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  ang/Class.html 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.

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