Learning Java

Reflection

In this section, we’ll take a look at the Java reflection API, supported by the classes in the java.lang.reflect package. As its name suggests, reflection is the ability for a class or object to examine itself. Reflection lets Java code look at an object (more precisely, the class of the object) and determine its structure. Within the limits imposed by the security manager, you can find out what constructors, methods, and fields a class has, as well as their attributes. You can even change the value of fields, dynamically invoke methods, and construct new objects, much as if Java had primitive pointers to variables and methods. And you can do all of this on objects that your code has never even seen before.

We don’t have room here to fully cover the reflection API. As you might expect, the reflect package is complex and rich in details. But reflection has been designed so that you can do a lot with relatively little effort; 20 percent of the effort will give you 80 percent of the fun.

The reflection API is used by JavaBeans to determine the capabilities of objects at runtime. It’s also used at a lower level by object serialization to tear apart and build objects for transport over streams or into persistent storage. Obviously, the power to pick apart objects and see their internals must be zealously guarded by the security manager. Your code is not allowed to do anything with the reflection API that it couldn’t do with static (ordinary, compiled) Java code. In short, ...