Software Design8.1

XML and XSLT and other TLA's

XML: more than buzzword compliancy Future of information interchange Leverages Internet and TCP/IP Readable by humans and machines

What does XSLT do? Transform XML to: HTML, XML, … Programming Language

How to use these? With Java (or other languages) With IE

Software Design8.2

Design Patterns: Abstract Factory

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 Design8.3

Design Patterns: Decorator

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

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 Design8.4

How Java works

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?

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 Design8.5

From JITs to Deoptimization

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

…? What does the class loader do?

Software Design8.6

Loading .class files 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.comhttp://java.sun.com/sfaq/verifier.html

Software Design8.7

The ClassLoader 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?)

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

Why implement a custom loader? Work at Duke with JOIE

Software Design8.8

Applications and Applets

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

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 Design8.9

CGI: Common Gateway Interface 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 Design8.10

Setting up CGI programs 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

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