Applets and Applications Developing Applets and Applications - - PowerPoint PPT Presentation

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Applets and Applications Developing Applets and Applications - - PowerPoint PPT Presentation

Sep 08, 2022 195 likes •260 views

Applets and Applications Developing Applets and Applications Application run by user, double-clickable/command-line Create a JPanel with the guts of the GUI/logic No restrictions on access, reads files, URLS, What will be in the

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SLIDE 1

Software Design 7.1

Applets and Applications

 Application run by user, double-clickable/command-line

  • No restrictions on access, reads files, URLS, …
  • GUI applications typically include a Jframe
  • Has title, menus, closeable, resizeable

 Applet is downloaded via the web

  • Runs in browser, not trusted (but see policy later)
  • Can't read files on local machine (but see policy)
  • Can't be resized within browser
  • Uses jar file to get all classes at once
  • Alternative? Establish several connections to

server

Software Design 7.2

Developing Applets and Applications

 Create a JPanel with the guts of the GUI/logic

  • What will be in the content pane of both deployments
  • Makes GUI very simple, see SameGame refactoring
  • Use JPanel in both Applet and Application

 Test with application first, easier to read files/resources

  • Migrate to Applet, test first with appletviewer
  • Migrate to web, may need to clear cache/reload

 Ideally first cleanly into OOGA architecture

  • Gui isn't the view, what about interfaces?

Software Design 7.3

Packages, JAR files, deployment

http://java.sun.com/docs/books/tutorial/jar/basics/index.html

 Java packages correspond semantically to modules

(related classes) and syntactically to a directory structure

  • Class names correspond to file names
  • Package names correspond to directories
  • Related classes belong together, easier to develop,

easier to deploy

  • Leverage default/package access, use properties of

protected which is subclass and package access

Software Design 7.4

Packages, javac, java, javadoc

 In moderately big programs packages are essential

  • Can’t easily live in a directory with 50 .java files
  • Can’t easily co-exist in such a directory
  • Harder to use tools like Make and Ant

 Each of javac, java, javadoc is slightly different with

packages, all must co-exist with CLASSPATH

  • File system vs. compiler vs. runtime
  • Source of confusion and problems
  • IDEs can manage Make/CLASSPATH issues
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SLIDE 2

Software Design 7.5

CLASSPATH and related concepts

 The default CLASSPATH is . current directory

  • Works fine with default/unnamed packages
  • Will not work with named packages

 Set CLASSPATH to directory in which packages live also

include current dir

  • setenv CLASSPATH "~ola:."
  • setenv CLASSPATH "`pwd`:."
  • On windows machines change registry variable,

separator is semi-colon rather than colon

 All problems are CLASSPATH problems

Software Design 7.6

More package details

To compile

  • Can cd into directory and type javac *.java
  • Can also type javac ooga/*.java from one level up
  • If CLASSPATH isn't set, the second won't work

To run

  • java ooga.TicTac will work, you must specify the "real"

name of the class being used.

  • Reading files requires full-paths or run from directory in

which file lives

To document

  • http://java.sun.com/j2se/javadoc/faq.html
  • Don't need to use –sourcepath, but can
  • javadoc –d doc ooga ooga.timer ooga.game …

Software Design 7.7

javadoc for packages

 See the javadoc faq

http://java.sun.com/j2se/javadoc/faq.html

  • For each package create a package.html file
  • Not in /** */ javadoc format, strictly html
  • First sentence after <body> is main description; a

sentence ends with a period.

  • The package.html file should provide complete

instructions on how to use the package. All programmer documentation should be accessible

  • r part of this file, e.g., in the file or linked to the

file

  • Use the {@link foo.bar bar} tag appropriately.
  • See the FAQ, or the source for the elan package
  • nline

 You may want to keep .java and .class files separate, see

sourcepath and classpath as commandline args to java

Software Design 7.8

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, http://java.sun.com/products/hotspot/

  • No method inlining, but uses dynamic deoptimization
  • Program loads new subclass, compiled code invalid, so

…?

 What does the class loader do?

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SLIDE 3

Software Design 7.9

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

Software Design 7.10

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
  • n 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 Design 7.11

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 JDK1.2 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 Design 7.12

Running an Applet

 An applet has an init() method

  • similar to constructor, called only once, when the

applet is first loaded

 An applet has a start() method

  • called each time the applet becomes “active”, run the

first time, or revisited e.g., via the back button in a browser

 An applet has a stop() method

  • called when applet is invisible, e.g., user scrolls or

goes to another web page

 other methods in an applet

  • destroy, getAppletInfo, getParameterInfo

 Applet subclasses Panel, so it is an Container/Component

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SLIDE 4

Software Design 7.13

Security Manager

Applets use a SecurityManager

  • Query for permissions
  • Supported by browsers by

convention (would you use an “untrusted” browser)

The picture shows JDK 1.0 model, “sandbox” restrictions supported by SecurityManager

  • Untrusted code restricted to

the sandbox

  • All downloaded/applets are

untrusted

  • Severely limits what a

downloaded program can do

Software Design 7.14

SecurityManager changes in JDK 1.1

 Applets support signing

using digital signatures

  • Signature stored with

code in JAR file that’s downloaded

  • Clients support
  • pen/full access to

“trusted” applets, some signatures ok

 Still “all-or-nothing”, an

applet is untrusted or completely trusted

  • What might be

preferable?

