CSC/ECE 517 Fall 2010/ch3 1c AE

Definition

Reflection is the integral quality of a program to dynamically manipulate its own state. More specifically, the program can infer and modify itself by making use of the representation of its own state. The state of a program can be its implementation aspects like syntax, semantics, structure, etc. The program acts through its self-accessible state and takes itself as its domain to be acted upon. Programming languages that possess this quality are said to be reflective.

Most object-oriented languages support some form of reflection. Smalltalk, Ruby, and Python in particular have very powerful and flexible reflection mechanisms. Java also supports reflection, but not in as flexible and dynamic fashion as the others. C++ does not support reflection as we've defined it here, but it does supported a limited form of run-time type information that allows a program to determine the type of an object at run-time. Eiffel also has support for a limited form of reflection, although it is much improved in the most recent versions of Eiffel, including the ability to determine the features contained in an object.

Consider a simple example of Reflection in Java below :

public class HelloWorld {
  public void printName() {
     System.out.println(this.getClass().getName());
  }
}

The line

(new HelloWorld()).printName();

sends the string 'HelloWorld' to the standard output. Let obj be an object of HelloWorld or one of its subclasses. When the program reaches the line,

obj.printName();

it sends the name of the class to the standard output.

This simple example denotes an effective feature. The printName method looks at the object obj for its class (this.getClass()). It decides what to print by delegating to the object's class in runtime.(i.e.,) the printName method behaves differently for each subclass of HelloWorld. The method is so flexible that it dynamically adapts itself to the class or subclass that inherits it.

Reflection being an effective language feature enables metaprogramming. A reflective system would possess the feature of allowing programmers to design, create make use of the objects to act dynamically based on an existing part of the running system. Thus, reflection can also be defined as a feature where a programming language behaves like its own metalanguage. It contains metadata and associated operations to manipulate it in runtime.

Pros and cons of reflection

Simplicity: Generated code is generally more readable and debuggable for developers.	Maintenance: With passive code generation, changing the objects to be loaded may require the loading class to be updated or regenerated. If the classes are regenerated, then customizations may be lost.
Compile time errors: Reflexive procedures fail more often at runtime than during compilation. For instance, changing the object to be loaded will probably cause the generated loading class to throw a compilation error, but the reflexive procedure won't see any difference until the class is encountered during runtime.	Reflection involves dynamic resolving , certain Java virtual machine optimizations can not be performed. Consequently, reflective operations have slower performance than their non-reflective counterparts, and should be avoided in sections of code which are called frequently in performance-sensitive applications.
Compile time errors: Reflexive procedures fail more often at runtime than during compilation. For instance, changing the object to be loaded will probably cause the generated loading class to throw a compilation error, but the reflexive procedure won't see any difference until the class is encountered during runtime.	Reflection requires a runtime permission which may not be present when running under a security manager. This is in an important consideration for code which has to run in a restricted security context, such as in an Applet. Exposure of Internals.
Without it, our solutions are messy, cumbersome, and fragile. Consider the following scenarios:	Since reflection allows code to perform operations that would be illegal in non-reflective code, such as accessing private fields and methods, the use of reflection can result in unexpected side-effects, which may render code dysfunctional and may destroy portability. Reflective code breaks abstractions and therefore may change behavior with upgrades of the platform.
More flexible	Performance Overhead & Security Restrictions

Reflective mechanisms

Types of reflection like introspection, intercession, structural & behavioral -- Reflection at Different Stages of the Program Lifecycle

Categorizing Reflection We identify two main criteria to categorize reflective systems. These criteria are when reflection takes place and what may be reflected. If we take what may be reflected as a criterion, we can distinguish: · Introspection: The system structure can be accessed but not modified. If we take Java as an example, with its java.lang.reflect package, we can get information about classes, objets, methods and fields at runtime [4]. · Structural Reflection: The system structure can be dynamically modified. An example of this kind of reflection is the addition of object’s fields. · Computational (Behavioral) Reflection: The system semantics (behavior) can be modified. In the standard Java API v.1.3, the class java.lang.reflect.Proxy has been added [5]; it can be used to modify the dispatching method mechanism, being handled by a proxy object. · A reflective programming language can be capable of changing itself, i.e. changing its own lexical or syntactical specification; that is what we call Linguistic Reflection. As an example, with OpenJava reflective language, the own language can be enhanced to be adapted to specific design patterns [6].

Reflective languages

Languages like LISP, CLOS, etc

Reflection Packages in OO-languages (java ‘s reflex package)

Java is so well crafted and its reflection API so carefully constrained that security is controlled simply. By learning when to use reflection and when not to, we will avoid unnecessarily complex code that can often be the result of amateurish use of reflection. In addition, you will learn to evaluate the performance of your designs, thereby ensuring the resulting code satisfies its performance requirements.

This introduction describes reflection, but scarcely reveals its value in this limited space. In Java, reflection enables to discover information about the loaded classes specifically about the Fields,Methods and constructors,Generics information,Metadata annotations.The forcoming section will cover:

Introspective Feature of Reflective Package in Java

Basically reflection in programming is an introspect upon itself and manipulate internal properties of the program. It also creates a way to obtain properties of Java components as they are dynamically loaded. Using the concept we can have all the information related with a class i.e. its public,private methods,fields,constructors and can create new objects ,arrays and invoke methods. We shall see these methods below and for these to work Java needs reflect package. Hence Java has reflection through package features.

