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 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 of dynamic loading . For example, even the simplest "Hello Java" program has to load the class file for your HelloJava class, the class file for its parent (usually java.lang.Object), the class for PrintStream (since you used System.out), the class for PrintStream's parent, and so on. To see this in action, try something like:

java -verbose HelloJava | more

To take another example, a browser can download an applet's bytecode file over the Internet and run it on your desktop. How does it load the class file into the running JVM? We discuss this little bit of Java magic in Recipe 25.3. The chapter ends with replacement versions of the JDK tools javap and AppletViewer the latter doing what a browser does, loading applets at runtime and a cross-reference tool that you can use to become a famous Java author by publishing your very own reference to the complete Java API.