Re: Why is dynamic polymorphism so useful?

Owen Jacobson <>
Mon, 31 Dec 2007 13:47:26 -0800 (PST)
On Dec 31, 11:05 am, wrote:

On Dec 28, 8:56 pm, Lew <> wrote: wrote:

I also realize, that with polymorphism we can run a program and while
program is already running, we can introduce a totally new class ( let=

us call this new class N_C ) and thanx to polymorphism this program
may operate on instances of N_C without the need to recompile or even
without the need to restart this program.

But how to write a code for this program in such flexible way that
program will KNOW that new class is present and operate on it is
beyond me.

Java has basically three ways to accomplish this: classloaders, reflecti=

on and

the debugger API.

And one method that is relatively "nice", using a fairly simple subset
of reflection, is serialization.

Say you have a plugin architecture with, say, pluggable codecs. You
employ the strategy pattern and create a Codec interface that has
encode() and decode() methods, among others, and supply some
implementations. But you also provide code that runs at startup or
when a user picks "Scan for plugins" from a menu that looks in a /
plugins directory someplace looking for ".codec" files, which are
actually serialized instances of Codec objects. It reads these one by
one with an ObjectInputStream and casts each to Codec; if a
ClassCastException is thrown, it's ignored and so is the object that
was loaded; same if any of the various serialization exceptions or
IOExceptions are thrown. NoClassDefFound? Discard.
ObjectStreamException? Discard. The surviving objects are added to the
collection of Codec objects the program maintains. Perhaps there's an
initialize() method on these, and the ones that are newly added (using
a Set allows detecting which are really new) get this run, adding
particular "Save As" options that go through their respective encode()
methods. Perhaps the program tries to decode files opened with "Open"
by invoking each one's "decode()" in turn until one of them returns
something other than null (or doesn't throw some exception); the new
ones now get a crack at decoding any opened file.

This requires, on the programmer's part, no nastier reflection code
than code to deserialize objects (and, in some SDK somewhere, to
serialize them so plugins can be made). A plugin's installation
requires only that some .class files (perhaps wrapped in a .jar) go
into the application's classpath and a .codec file go in the
application's plugins directory. Likely that plugins directory is on
the app's class path and contains the plugin classes as well.

The Java Beans specification uses the extension ".ser" for this and
provides some hooks in the .jar format and java.beans API for finding
and loading serialized bean instances. See sections 10.3 and 11.8 of
the spec:

and the related API docs:

and the Java-Bean MANIFEST.MF entry described in:


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