Cracking the HTML Persistence Barrier
The applet or object that is continually being reloaded does not need to be entirely identical from page to
page. In particular, it may have different parameters, as expressed in varying <PARAM> tags under the
<APPLET> tag. Clever use of this freedom with parameters can enable massive changes in the classes that
are instantiated from one page to the next, and thus generate genuine alterations in the code itself. The fact
that init() and start() are called when a page is entered, and stop() is called when the page is left, can be
used to do some very useful things – the garbage collector is always there to get rid of discarded elements.
The program can vary in ways that would never be possible if it were not being continually re-activated.
Alternatively, boolean variables can be set or cleared in an abstract class, or in some other non-instantiated
region, which also determine ways in which the said applet or Active X object re-incarnates itself.
It is not necessary to load an applet or object into every page of a site in order to gain persistence. It is
enough to load the said applet or object into those pages in which access to the accumulated information is
necessary. Browser mechanisms, and the functionality of JAVA, make sure that the information is
maintained - for as long as memory permits - until it is needed, even through extended access to pages or
sites which know nothing about the applet or object. The applet simply sleeps, in an inactive state, until
reloaded through a relative call by some HTML page that desires access to its program code or memory.
Of course, if the applet is inactive, and if memory is needed for other JAVA programs, then the virtual
machine will eventually overwrite the inactive program. There is room on the web, therefore, for only a
limited number of programs that use this technique. Any particular intranet of course would have its own
separate limitation.
Persistence of memory throughout other pages or sites is not a security issue because an applet or Active X
object is resurrected – or made accessible to other programs - only if it resides at the same codebase, or at
some relative offset.
Let us now look at a second method for developing persistence between HTML pages. It uses abstract
classes (although, as we will see, that limitation can be bypassed). In nature, we observe objects like robins
and crows, but never things such as ‘birds.’ Robins and crows are birds, but birds as such do not exist; they
are an abstraction. Similarly, abstract classes in JAVA are a tool for dealing with things that are common to
various objects – in nature it would be wings, or beaks – they are not themselves instantiated. What we are
going to do, in general terms, is take an abstract class which is an abstraction of nothing, and inject code
into it – just as a virus inserts its DNA into a cell and takes over the machinery for its own purposes. An
infected cell turns out copies of the virus; the abstract class, in our case, simply chooses to ‘live forever.’ It
is all done, as you will see, with completely standard JAVA coding. The principle will then be extended,
through standard coding, to classes which are not abstract.
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Lane Friesen
lanelise@dowco.com
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