Reflection and Introspection

REFLECTION AND INTROSPECTION

Muhammad Adil Raja

Roaming Researchers, Inc.

cbna

April 24, 2015

Reflection and Introspection

OUTLINE I

1 INTRODUCTION

2 REFERENCES

Reflection and Introspection

Introduction

INTRODUCTION

Reflection and Introspection imply a program that candynamically learn about or modify itself.Learn the class of objects.Learn what methods an object responds to.Creating instances from class objects.Dynamically adding new classes.Dynamically adding new methods to existing classes.Not all abilities are found in all languages.Some languages do not have any support for reflection orintrospection.

Reflection and Introspection

Introduction

CLASS OBJECTS

In most languages, introspection begins with an object thatrepresents the class.Typically this object has class Class.

Reflection and Introspection

Introduction

THINGS YOU CAN DO WITH A CLASS VARIABLE

One operation that can be performed with a value of type classis to get its parent class

EXAMPLE

Class parentClass = aClass . getSuperc lass ( ) ; / / Java

Another operation is to get the list of all subclasses for a class.

aSet <− aClass subclasses " Sma l l t a l k "

You can also get the name of a class as a string.

char ∗ name = t y p e i n f o ( aVar iab le ) . name ( ) ; / / C++

Reflection and Introspection

Introduction

GETTING THE CLASS FROM A STRING

In some languages, you can take the name of a class (a string)and from this get the class object.

EXAMPLE

Class aClass = Class . forName ( " classname " ) ; / / Java

The class does not even have to be loaded.The class will automatically be loaded for you.

Reflection and Introspection

Introduction

TESTING IF AN OBJECT IS AN INSTANCE OF A CLASS

We have seen already that most languages provide a way totest if an object is an instance of a class.

EXAMPLE

i f Member ( aVar iable , Ch i ld ) thenaChi ld = Chi ld ( aVar iab le ) (∗ Object Pascal ∗)

This is needed for downcasting.However, if you find yourself using this a lot, then youaren’t thinking in an object-oriented way, since often suchsituations can be better written using polymorphism.

Reflection and Introspection

Introduction

METHODS AS OBJECTS

The next level of complexity is treating a method as anobject.First step is to get a collection of methods from a class

EXAMPLE

Method [ ] methods = aClass . getDeclaredMethods ( ) ; / / Java

aSet <− aClass s e l e c t o r s . / / Sma l l t a l k

Reflection and Introspection

Introduction

USING A METHOD

EXAMPLE

System . out . p r i n t l n ( methods [ 0 ] . getName ( ) ) ;Class c = methods [ 0 ] . getReturnType ( ) ;

Reflection and Introspection

Introduction

DYNAMIC EXECUTION

You can also invoke the method, passing it the receiver and anarray of arguments:

EXAMPLE

Class sc = S t r i n g . class ;Class [ ] paramTypes = new Class [ 1 ] ;paramTypes [ 0 ] = sc ;t ry {

Method mt = sc . getMethod ( " concat " , paramTypes ) ;Object mtArgs [ ] = { " xyz " } ;Object r e s u l t = mt . invoke ( " abc " , mtArgs ) ;System . out . p r i n t l n ( " r e s u l t i s " + r e s u l t ) ;

} catch ( Except ion e ) {System . out . p r i n t l n ( " Except ion " + e ) ;

}

Here we dynamically look up a method, based both on nameand type signature, then create an array of arguments, thenexecute the method.

Reflection and Introspection

Introduction

CLASS LOADING

Java has a class named ClassLoader that allows you todefine a class from an array of bytes:

EXAMPLE

class SimpleClassLoader extends ClassLoader {public Class getClass ( S t r i n g name) {

Class theClass = nul l ;t ry {

F i l e f = new F i l e (name ) ;InputStream i s = new Fi le InputSt ream ( f ) ;i n t bu fs i ze = ( i n t ) f . l eng th ( ) ;byte buf [ ] = new byte [ bu fs i ze ] ;i s . read ( buf , 0 , bu fs i ze ) ;i s . c lose ( ) ;theClass = def ineClass ( null , buf , 0 , buf . leng th ) ;

} catch ( Except ion e ) {System . e r r . p r i n t l n ( " E r ro r dur ing load " + e ) ;System . e x i t ( 1 ) ;

}return theClass ;

}}

Once you have a class, you can create instances, or executemethods.

Reflection and Introspection

Introduction

METACLASSES – STRANGE LOOPS

You can get into some interesting places by asking theright questions.

A class is an object.All objects are instances of a class.What is the class of a class?

In Java this path circles back on itself very quickly; Class isan intance of itself.In Smalltalk, the path goes on a bit longer before it circlesback on itself.

Reflection and Introspection

Introduction

RATIONALE FOR METACLASSES

Remember that by introducing new classes that representclasses, Smalltalk was able to solve the following problemHow do you give unique behavior to just one instance of aclass?(For example, the behavior to initialize newly createdinstances of a class).The answer was, you don’t.You add a new child class that defines the behavior youwant, and put this between the object and the true parent.

Reflection and Introspection

Introduction

RELATIONSHIP TO COMPONENTS

Relection and Introspection have taken on increasingimportance as the field moves from object-basedcomputing to component-based computing (COM, Corba,JavaBeans, etc).This is because components must often be dynamicallyloaded and executed.And reflection is the underlying mechanism that permitsthis.

Reflection and Introspection

References

REFERENCES

Images and content for developing these slides have beentaken from the follwoing book with the permission of theauthor.An Introduction to Object Oriented Programming, TimothyA. Budd.This presentation is developed with Beamer:

JuanLesPins, beetle.