10/18/08 Matt Swatzell

What is Reflection?

Some Definitions…. Reflection is the process by which a

program can modify its own behavior. A program that can analyze and modify

its own parameters at runtime is considered reflective.

Tools for manipulating code at runtime. In other words… A way for a

program/application to learn about itself, change itself, and perhaps even add to itself.

Some Examples

Reflection is used for many things: IDE’s / software analysis tools

Debuggers Server Side Code i.e. ASP.NET Plug-ins Frameworks i.e. Java Beans Imitating function pointers in Java

Code Demo

Code Demo How would you go about doing this?

The java.lang.Class Class http://java.sun.com/j2se/1.5.0/docs/a

pi/java/lang/Class.html JVM always maintains Runtime

Type Definition for all objects. Determines correct methods to call

Class is used for this purpose. Unique Class object for each type.

java.lang.Class cont.

3 ways to get the Class class of an object (any Object).getClass(); For objects that

already exist Scanner scan = new Scanner(System.in); Class c = scan.getClass();

(Any type).class When you only know the type Works for primitives too Int.class; Scanner.class;

Class.forName(“fully qualified path”); Know the name of class you want Class c1 = Class.forName(“java.util.Scanner”);

Useful methods in Class

Public string Name(); returns name of class

Constructor[] getConstructors() an Array of constructors (more on these later)

Method[] getMethods() an Array of methods

Field[] getFields() an Array of fields

Class[] getInterfaces() an Array of Interfaces

<T> newInstance() An instance of the class, only works in certain

cases

java.lang.reflect.Constructor

Represents a constructor for a class object Important Methods

Class getDeclaringClass(); Int getModifiers();

Why int? Class[] getParameterTypes(); Object newInstance(Object[] args);

Creates a new class if parameters are needed

Java.lang.reflect.Method

Represents a method in a class Useful Methods in Method

Class[] getExceptionTypes(); Int getModifiers(); String getName(); Class[] getParameterTypes(); Class getReturnType(); Invoke(Object o, Object[] args);

Exceptions, Exceptions, Exceptions

Java.lang.reflect.Field

Represents a Field within a Class get(type)(Object obj);

Returns the value of the field in Object obj set(Type)(Object o)

Sets the value

Two more java.lang.reflector.*’s

Java.lang.reflect.AccessibleObject Base for Constructor, method, field

Allows you to modify accessibility at runtime. isAccessible(); setAccessible(boolean flag);

Breaks with SecurityManagers java.lang.reflect.Modifier

Takes int and returns facts about modifers on it isPrivate(int i); toString(int i);

Can I do it with Arrays?

Java.lang.reflect.Array get(type)(Object Array, int index); set(Type)(Object array, int Index, type

value); newInstance(Class type, int length); New Instance(Class type, int[]

dimensions);

Function Pointers (in a way…)

Cannot pass location of method in java In C/C++ this is possible

Passing functions as parameters Void doAwesomeness(function awesome);

Reflection provides a handy workaround if you need this functionality Simply pass method object and call invoke

Before you get too excited… It not in Java for a reason Inheritance / Polymorphism usually safer.

Problems With Reflection

Slow….. Security Problems If not security problems, than

security risks

A brief intro to ClassLoader Q: So, other than object analyzers,

generic toStrings, and function pointers what is Reflection good for?

A: Loading classes dynamincally Sometimes you will not know all the

classes that will be needed. Used by your application

Sometimes the user will want to add their own

Think of VS, Eclipse, some games

ClassLoader cont.

ClassLoader is responsible for loading classes

Every class has reference to ClassLoader the created it.

Only object w/o class loader are generic Objects in arrays.

Primitives have no class loader The original ClassLoader, (called the

bootstrap class loader) has no parent class loader all others, as objects, do

Boot strap Class Loader

Where does JVM look for classes? Some are included with application

Custom Defined Ones Are all of them? JVM looks in ClassPath for classes, then

in program / application What if you want to look elsewhere?

On your own machine, you could add to ClassPath

Can’t do this on end-user’s

Other Options

Only one really… Create a new ClassLoader that

extends abstract ClassLoader. Override loadClass

Get location of file Get byte array of the class data Call define class and return the result You now have a class of a type that was

unknown until runtime Problems with inheritance

An example custom Class Loader Public class customLoader extends ClassLoader {

public class findClass(String name) { Byte[] b = openFileAndGetByteArray(name); Return defineClass(name,b,0,b.length);

}

}

Citations

Horstmann , Cay S., and Gary Cornell. Core Java 2 . Volume 1. Santa Clara, CA: Sun Microsystems, 2005.

"The Reflection API." java.sun.com. Sun Microsystems, Web. 19 Nov 2009.