CSC/ECE 517 Fall 2011/ch4 4f ss

Reflection

Reflection allows program entities to discover things about themselves through introspection. In other words,Reflection is the ability of a program to determine information about an object at runtime. The information may include the following :

• The type of the object
• Inheritance structure
• Methods it contains.
• Number of parameters of the methods,types of parameters, return types.
• Names and types of the attributes of the object.

Reflection is supported by many Object-oriented programminglanguages in various forms. Powerful and flexible reflection mechanisms are exhibited by Smalltalk, Ruby, and Python . Java also supports reflection. But reflection in java is verbose and is not as flexible and dynamic as these languages. C++ allows a program to determine the type of an object at run-time. Thus it does support reflection but in a limited form only. Eiffel also has support for a limited form of reflection which includes the ability to determine the features contained in an object.

Reflection in Ruby

In Ruby, reflection is simpler because it is done using the language mechanisms ie .methods gives the list of methods. In contrast reflection as we shall see later is much more verbose because we need to use class names,method names,parameter names etc.

Examples of Reflection in Ruby

3.1419.methods #gives the list of methods of the float number.
irb(main):005:0> 3.1419.methods
=> [:to_s, :coerce, :-@, :+, :-, :*, :/, :quo, :fdiv, :%, :modulo, :divmod, :**,
 :==, :===, :<=>, :>, :>=, :<, :<=, :eql?, :hash, :to_f, :abs, :magnitude, :zero
?, :to_i, :to_int, :floor, :ceil, :round, :truncate, :nan?, :infinite?, :finite?
, :numerator, :denominator, :to_r, :rationalize, :arg, :angle, :phase, :singleto
n_method_added, :i, :+@, :div, :remainder, :real?, :integer?, :nonzero?, :step,
:to_c, :real, :imaginary, :imag, :abs2, :rectangular, :rect, :polar, :conjugate,
 :conj, :between?, :nil?, :=~, :!~, :class, :singleton_class, :clone, :dup, :ini
tialize_dup, :initialize_clone, :taint, :tainted?, :untaint, :untrust, :untruste
d?, :trust, :freeze, :frozen?, :inspect, :methods, :singleton_methods, :protecte
d_methods, :private_methods, :public_methods, :instance_variables, :instance_var
iable_get, :instance_variable_set, :instance_variable_defined?, :instance_of?, :
kind_of?, :is_a?, :tap, :send, :public_send, :respond_to?, :respond_to_missing?,
 :extend, :display, :method, :public_method, :define_singleton_method, :__id__,
:object_id, :to_enum, :enum_for, :equal?, :!, :!=, :instance_eval, :instance_exe
c, :__send__]

This is an example of discovering things about 3.1459 at runtime.(Reflection).We can act on information during runtime without compiling it in. Information needed can be found out at run-time instead of writing code at compile time.

Example1:

puts "hello".class  #returns the class of object "hello" 

Output:
String

Example 2:

puts "hello".class.superclass  #returns the superclass of the String class

Output:
Object

Example 3:

puts string.ancestors #returns the ancestors of the string class

Output:
String
Comparable
Object
Basic Object
Kernel

Comparable which is an ancestor of the String class is a mixin.It is used to provide the comparable functionality (operations like <,>,<=,>= etc)to the String class based on the definition of the rocket method.

Object is the superclass of all classes.Its methods are available to all classes unless explicitly overridden. It provides methods like to_s,eql? etc.

Basic Object is the parent class of all classes in ruby including Object class. It is an explicit blank class.

Kernel is a module is included by class Object(hence a mixin). Its methods are available in all Ruby objects.

 Fixnum.ancestors #returns the ancestors of Fixnum.
=> [Fixnum, Integer, Numeric, Comparable, Object, Kernel, BasicObject]

Integer is the basic for Fixnum and Bignum which are two concrete classes that hold whole numbers in ruby.

Numeric is a class which is used to provide methods like abs,floor,ceil,modulo,unary +,unary - etc. ie methods which can be performed on numbers in general.

