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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

JAR Files

Java Archive (JAR) files are Java’s suitcases. They are the standard and portable way to pack up all the parts of your Java application into a compact bundle for distribution or installation. You can put whatever you want into a JAR file: Java class files, serialized objects, data files, images, audio, etc. A JAR file can also carry one or more digital signatures that attest to its integrity and authenticity. A signature can be attached to the file as a whole or to individual items in the file.

The Java runtime system can load class files directly from an archive in your CLASSPATH, as described earlier. Nonclass files (data, images, etc.) contained in your JAR file can also be retrieved from the classpath by your application using the getResource() method (described in Chapter 12). Using this facility, your code doesn’t have to know whether any resource is in a plain file or a member of a JAR archive. Whether a given class or data file is an item in a JAR file or an individual file on the classpath, you can always refer to it in a standard way and let Java’s class loader resolve the location.

File Compression

Items stored in JAR files are compressed with the standard ZIP file compression. Compression makes downloading classes over a network much faster. A quick survey of the standard Java distribution shows that a typical class file shrinks by about 40 percent when it is compressed. Text files such as HTML or ASCII containing English words often compress to one-tenth their original size or less. (On the other hand, image files don’t normally get smaller when compressed as most common image formats are themselves a compression format.)

Java also has an archive format called Pack200, which is optimized specifically for Java class bytecode and can achieve over four times greater compression of Java classes than ZIP alone. We’ll talk about Pack200 later in this chapter.

The jar Utility

The jar utility provided with the JDK is a simple tool for creating and reading JAR files. Its user interface isn’t particularly friendly. It mimics the Unix tar (tape archive) command. If you’re familiar with tar, you’ll recognize the following incantations:

jar -cvf jarFile path [ path ] [ ... ]

Create jarFile containing path(s).

jar -tvf jarFile [ path ] [ ... ]

List the contents of jarFile, optionally showing just path(s).

jar -xvf jarFile [ path ] [ ... ]

Extract the contents of jarFile, optionally extracting just path(s).

In these commands, the flag letters c, t, and x tell jar whether it is creating an archive, listing an archive’s contents, or extracting files from an archive. The f means that the next argument is the name of the JAR file on which to operate. The optional v flag tells jar to be verbose when displaying information about files. In verbose mode, you get information about file sizes, modification times, and compression ratios.

Subsequent items on the command line (i.e., anything aside from the letters telling jar what to do and the file on which jar should operate) are taken as names of archive items. If you’re creating an archive, the files and directories you list are placed in it. If you’re extracting, only the filenames you list are extracted from the archive. (If you don’t list any files, jar extracts everything in the archive.)

For example, let’s say we have just completed our new game, spaceblaster. All the files associated with the game are in three directories. The Java classes themselves are in the spaceblaster/game directory, spaceblaster/images contains the game’s images, and spaceblaster/docs contains associated game data. We can pack all this in an archive with this command:

    % jar -cvf spaceblaster.jar spaceblaster

Because we requested verbose output, jar tells us what it is doing:

    adding:spaceblaster/ (in=0) (out=0) (stored 0%)
    adding:spaceblaster/game/ (in=0) (out=0) (stored 0%)
    adding:spaceblaster/game/Game.class (in=8035) (out=3936) (deflated 51%)
    adding:spaceblaster/game/Planetoid.class (in=6254) (out=3288) (deflated 47%)
    adding:spaceblaster/game/SpaceShip.class (in=2295) (out=1280) (deflated 44%)
    adding:spaceblaster/images/ (in=0) (out=0) (stored 0%)
    adding:spaceblaster/images/spaceship.gif (in=6174) (out=5936) (deflated 3%)
    adding:spaceblaster/images/planetoid.gif (in=23444) (out=23454) (deflated 0%)
    adding:spaceblaster/docs/ (in=0) (out=0) (stored 0%)
    adding:spaceblaster/docs/help1.html (in=3592) (out=1545) (deflated 56%)
    adding:spaceblaster/docs/help2.html (in=3148) (out=1535) (deflated 51%)

jar creates the file spaceblaster.jar and adds the directory spaceblaster, adding the directories and files within spaceblaster to the archive. In verbose mode, jar reports the savings gained by compressing the files in the archive.

