Reflection

Reflection
in Java

Computer Science Department

bernstdh@jmu.edu

Introduction

Reflection Defined:
- An object looking at its internal details at runtime (a.k.a., introspection)
Reflection in Java:
- Because Java bytecode is interpreted at runtime, reflection is fairly straightforward

Important Classes

In java.lang:
- The Class class
In java.lang.reflect:
- The Field class
- The Method class
- The Modifier class

An Important Method

An Example

Using Reflection on a String

javaexamples/reflection/Example1.java

Using Reflection with Arrays

Review of Arrays:
- An array contains a fixed and known number of homogeneous elements
- An array (normally) is a contiguous block of memory
Arrays in Java:
- An array is an object
- The elements are (references of) Object objects
- Since all classes extend Object, an array can contain references to objects of different classes

Using Reflection with Arrays (cont.)

Using Reflection with an Array of References to Objects in Different Classes

javaexamples/reflection/Example2.java

Using Reflection with Arrays (cont.)

An Observation:
- You will see many examples of this kind, and many people advocating its use
The State of the Practice:
- This is an inappropriate practice

Using Reflection for Serialization

An Observation:
- At runtime, an object can look at its internal structure in order to "freeze" it
A Cautionary Note:
- As we noted in our discussion of serialization, this can be more complicated than it seems

Using Reflection for Serialization

Serializing a Simple Class in XML

javaexamples/reflection/Customer.java

javaexamples/reflection/XMLWriter.java

Some Other Uses of Reflection

There's Always More to Learn