Comparable which is an ancestor of the Fixnum class is a mixin.It is used to provide the comparable functionality (operations like <,>,<=,>= etc)to the FixNum class based on the definition of the rocket method.

We have discussed the Object and BasicObject classes in the above example

Note: It is useful to print out the name of a class while debugging.A class of an object should not be tested while debugging in objected oriented programming. It is advised to use polymorphism to replace code that tests the class of an object and performs certain things.

eg: if s.kind_of? Integer then this_method else that_method end #this is not a good practice. #Should be replaced by polymorphism.

In some languages like C, C++, or Ada, an assignment statement like x = y is interpreted as copying the contents of variable y into the variable x. It is generally required that x,y should be of the same type and size. But it is allowed in certain cases when the variables are of similar types.In C++, if the sizes are different say size of x is less than the size of y there will be a loss of data during assignment.(Slicing).

Ruby exhibits dynamic binding, not static binding ie the type of the object in a variable is determined at runtime but not at compile time.

Therefore in ruby, x = y can be 'interpreted as bind x to the same object that y is bound to'.The assignment operation is implemented by copying the reference stored in y into x.

Thus in ruby,assignments physically copy references, but not the objects.

Obtaining Reference to a method in Ruby

A reference to a method can be obtained by using the method method in the class Object. It is available to all classes, since Object is the superclass of all classes. The reference thus obtained can be used to invoke that particular method. eg:

str = "Hello, World"
#The name of the method has to be passed as a symbol to method:
m = str.method(:upcase) # returns a Method object which is  a closure.
puts m.call

Note that in the above example name of the method passed as a symbol.Every instance of a particular symbol is the same symbol. Every instance of a particular symbol is the same symbol.

irb(main):009:0> puts :mysymbol.object_id
332648
=> nil
irb(main):010:0> puts :mysymbol.object_id #Note that 2 occurrences of the same symbol yielded 
332648                                    #same object id unlike strings (shown below)
=> nil
irb(main):011:0> puts "mystring".object_id
21196488
=> nil
irb(main):012:0> puts "mystring".object_id
20959140

Applications of passing methods as parameters

Quadrature integration

Suppose we need to compute the area under a curve described by some function, e.g., x2 + 2x + 3.

Then we can define class Float

 def poly
   self*self + 2*self + 3
 end
end

And to compute the area,we pass poly to an integration method which takes the method reference,upper bound and lower bound between which the area is to be computed as the parameters area = integrate(:poly, 10, 20) #calculates the area of the curve x2+2x+3 between the limits x=10 to x=20.

Intercepting calls to undefined methods

Ruby provides an option to intercept the call when a call to an undefined method is made on an object. Implementation of the method method_missing within the class definition provides this facility. Ruby passes as parameters the name of the method called and the arguments passed to it. class Duck

 def quack
   puts "Quack"
 end  
 def method_missing(meth, *args)
   puts "Sorry, I do not #{meth}"
 end

end d = Duck.new d.quack >>Quack d.bark >> Sorry, I do not bark

Example of an interesting usage of method_missing: “Roman method_missing”.

Example 4: Let us now consider the example as discussed in the class:

puts 100.class # print the class of 1
>> Fixnum
puts 98343098452405890454.class #prints the class of 98343098452405890454
>> Bignum
puts 123456789012345.kind_of? Integer
>> true
puts 123456789012345.instance_of? Integer
>> false
puts 123456789012345.instance_of? Bignum
>> true

Difference between Fixnum and Bignum in Ruby

A Fixnum stores Integer values which can be represented in a native machine word (minus 1 bit). If an operation on a Fixnum results in a value that exceeds the range, the value is automatically converted to a Bignum.

When Fixnum objects are assigned or passed as parameters, the actual objects are passed, rather than references to the objects. There is only one Fixnum object instance for a given integer value

Bignum objects are used to store values of integers outside the range of Fixnum. They are created automatically when encountered with integer calculations that would otherwise overflow a Fixnum. When a calculation involving Bignum objects returns a result that will fit in a Fixnum, the result is automatically converted.