We can unpack the archive with this command:

    % jar -xvf spaceblaster.jar

Likewise, we can extract an individual file or directory with:

    % jar -xvf spaceblaster.jar filename

But, of course, you normally don’t have to unpack a JAR file to use its contents; Java tools know how to extract files from archives automatically. We can list the contents of our JAR with the command:

    % jar -tvf spaceblaster.jar

Here’s the output; it lists all the files, their sizes, and their creation times:

    0 Thu May 15 12:18:54 PDT 2003 META-INF/
      1074 Thu May 15 12:18:54 PDT 2003 META-INF/MANIFEST.MF
         0 Thu May 15 12:09:24 PDT 2003 spaceblaster/
         0 Thu May 15 11:59:32 PDT 2003 spaceblaster/game/
      8035 Thu May 15 12:14:08 PDT 2003 spaceblaster/game/Game.class
      6254 Thu May 15 12:15:18 PDT 2003 spaceblaster/game/Planetoid.class
      2295 Thu May 15 12:15:26 PDT 2003 spaceblaster/game/SpaceShip.class
         0 Thu May 15 12:17:00 PDT 2003 spaceblaster/images/
      6174 Thu May 15 12:16:54 PDT 2003 spaceblaster/images/spaceship.gif
     23444 Thu May 15 12:16:58 PDT 2003 spaceblaster/images/planetoid.gif
         0 Thu May 15 12:10:02 PDT 2003 spaceblaster/docs/
      3592 Thu May 15 12:10:16 PDT 2003 spaceblaster/docs/help1.html
      3148 Thu May 15 12:10:02 PDT 2003 spaceblaster/docs/help2.html

JAR manifests

Note that the jar command automatically adds a directory called META-INF to our archive. The META-INF directory holds files describing the contents of the JAR file. It always contains at least one file: MANIFEST.MF. The MANIFEST.MF file can contain a “packing list” naming the files in the archive along with a user-definable set of attributes for each entry.

The manifest is a text file containing a set of lines in the form keyword: value. The manifest is, by default, empty and contains only JAR file version information:

Manifest-Version: 1.0
Created-By: 1.7.0_07 (Oracle Corporation)

It is also possible to sign JAR files with a digital signature. When you do this, digest (checksum) information is added to the manifest for each archived item (as shown next) and the META-INF directory holds digital signature files for items in the archive.

    Name: com/oreilly/Test.class
    SHA1-Digest: dF2GZt8G11dXY2p4olzzIc5RjP3=
    ...

You can add your own information to the manifest descriptions by specifying your own supplemental, manifest file when you create the archive. This is one possible place to store other simple kinds of attribute information about the files in the archive, perhaps version or authorship information.

For example, we can create a file with the following keyword: value lines:

    Name: spaceblaster/images/planetoid.gif
    RevisionNumber: 42.7
    Artist-Temperament: moody

To add this information to the manifest in our archive, place it in a file called myManifest.mf and give the following jar command:

    % jar -cvmf myManifest.mf spaceblaster.jar spaceblaster

We included an additional option, m, which specifies that jar should read additional manifest information from the file given on the command line. How does jar know which file is which? Because m is before f, it expects to find the manifest information before the name of the JAR file it will create. If you think that’s awkward, you’re right; get the names in the wrong order, and jar does the wrong thing.

An application can get this manifest information from a JAR file using the java.util.jar.Manifest class.

We’ll see more examples of adding information to the JAR manifest in Chapter 22. The JavaBeans APIs use manifest information to designate which classes are “beans” using a Java-Bean attribute. This information is used by IDEs that work with JavaBeans.

Making a JAR file runnable

Aside from attributes, you can put a few special values in the manifest file. One of these, Main-Class, allows you to specify the class containing the primary main() method for an application contained in the JAR:

    Main-Class: com.oreilly.Game

If you add this to your JAR file manifest (using the m option described earlier), you can run the application directly from the JAR:

    % java -jar spaceblaster.jar

More importantly, under Mac OS X, Windows, and other GUI environments, you can simply double-click on the JAR file to launch the application. The interpreter looks for the Main-Class value in the manifest, then loads the designated class as the application’s startup class.

The pack200 Utility

Pack200 is an archive format that is optimized for storing compiled Java class files. Pack200 is not a new form of compression, but rather a super-efficient layout for class information that eliminates many types of waste and redundancy across related classes. It is effectively a bulk class-file format that deconstructs many classes and reassembles their parts efficiently into one catalog. This then allows a standard compression format like ZIP to work at maximum efficiency on the archive, achieving four or more times greater compression. The Java runtime does not understand the Pack200 format, so you cannot place archives of this type into the classpath. Instead, it is mainly an intermediate format that is very useful for transferring application JARs over the network for applets or other kinds of web-based applications.

You can convert a JAR to and from Pack200 format with the pack200 and unpack200 commands supplied with the JDK.

For example, to convert foo.jar to foo.pack.gz, use the pack200 command:

    % pack200 foo.pack.gz foo.jar

To convert foo.pack.gz to foo.jar:

    % unpack200 foo.pack.gz foo.jar

Note that the Pack200 process completely tears down and reconstructs your classes at the class level, so the resulting foo.jar file will not be byte-for-byte the same as the original.

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