Java in a Nutshell, 9th Edition

Chapter 13. Classloading, Reflection, and Method Handles

In Chapter 4, we met Java’s Class objects, a way of representing a live type in a running Java process. In this chapter, we will build on this foundation to discuss how the Java environment loads and makes new types available at runtime.

In the second half of the chapter, we’ll introduce Java’s introspection capabilities, both the original Reflection API and the newer Method Handles capabilities.

These capabilities are important because they provide the dynamic features of the Java platform. Remember back in Chapter 1 we said that although Java source code looks like C/C++ and it seems to be a compiled language, at runtime it is a different story?

This chapter will provide an introduction to how this is achieved—and why Java is capable of being as flexible and dynamic as Python or JavaScript or Ruby. Let’s get started by taking a closer look at class files and Class objects.

Class Files, Class Objects, and Metadata

Class files, as we saw in Chapter 1, are the result of compiling Java source files (or, potentially, other languages) into the intermediate form used by the JVM. These are binary files that are not designed to be human-readable. At runtime these files are represented by class objects that contain metadata about the Java type defined in the file.

Examples of Class Objects

You can obtain a class object in Java in several ways. The simplest is:

Class<?> myClass = getClass();

This returns the class object of the ...