CSC/ECE 517 Fall 2010/ch3 1c AE
Reflective Language Features vs. Reflective Packages
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
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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 in OO like JAVA
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:
Reflection basics: 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)
Using methods reflectively
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
It also enables to use these metaobjects to their instances in run time environment. E.g. Method.invoke(Object o, Object… args) With the Java reflection API, you can interrogate an object to get all sorts of information about its class.
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