1. 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

2. The java.lang.Class Class
ang/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.

3. 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”);

4. 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

5. 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

6. 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

7. 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

8. 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);

9. 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);

10. 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.

11. Problems With Reflection
Slow…..
Security Problems
If not security problems, than security risks

12. 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

13. 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

14. 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

15. 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

16. 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); }

17. 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.