XML and XSLT and other TLA's

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

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

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

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

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?

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.com http://java.sun.com/sfaq/verifier.html

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

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

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

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