03G-1 Reflection In Java “Reflection is the ability of a program to manipulate as data something representing the state of the program during its own execution.” [Demers and Malenfant] For every loaded class, the JVM maintains an associated Class object The Class object “ reflects ” the class it represents The primitive Java types are also represented as Class objects Instances of the class Class store information about classes Class name Inheritance Interfaces implemented Methods, members, etc. Can look up instances: By name or From an object

03G-2 Accessing the Class object To get the Class object for an object mystery: Class c = mystery.getClass(); Or, using the class type: Class c = mysteryClass.class; Or, using the class name: Class c = Class.forName(“mysteryClass”); Getting the superclass of MysteryClass: Class s = c.getSuperclass(); Getting the class name: String s = c.getName(); Discovering the interfaces implemented by a class: Class[] interfaces = c.getInterfaces(); Discovering the fields of a class: Field[] fields = c.getFields(); Discovering the methods of a class: Method[] methods = c.getMethods();

03G-3 Example – exploring a type public static void showType(String className) throws ClassNotFoundException { Class thisClass = Class.forName(className); String flavor = thisClass.isInterface() ? "interface" : "class"; System.out.println(flavor + " " + className); Class parent = thisClass.getSuperclass(); if (parent != null) { System.out.println("extends " + parent.getName()); } Class[] interfaces = thisClass.getInterfaces(); for (int i=0; i<interfaces.length; ++i) { System.out.println("implements "+ interfaces[i].getName()); }

03G-4 The Output class java.lang.Object class java.util.HashMap extends java.util.AbstractMap implements java.util.Map implements java.lang.Cloneable implements java.io.Serializable class Point extends java.lang.Object

03G-5 Displaying Methods static void showMethods(Object o) { Class c = o.getClass(); Method[] theMethods = c.getMethods(); for (int i = 0; i < theMethods.length; i++) { String methodString = theMethods[i].getName(); System.out.println("Name: " + methodString); System.out.println(" Return Type: " + theMethods[i].getReturnType().getName()); Class[] parameterTypes = theMethods[i].getParameterTypes(); System.out.print(" Parameter Types:"); for (int k = 0; k < parameterTypes.length; k ++) { System.out.print(" " + parameterTypes[k].getName()); } System.out.println(); }

03G-6Output Output for a call of the form: Polygon p = new Polygon(); showMethods(p);. Name: equals Return Type: boolean Parameter Types: java.lang.Object Name: getClass Return Type: java.lang.Class Parameter Types: Name: intersects Return Type: boolean Parameter Types: double double double double

03G-7 Main Java reflection Classes u Class ( java.lang.Class ) l Instances of the class Class represent classes and interfaces in a running Java application, every object is represented by a Class object. u Package java.lang.reflect: l Member ( java.lang.reflect.Member ) An Interface that reflects identifying information about a single member (a field or a method) or a constructor. l Method ( java.lang.reflect.Method ) Implements Member Interface provides information about, and access to, a single method on a class or interface. Represents instance methods and class methods ( static ) l Field Implements Member Interface provides information about, and dynamic access to, a single field ( also for static fields ) Provides access and modification ( set, get ) methods.

03G-8 Main Java reflection Classes l Constructor provides information about, and access to, a single constructor for a class. l Package Package objects contain version information about the implementation and specification of a Java package l Modifier provides static methods and constants to decode class and member access modifiers. The sets of modifiers are represented as integers with distinct bit positions representing different modifiers

03G-9 Reflection API Interface - example class Class{ static Class forName(String name); Object newInstance(); Field getField(String name); Method getMethod(String name, Class[] paramTypes); //other methods: getName(), getInterfaces(), // getSuperclass(), getModifiers(), // getFields(), getMethods(), … } class Field extends AccessibleObject { Object get(Object obj); void set(Object obj, Object val); //other methods: getType(), getDeclaringClass(),... }

03G-10 Exploring the Instantiation Hierarchy public static void traverse(Object o){ for (int n = 0; ; o = o.getClass()) { System.out.println("L"+ ++n + ": " + o + ".getClass() = " + o.getClass()); if (o == o.getClass()) break; } public static void main(String[] args){ traverse(new Integer(3)); } L1: 3.getClass() = class java.lang.Integer L2: class java.lang.Integer.getClass() = class java.lang.Class L3: class java.lang.Class.getClass() = class java.lang.Class