 Listing methods in a class (Class fundamentals)
 import java.lang.reflect.*;
 public class ClassDisplay{
       public static void main(String args[]){
            try{
                Class c=Class.forName("java.util.Stack");
                Method m[]=c.getDeclaredMethods();
                for(int i=0;i<m.length ;i++)
                    System.out.println(m[i].toString());
               }catch(Throwable e){System.err.println(e); }
       }
 }

  Output displays the return object type and parameter object type together with the method name
  public synchronized java.lang.Object java.util.Stack.pop()
  public java.lang.Object java.util.Stack.push(java.lang.Object)
  public boolean java.util.Stack.empty()
  public synchronized java.lang.Object java.util.Stack.peek()
  public synchronized int java.util.Stack.search(java.lang.Object)

Methods reflection

   import java.lang.reflect.*;
   public class MethodDisplay{
       private int f1(Object p,int x) throws NullPointerException {
           if(p==null){
                 throw new NullPointerException();}
             return x;
       }  
        
       public static void main(String args[]){
           try{
                  Class cls= Class.forName("MethodDisplay");
                  Method meth[]= cls.getDeclaredMethods();
                  for(int i=0;i<meth.length;i++)
                     {
                         Method m=meth[i];
                         System.out.println("Method Name: "+m.getName());
                         System.out.println("Method Declared class : "+m.getDeclaringClass());
                         Class prec[]=m.getParameterTypes();
                         System.out.println("Method Exception Types : "+m.getExceptionTypes());
                         System.out.println("Method Return Type : "+ m.getReturnType());
                     }
              }catch(Throwable e){System.err.println(e);}
        }   
    }        
       
  Output for above code which displays the methods declared :
  Method Name: f1
  Method Declared class : class MethodDisplay
  Method Exception Types : [Ljava.lang.Class;@3e25a5
  Method Return Type : int
  Method Name: main
  Method Declared class : class MethodDisplay
  Method Exception Types : [Ljava.lang.Class;@19821f
  Method Return Type : void         

Similarly we can get information about the fields declared in a class with the help of methods like getField(String name),getFields(),getDeclaredField(String name),getDeclaredFields().The same technique used for Constructor infomration retrieval.

Intercessive Feature of Reflective Package in Java

Intercessive is process of invoking dynamically on a class at run time. The previous section just talked about the ways to get information about a class. It enables to use the metaobjects to their instances in run time environment. E.g. Method.invoke(Object o, Object… args) With the Java reflection API, we can interrogate an object to get all sorts of information about its class.Lets see the technique how Java helps to invoke a method defined in a class at runtime using reflection API.

Method Invocation

 import java.lang.reflect.*;
 public class MethodInvoke{
       public int add(int a,int b){
             return a+b;
       }
       
       public static void main(String args[]){
            try{
                  Class cls= Class.forName("MethodInvoke");
                  Class partypes[]= new Class[2];
                  partypes[0]= Integer.TYPE;
                  partypes[1]=Integer.TYPE;
                  Method meth=cls.getDeclaredMethod("add",partypes);
                  MethodInvoke MI = new MethodInvoke();
                  Object arglist[]=new Object [2];
                  arglist[0]=new Integer(37);
                  arglist[1]=new Integer(47);
                  Object retobj = meth.invoke(MI, arglist);
                  Integer retval = (Integer)retobj ;
                  System.out.println("Value :"+ retval);
            }catch (Exception e){}
       }
 }

First we get the method declared and then we invoke the method by calling Method.invoke(*) . This accepts a class object where the method is declared along with the parameter list defined in an array.

Dynamic array Manipulation

 import java.lang.reflect.*;
 public class ArrayManipulation {
     public static void main(String args[]){
   	  try{
   		  Class cls= Class.forName("java.lang.String");
       	  Object arr= Array.newInstance(cls, 10);
       	  Array.set(arr,5,"this is a reflection test");
       	  String s =(String)Array.get(arr,5);
       	  System.out.println("Returned String :" + s);   		  
   	     }catch (Exception e){}    	     	  
     }
 }

References

http://www.springerlink.com/content/6l63883r9224q186/ http://www.cs.indiana.edu/~jsobel/rop.html http://publications.csail.mit.edu/lcs/pubs/pdf/MIT-LCS-TR-272.pdf http://www.di.uniovi.es/ooosws2001/papers/ortin_final.pdf http://download.oracle.com/javase/1.4.2/docs/api/java/lang/reflect/package-summary.html http://scg.unibe.ch/archive/papers/Duca06dMOOSEMODELS2006.pdf file:///C:/Users/Summer/Downloads/10.1.1.158.2012.pdf - good link .. http://www.ub.uni-konstanz.de/kops/volltexte/2002/803/pdf/DissertationEgbertAlthammer.pdf http://www.laputan.org/reflection/ooffrmla.html