03G-1

Reflection In JavaReflection 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 objectAccessing the Class objectTo 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 typeExample – exploring a typepublic 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 OutputThe 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 MethodsDisplaying Methodsstatic 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-6

OutputOutputOutput for a call of the form:

Polygon p = new Polygon();showMethods(p);

.

. Name: equals Return Type: boolean Parameter Types: java.lang.ObjectName: getClass Return Type: java.lang.Class Parameter Types:Name: intersects Return Type: boolean Parameter Types: double double double double

03G-7

Main Java reflection ClassesMain Java reflection Classes Class ( java.lang.Class )

Instances of the class Class represent classes and interfaces in a running Java application, every object is represented by a Class object.

Package java.lang.reflect: Member ( java.lang.reflect.Member )

An Interface that reflects identifying information about a single member (a field or a method) or a constructor.

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 )

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 ClassesMain Java reflection Classes Constructor

provides information about, and access to, a single constructor for a class.

PackagePackage objects contain version information about the

implementation and specification of a Java package 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 - exampleReflection API Interface - exampleclass 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 HierarchyExploring the Instantiation Hierarchypublic 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.IntegerL2: class java.lang.Integer.getClass() = class java.lang.ClassL3: class java.lang.Class.getClass() = class java.lang.Class

03G-11

Using ArraysUsing Arrays 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 ExampleA Java Reflection Example Illustrates Four Issues:

Runtime type Information (RTTI) Introspection Invoking Method Objects Dynamic Instantiation

03G-13

Back to our Employee ExampleBack to our Employee Example……Employee

number

level

print()

MonthlyEmployee

print()

HourlyEmployee

print()

Only partial view of theclasses…

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 BindingReflection and Dynamic BindingWhat 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 = MonthlyEmployeeclass of e = HourlyEmployee

03G-15

SuperClassSuperClassWhat 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 = Employeebase of base class of e = java.lang.Object

03G-16

Getting and setting fieldsGetting and setting fields

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= 111level= 12

Field f = c.getField("level");f.setInt(e,f.getInt(e)+1);

03G-17

Examining ModifiersExamining ModifiersWhat 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 ObjectsInvoking 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 InstantiationDynamic InstantiationThe 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 ReflectionImplementing Java Reflection At runtime, JVM loads class files and creates

objects representing the classes. 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 - WhatReflection - What’’s missings missing Reflection is introspection only

Cant add / modify fields (structure), methods (behavior) Implementation is not available

Program logic is not reflected Major performance impact

Much slower then doing the same operations directly… Complex code

03G-22

Java SerializationJava Serialization the process of converting objects into a linear

stream of bytes 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();