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.
Another important concept that is closely associated with reflection is reification. Reification is the process of packaging the information about the environment suitable so that it can be manipulated(by reflection).
Lets look into the pros and cons of reflection and then the languages that sport and support reflection.
Pros and cons of reflection
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Reflective mechanisms
It is a feature that is made available in a language that facilitatates either reification or reflection or both.
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 and Ruby
Reflection in OO-languages
Introspective Feature
Class Based Reflection
Java
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:
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){}
}
}
Arrays are declared and initiliazed dynamically with the help of Reflection. An new array instance cannot be created till we know the size of the array. But this is not the case with reflection. It helps us to dynamically create arrays. Reflection package provides with Array class which has newInstance method that allows to create arrays for a particular class and size. This is one of the major merits of reflection in Java.
Security in Reflection
Security can be a complex issue when dealing with reflection.Uncontrolled access can create major security risks in other cases, such as when code is executed in an environment shared by untrusted code.The Java programming language defines a multi-level approach to handling reflection security. The basic mode is to enforce the same restrictions on reflection as would apply for source code access i.e public components can be accessed anywhere, and accessing of private components is limited or allowed with permission and limited access to protected and package (default access) components can be done by the java.lang.reflect.AccessibleObject class. This class defines a setAccessible method that lets turn the access checks on or off for an instance of one of these classes. The only catch is that if a security manager is present, it will check that the code turning off access checks has permission to do so. If there's no permission, the security manager throws an exception.
import java.lang.reflect.*;
class Securityclass {
private String m_s1, m_s2;
public Securityclass(String s1, String s2) {
m_s1 = s1;
m_s2 = s2;
}
}
public class SecurityReflection{
public static void main(String[] args) {
try {
Securityclass ts = new Securityclass("a", "b");
Class clas=Class.forName("Securityclass");
Field field = clas.getDeclaredField("m_s1");
field.setAccessible(true);
System.out.println("Retrieved value is " + field.get(ts));
} catch (Exception ex) {
ex.printStackTrace(System.out);
}
}
}
In the above example if the line field.setAccessible(true) is commented out then the program will not be able to access the private string in other class. By giving access permission the private member can be accessed.
Performance Issues
Eventhough Java provides Reflection package which is a powerful tool some issues need to be considered which are a few drawbacks. One of the main drawbacks is the effect on performance. Using reflection is basically an interpreted operation, where we tell the JVM what we want to do and it does it for us. This type of operation is always going to be slower than just doing the same operation directly.
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