Java Reflection: Tools for Dynamic Code Manipulation

1. 10/18/08 Matt Swatzell Reflection In Java

2. 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.

3. 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

4. Code Demo • Code Demo • How would you go about doing this?

5. The java.lang.Class Class • http://java.sun.com/j2se/1.5.0/docs/api/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.

6. 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”);

7. 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

8. 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

9. 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

10. 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

11. 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(inti); • toString(inti);

12. 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);

13. 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.

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

15. 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

16. 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

17. 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

18. 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

19. 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); • } • }

20. 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.