OO Languages distinguish between a primitive and reference with help of a leading or trailing bit. ie One bit indicates if the remaining bits store a primitive or a reference and the remaining bits represent the value. In numeric calculations, most of numbers are primitives so primitives can be used directly avoiding the overhead of indirection. One main difference between Fixnum and Bignum is that Fixnum can be stored in 1 machine word. If the object cannot be stored in a machine word then Bignum is used. In case of Bignum a reference to the actual object is stored.

In contrast to Fixnum objects, in case of objects references to objects are used for parameter passing.

Reflection in Java

Reflection in Java is much more verbose than in ruby.It is an advanced feature of the Java environment. Reflection is achieved using the Reflection API.The verbosity of reflection in java may be associated with the usage of the library for reflection.

Big industrial-strength protocols like SOAP and JavaBeans wouldn't be possible if it weren't for Reflection. Every time you drag-and-drop an object in your favorite IDE into a form, Reflection is orchestrating the action behind the scenes. Actually, most sophisticated Java applications rely on Reflection in one way or another.

Reflection is an advanced feature of the Java environment. It gives runtime information about objects, classes, and interfaces. Some of the questions that can be answered using reflection in java:

*   Which class does an object belong to?
*   What is the description of a given class name?
*   What are the fields in a given class?
*   What is the type of a field?
*   What are the methods in a class?
*   What are the parameters of a method?
*   What are the constructors of a given class? 
*   Constructing an object using a given constructor
*   Invoking an object's method using such-and-such parameters
*   Assigning a value to an object's field
*   Dynamically creating and manipulating arrays  

Java Reflection Classes

All Reflection classes (With the exception of the class Class that resides in the default Java package) are contained in the package java.lang.reflect.

Following are some of the classes used to various represent members of a class.: Classes-'Class' class Fields-'Field' class methods-'Method' class constructors-'Constructor' class arrays-'Array' class.

Illustration of Reflection in Java through Examples

• Class

Each class or interface in Java is described by a Class object.It contains methods to obtain details about the class like name, parent class, constructors, fields, methods, interfaces implemented etc.

To obtain the class that an object belongs to, invoke the method Class getClass()(returns 'Class') of the 'Object' class (superclass of all the Java classes hierarchy)

String myStr = "HelloWorld";
Class theClass = myStr.getClass(); // 'theClass' now contains 'String'

The property ".class"(available for every java class) returns a Class object for the class.

String myStr = "HelloWorld";
if (myStr.getClass()==String.class)
System.out.println("The object is a String"); // prints "The object is a String"

The wrapper classes (Integer, Boolean, Double,...) contain a ".TYPE" property that returns the Class object representing the primitive type.

Class myClass = Integer.TYPE; //

• Field

The Field class describes the different attributes of a Java class field. Information such as a field name, its type, and its accessibility can be obtained from a field object. It also contains methods to set and get the field's value for a given object Example

   import java.lang.reflect.*;
        
   public class MyfieldClass {
      int i = 100;
      String s = "Hello World";
        
      public static void main(String args[])
      {
         try {
            Class cls = Class.forName("MyfieldClass");
        
            Field flist[]= cls.getDeclaredFields();
            for (int i = 0; i < flist.length; i++) {
              
               Field f = flist[i];
               System.out.println("name
                  = " + f.getName());
               System.out.println("decl class = " +
                           f.getDeclaringClass());
               System.out.println("type
                  = " + f.getType());
               
            }
          }
          catch (Throwable e) {
             System.err.println(e);
          }
       }
   }

The output of the program is:

  name = i
   decl class = MyfieldClass
   type = double
   
   name = s
   decl class = MyfieldClass
   type = class java.lang.String
   
  

• Method

The Method class is used to obtain information(the method name, its type, its accessibility, and its parameter types) about class methods.We can even invoke the method on a particular object and pass a set of parameters to it.

   import java.lang.reflect.*;

   public class myMethodClass {
      private int myMethod(int y, int x) 
      {
         return x+y;
      }
        
      public static void main(String args[])
      {
         try {
           Class cls = Class.forName("myMethodClass");
        
