Java Cookbook

Chapter 25. Introspection, or “A Class Named Class”

Introduction

The class java.lang.Class 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 machine.

The JVM Interpreter 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, and could study the JVM 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 ...