Chapter17

Chapter 17: Reflection, or “A Class Named Class”

17.1 Getting a Class Descriptor

Problem

You want to get a Class object from a class name or instance.

Solution

If the type name is known at compile time, you can get the class instance using the compiler keyword .class, which works on any type that is known at compile time, even the eight primitive types.

Otherwise, if you have an object (an instance of a class), you can call the java.lang.Object method getClass(), which returns the Class object for the object’s class.

17.2 Finding and Using Methods and Fields

Problem

You need to find arbitrary method or field names in arbitrary classes.

Solution

Use the reflection package java.lang.reflect.

17.4 Loading and Instantiating a Class Dynamically

Problem

You want to load classes dynamically, just like web servers load your servlets.

Solution

Use class.forName("ClassName"); and the class’s newInstance() method.

17.5 Constructing a Class from Scratch with a ClassLoader

Problem

You need to load a class from a nonstandard location and run its methods.

Solution

Examine the existing loaders such as java.net.URLClassLoader. If none is suitable, write and use your own ClassLoader.

17.6 Constructing a Class from Scratch with JavaCompiler

Problem

You’d rather construct a class dynamically by generating source code and compiling it.

Solution

Use the JavaCompiler from javax.tools.