Transcript Designing Classes and Programs

XML and XSLT and other TLA's
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XML: more than buzzword compliancy
 Future of information interchange
 Leverages Internet and TCP/IP
 Readable by humans and machines
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What does XSLT do?
 Transform XML to: HTML, XML, …
 Programming Language
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How to use these?
 With Java (or other languages)
 With IE
Software Design
8.1
Design Patterns: Abstract Factory
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Provide an interface for creating families of related or
dependent objects without specifying their concrete
classes.
 System is independent of how its products are created,
composed, represented
 Configured with one of multiple families of products
 Family of products designed to be used together, this
constraint should be enforced
 Provide library, but reveal interfaces, not
implementations
Makes exchanging product families simple
Supporting new products is difficult
Software Design
8.2
Design Patterns: Decorator
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Attach additional responsibilities to an object
dynamically. Decorators provide an alternative to
subclassing
 Add responsibilities dynamically and transparently,
without affecting other objects
 Withdraw responsibilities
 Extension by subclass might be impractical
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Decorate a component so that the component and the
decorator (container) share the same interface
 Lots of little objects that look alike, easy to customize
if you understand, but hard to learn
Software Design
8.3
How Java works
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The java compiler takes a .java file and generates a .class file
 The .class file contains Java bytecodes, the assembler
language for Java programs
 Bytecodes are executed in a JVM (java virtual machine),
the valid bytecodes are specified by Sun
• What if third parties create platform/OS specific codes?
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IBM has Jikes, a lightning fast compiler, others?
The JVM interprets the bytecodes
 JVM is platform/OS specific, must ultimately run the code
 Different JVMs will have different performance
 Sun has HotSpot, previously the big thing was JITs
• Adaptive compilation vs. compile when executed
Software Design
8.4
From JITs to Deoptimization
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JITs compile bytecodes when first executed
 If we can cache translated code we can avoid re-translating
the same bytecode sequence
 Spend time compiling things that aren’t frequently
executed (optimistic optimization?)
 Errors indicate “compiled code” rather than line number
Sun’s HotSpot VM uses a different strategy for performance
 Adaptive compilation: save time over JIT, compile
“hotspots” rather than everything, uses less memory, starts
program faster, …
 No method inlining, but uses dynamic deoptimization
• Program loads new subclass, compiled code invalid, so …?
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What does the class loader do?
Software Design
8.5
Loading .class files
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The bytecode verifier “proves theorems” about the bytecodes
being loaded into the JVM
 These bytecodes may come from a non-Java source, e.g.,
compile Ada into bytecodes (why?)
This verification is a static analysis of properties such as:
 .class file format (including magic number 0xCAFEBABE)
 Methods/instances used properly, parameters correct
 Stack doesn’t underflow/overflow
 …
Verification is done by the JVM, not changeable
 Contrast ClassLoader, which is changeable, can modify
classes before they’re loaded into the JVM
http://securingjava.com
http://java.sun.com/sfaq/verifier.html
Software Design
8.6
The ClassLoader
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The “boot strap” loader is built-in to the JVM
 Sometimes called the “default” loader, but it’s not
extensible or customizable the way other loaders are
 Loads classes from the platform on which the JVM runs
(what are loader and JVM written in?)
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Applet class loader, RMI class loader, user loaders
 Load .class files from URLs, from other areas of platform
on which JVM runs
 A class knows how it was loaded and new instances will
use the same loader
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Why implement a custom loader?
 Work at Duke with JOIE
Software Design
8.7
Applications and Applets
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An applet is a program delivered via the web
 security issues, sandbox model
 where does code/images/etc come from? How is it
delivered?
 what browsers support JDK 2.0 out-of-the box?
 Use IE/Netscape with plugin, use Opera as is, use
appletviewer for debugging, testing
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Possible to wrap up lots of classes in a .jar file
 java archive, similar to a tar file, possible to include
.class files, .au, .gif, etc, so all code transferred at once
Software Design
8.8
CGI: Common Gateway Interface
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http://hoohoo.ncsa.uiuc.edu/cgi/overview.html
How do programs running on the server communicate
with clients over the web?
 Can do simple UI stuff in HTML: textfield, buttons,
radio buttons, select/choicebox, checkbox, …
 Communicate choices to client program via
“parameters”
The communication is done by encoding text in a string
that’s passed to program running on server
 Server program can be written in Perl, C, C++, Java, …
 Two forms of communication: get and post
• Post is more robust, no limit on size of string sent
Software Design
8.9
Setting up CGI programs
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http://www.oit.duke.edu/sa/cgi/
 Request a CGI key
 Create a directory and give cgi.ola rliw permission
 Put code in the directory, html pages can live anywhere
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Debugging is tricky, probably want to debug from the
command-line rather than over the web
Be sure to send appropriate header to start HTML page from
server program when sending to client/browser
 Missing header causes “internal server” or other error
See code examples and HTML sample for details
 Server must parse arguments
 Replace “+” with “ “, look for %xx args, replace them
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Software Design
8.10