and the reflection package
java.lang.reflect, provide a number of
mechanisms for gathering information from the Java Virtual Machine.
Known collectively as introspection or
reflection , these facilities allow you to load classes on the fly,
to find methods and fields in classes, to generate listings of them, and
to invoke methods on dynamically loaded classes. There is even a
mechanism to let you construct a class from scratch (well, actually,
from an array of bytes) while your program is running. This is about as
close as Java lets you get to the magic, secret internals of the Java
The JVM implementation is a large program, normally written in C and/or C++, that implements the Java Virtual Machine abstraction. You can get the source for Sun's and other JVMs via the Internet, which you could study for months. Here we concentrate on just a few aspects, and only from the point of view of a programmer using the JVM's facilities, not how it works internally; that is an implementation detail that varies from one vendor's JVM to another.
I'll start with loading an existing class dynamically,
move on to listing the fields and methods of a class and invoking
methods, and end by creating a class on the fly using a
ClassLoader. One of the more interesting aspects of Java, and one that accounts for both its flexibility (applets, servlets) and part of its perceived speed problem, is the notion ...