03G-11 Using Arrays u Reflection can be used to create and manipulate arrays whose size and component type are not known until runtime public static void testArray(){ Class cls = String.class; int i=10; Object arr = Array.newInstance(cls, i); Array.set(arr, 5, "this is a test"); String s = (String)Array.get(arr, 5); System.out.println(s); }

03G-12 A Java Reflection Example u Illustrates Four Issues: l Runtime type Information (RTTI) l Introspection l Invoking Method Objects l Dynamic Instantiation

03G-13 Back to our Employee Example … Employee number level print() MonthlyEmployee print() HourlyEmployee print() Only partial view of the classes … public final class MonthlyEmployee extends Employee { public void print() { System.out.println("I’m a Monthly Employee"); } public final class HourlyEmployee extends Employee { public void print() { System.out.println("I’m a Hourly Employee"); }

03G-14 Reflection and Dynamic Binding What is the output of the following test code: Employee e; e = new MonthlyEmployee(); Class c = e.getClass(); System.out.println("class of e = " + c.getName()); e = new HourlyEmployee(); c = e.getClass(); System.out.println("class of e = " + c.getName()); class of e = MonthlyEmployee class of e = HourlyEmployee

03G-15SuperClass What is the output of the following test code: c = c.getSuperclass(); System.out.println("base class of e = " + c.getName()); c = c.getSuperclass(); System.out.println("base of base class of e = " + c.getName()); base class of e = Employee base of base class of e = java.lang.Object

03G-16 Getting and setting fields u Non public fields are not printed! getDeclaredFields returns all fields declared by the class, but excludes inherited ones Field fields[] = c.getFields(); for(int i = 0; i < fields.length; i++) { System.out.print(fields[i].getName() + "= "); System.out.println(fields[i].getInt(e)); } e is an instance of Employee or its subtype The output produced: number= 111 level= 12 Field f = c.getField("level"); f.setInt(e,f.getInt(e)+1);

03G-17 Examining Modifiers What is the output of the following test code: int m = c.getModifiers(); if (Modifier.isPublic(m)) System.out.println("public"); if (Modifier.isAbstract(m)) System.out.println("abstract"); if (Modifier.isFinal(m)) System.out.println("final"); public final

03G-18 Invoking Method Objects We can ask a method object to invoke the method it represents we must provide it with the implicit and explicit arguments Employee e = new HourlyEmployee(); Class c = e.getClass(); Method m = c.getMethod("print", null); m.invoke(e, null); the output produced: I’m a Hourly Employee

03G-19 Dynamic Instantiation The universal printer gets the employee type and invokes the print method. class UniversalPrinter { public void print(String empType) { Class c = Class.forName(empType); Employee emp = (Employee ) c.newInstance(); emp.print(); } - for calling other constructors (with arguments), use the Constructor class - Constructor c =... - Object newObject = c.newInstance( Object[] initArguments ) What is the output of the following code? UniversalPrinter p = new UniversalPrinter(); String empType; empType = "HourlyEmployee"; p.print(empType); empType = "MonthlyEmployee"; p.print(empType);

03G-20 Implementing Java Reflection u At runtime, JVM loads class files and creates objects representing the classes. l Object representing a class has a name (string field), list of fields (each of type Field), list of methods… class Field { String name; Class type; Class clazz; int offset; Object get(Object obj) { if (clazz.isInstance(obj)) { f = ((char*)obj) + offset; return (type.primitive = TRUE ? wrap(f) : (Object)f); } } } class Class { String name; Field[] fields; Method[] methods; boolean primitive; bool isInstance... Object newInstance.. }

03G-21 Reflection - What ’ s missing u Reflection is introspection only l Cant add / modify fields (structure), methods (behavior) u Implementation is not available l Program logic is not reflected u Major performance impact l Much slower then doing the same operations directly … u Complex code

03G-22 Java Serialization u the process of converting objects into a linear stream of bytes l Depends on reflection public class PersistentTime implements Serializable{ private Date time; public PersistentTime(){ time = Calendar.getInstance().getTime(); } public Date getTime(){return time;} } String filename = "time.ser"; PersistentTime time = new PersistentTime(); FileOutputStream fos = null; ObjectOutputStream out = null; fos = new FileOutputStream(filename); out = new ObjectOutputStream(fos); out.writeObject(time); out.close();