            Method mlist[] = cls.getDeclaredMethods();
            
              for (int i = 0; i < mlist.length;i++) 
              {  
               Method m = mlist[i];
               System.out.println("name= " + m.getName());
               System.out.println("decl class = " +m.getDeclaringClass());
                              
               Class pt[] = m.getParameterTypes();
               for (int j = 0; j < pt.length; j++)
                  System.out.println("param #" + j + " " + pt[j]);
                   
              
               System.out.println("return type = " +
                                  m.getReturnType());
               
            }
         }
         catch (Throwable e) {
            System.err.println(e);
         }
      }
   }

The output of the program is:

  name = myMethod
   decl class = class myMethodClass
   param #0 int
   param #1 int
   
   return type = int
   
   name = main
   decl class = class myMethodClass
   param #0 class [Ljava.lang.String;
   return type = void
   

• Constructor

The Constructor class can be used to obtain information(parameter types, number of parameters, and accessibility) about class constructors.We can also invoke the constructor to create new object instances

 import java.lang.reflect.*;
        
   public class Myconstructor {
      public Myconstructor()
      {
      }
        
     public static void main(String args[])
      {
         try {
           Class c = Class.forName("Myconstructor");
        
           Constructor clist[]= c.getDeclaredConstructors();
         
           for (int i = 0; i < clist.length; i++) {
               Constructor ct = clist[i];
               System.out.println("name= " + ct.getName());
               System.out.println("Declaring Class Name = " +
                            ct.getDeclaringClass());
               Class pTypes[] = ct.getParameterTypes();
               for (int j = 0; j <  pTypes.length; j++)
                  System.out.println("param #" 
                     + j + " " +  pTypes[j]);
               
              
            }
          }
          catch (Throwable e) {
             System.err.println(e);
          }
      }
   }

output:

Name=MyConstructor
Declaring Class name=MyConstructor

Applications of Reflection in Java

Many sophisticated Java applications rely on reflection. Apart from the general benefits of reflection, listed below are a few which are specific to Java in general.

• Reflection is used extensively in protocols like SOAP
• Whenever we drag-and-drop an object in an IDE to use it in a form, Reflection is used behind the scenes.
• Java Beans
• It is used in Debuggers and Test Tools which require examining the private members <ref>http://download.oracle.com/javase/tutorial/reflect/</ref>.
• Method can be easily invoked methods by their names, it is widely used in web, when invoking getters and setters by property names mentioned on jsp/jsf pages.

Advantages of Reflection

Reflection has many advantages some of which have been listed below:

• Extensibility Features: An application may make use of external, user-defined classes by creating instances of extensibility objects using their fully-qualified names.
• Class Browsers and Visual Development Environments A class browser will able to enumerate the members of classes and visual development environments can benefit from making use of type information available in reflection to help the developer in writing correct code.
• Debugging: Debuggers and Test Tools Debuggers need to be able to examine private members on classes. Test harnesses can make use of reflection to systematically call a discoverable set APIs defined on a class, to insure a high level of code coverage in a test suite.
• Reflection helps in keeping software robust
• It helps in making the applications more flexible and pluggable.
• It helps in making the source code more readable and easier to understand.
• It can simplify source code and design.
• Expensive conditional code can be reduced/removed.

Disadvantages of Reflection

• Performance Overhead: Reflection involves types that are dynamically resolved due to which certain Java virtual machine optimizations can not be performed. Consequently, reflective operations have slower performance than their non-reflective counterparts. Hence, they should be avoided in sections of code which are called frequently in performance-sensitive applications.

• Security Restrictions: 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 in Java.

• Exposure of Internals: 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.

• Compile-check features are lost. You can't be sure you're operating the right amount of parameters, their types, you even can't be sure the method you call exists.

MetaProgramming

The greek prefix 'Meta' refers to knowledge about knowledge. Metaprogramming means programs can create new code at runtime and run the code that is written. Program that writes a program.The related technique of metaprogramming allows one to create new program entities, such as methods or classes, at run time.

See more: http://download.oracle.com/javase/tutorial/reflect/