Software Design 7.15

SecurityManager changes in JDK 1.2

Policies are now supported

  • Allow more fine-grained

control of access, permission

  • Based on location (URL)

and/or digital signatures

  • Uses public/private key,

applets don’t need to be signed, can be from a trusted location

Set policies on a system wide basis using policytool

  • What about user-level

permissions?

Software Design 7.16

Networkable solutions

 Using sockets we can pass objects back and forth between

client and server, but we’re limited

  • Cannot call methods on objects between client/server,
  • nly send objects and respond to protocols
  • Conceivably we’ll run into firewall problems

 RMI, Remote Method Invocation, can fix both these

problems

  • Clients call methods across machines, on objects

running in other JVMs

  • Built on top of HTTP, so can (in theory) be used

through firewalls

 Requires bookkeeping and security precautions, non-

trivial to set up (not in theory, but in practice)

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SLIDE 5

Software Design 7.17

RMI ideas

Think of clients and servers, though this distinction is blurred

  • In jdk 1.2, server-side objects can call client object methods,

so really distributed computing

  • Still, an initial server/rmiregistry begins the RMI process

JVMs on different machines execute, objects communicate with each other between JVMs

  • Sockets used underneath, either TCP/IP or HTTP or …

customizable e.g., SSL

Classes can either be located in all JVMs, or transferred/downloaded using HTTP codebase

  • Codebase is also used in applets, but restricted to original

web page as root

Software Design 7.18

Applet codebase

 JVM executing in browser has different capabilities

than “regular” JVM

  • Looks in codebase as its CLASSPATH, also uses

client/browser side CLASSPATH

  • Codebase is relative to location of web page
  • riginating the applet for security reasons
  • Implications for downloading foo.jar?

Software Design 7.19

XML: Another TLA or the Future?

http://java.sun.com/xml/tutorial_intro.html

 XML is eXtensible Markup Language

  • It's a w3c standard (what does this mean?)
  • http://www.w3.org/XML/
  • It's simple, flexible, portable, extensible,

hierarchical, complicated (see simple)

  • Basis for XMP-RPC, SOAP, UDDI, WSDL

 Why do we need to understand XML?

  • It really is simple, and it really is powerful
  • It really is in-demand and one of the

FOTDay

Software Design 7.20

XML in brief

 Need some method to create XML

  • Exported by applications, e.g., iTunes
  • Edited by hand, this is a bad idea
  • Done programatically, see bullet 1

 XML can be validated with a DTD

  • Document Type Definition
  • Often provided by applications
  • Really necessary for complex XML data
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SLIDE 6

Software Design 7.21

What does it look like?

<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN "http://www.apple.com/DTDs/PropertyList-1.0.dtd"> <plist version="1.0"> <dict> <key>Major Version</key><integer>1</integer> <key>Minor Version</key><integer>1</integer> <key>Application Version</key><string>6.0.1</string> <key>Features</key><integer>1</integer> <key>MusicFolder</key> <string>file://localhost/Users/ola/Music/iTunes/iTunes%20Music/</string> <key>Library Persistent ID</key><string>2D2047AF6D8A9673</string> <key>Tracks</key> <dict> <key>40</key> <dict> <key>Track ID</key><integer>40</integer> <key>Name</key><string>Track 01</string> <key>Kind</key><string>AAC audio file</string> Software Design 7.22

Elements of XML

 There's an XML header, specifies global "stuff"  There's the content

  • Root element
  • Sub elements

 What is an element?

  • Tagged, case-sensitive, semantic
  • Can contain text, data, attributes

<project name="foo" type="java"> <file>Glob.java</file> <file>Blib.java</file> </project>

Software Design 7.23

Handling XML

 There are two ways to parse/read XML  SAX: Simple API for XML

  • Stream based, register callbacks that are

called when certain tags/data parsed

  • Can't move around, but can read and discard

 DOM: Document Object Model

  • Entire tree-like structure read into memory
  • Easy to move around

Software Design 7.24

Saving and restoring objects

Classes should implement Serializable, this is a tag interface, not necessary to implement a function (see Cloneable)

  • mark non-serializable fields as transient
  • platform specific objects like font sizes, these need to be reconstructed

rather than re-read

  • fields that aren’t needed when an object is deserialized
  • use ObjectOutputStream and ObjectInputStream, can

customize behavior using private?! functions below

private void writeObject(java.io.ObjectOutputStream out) throws IOException private void readObject(java.io.ObjectInputStream in) throws IOException, ClassNotFoundException;

  • also defaultReadObject() and defaultWriteObject()