress in the next section.) Finally, we call the DatagramPacket constructor with four arguments: the byte array containing our data, the length of the data, the destination address object, and the port number. We ask for the string to be encoded using the UTF-8 charset; we’ll use the same character set to decode it.

The remaining lines construct a default client DatagramSocket and call its send() method to transmit the DatagramPacket. After sending the datagram, we close the socket:

    DatagramSocket ds = new DatagramSocket();
    ds.send( pack );
    ds.close();

Two operations throw a type of IOException: the InetAddress.getByName() lookup and the DatagramSocket send() method. InetAddress.getByName() can throw an UnknownHostException, which is a type of IOException that indicates the hostname can’t be resolved. If send() throws an IOException, it implies a serious client-side communication problem with the network. We need to catch these exceptions; our catch block simply prints a message telling us that something went wrong. If we get one of these exceptions, we can assume the datagram never arrived. However, we can’t assume the inverse: even if we don’t get an exception, we still don’t know that the host is actually accessible or that the data actually arrived; with a DatagramSocket, we never find out from the API.

The Pulse server code

The Pulse server corresponds to the HeartBeat applet. First, it creates a DatagramSocket to listen on our prearranged port. This time, we specify a port number in the constructor; we get the port number from the command line as a string (argv[0]) and convert it to an integer with Integer.parseInt(). Note the difference between this call to the constructor and the call in HeartBeat. In the server, we need to listen for incoming datagrams on a prearranged port, so we need to specify the port when creating the DatagramSocket. The client just sends datagrams, so we don’t have to specify the port in advance; we build the port number into the DatagramPacket itself.

Second, Pulse creates an empty DatagramPacket of a fixed maximum size to receive an incoming datagram. This form of the constructor for DatagramPacket takes a byte array and a length as arguments. As much data as possible is stored in the byte array when it’s received. (A practical limit on the size of a UDP datagram that can be sent over the Internet is 8K, although datagrams can be larger for local network use—theoretically up to 64K.) Finally, Pulse calls the DatagramSocket’s receive() method to wait for a packet to arrive. When a packet arrives, its contents are printed by turning them to a string using UTF-8 encoding. We determine the actual number of received bytes from the packet’s getLength() method.

As you can see, DatagramSockets are slightly more tedious than regular Sockets. With datagrams, it’s harder to spackle over the messiness of the socket interface. The Java API rather slavishly follows the Unix interface, and that doesn’t help. But all in all, it’s not that hard to use datagrams for simple tasks.

InetAddress

The java.net.InetAddress class is the lowest-level Java API for working with IP addresses. Instances of InetAddress represent individual addresses and the InetAddress class provides the API for using the platform’s name service to map a string hostname to a numeric IP address. Most of our networking examples showed the use of hostnames to identify remote servers, but under the covers, Java utilized the static InetAddress.getByName() method to map the name to a physical IP address. Java normally uses the DNS to perform this lookup (and it caches the results for efficiency). Most Java networking APIs (such as Sockets) will accept either a hostname or an InetAddress as a destination. The InetAddress class can also be used to perform reverse IP lookups (get a name for an IP address) as well as to find the primary address of the local host via the static InetAddress.getLocalHost() method.

A useful feature of InetAddress is the method isReachable(), which attempts to use the ICMP ping protocol to determine whether a remote address can be reached over the network. The ping protocol is the standard mechanism used to check reachability and latency on networks. It is a low-level IP protocol (along with TCP and UDP) and is not guaranteed to be supported everywhere. If isReachable() can’t use ICMP, it attempts to use TCP to reach the echo service (port 7) on the remote host. For example:

    InetAddress server = InetAddress.getByName("myserver");
    If ( !server.isReachable( timeout ) ) // milliseconds
        pageSomeone();

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