Reflection (computer science)

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In computer science, reflection is the process by which a computer program can observe and modify its own structure and behavior. The programming paradigm driven by reflection is called reflective programming.

At the lowest level, machine code can be treated reflectively because the distinction between instruction and data becomes just a matter of how the information is treated by the computer. Normally, 'instructions' are 'executed' and 'data' is 'processed', however, the program can also treat instructions as data and therefore make reflective modifications. Reflection is most common in high-level virtual machine programming languages like Smalltalk, and less common in lower-level programming languages like C.

Common types of reflection are runtime and dynamic, but some programming languages support compile time or static reflection.

[edit] Reflection-Oriented Programming

Reflection-oriented programming, or reflective programming, is a functional extension to the object-oriented programming paradigm. Reflection-oriented programming includes self-examination, self-modification, and self-replication. However, the emphasis of the reflection-oriented paradigm is dynamic program modification, which can be determined and executed at runtime. Some imperative approaches, such as procedural and object-oriented programming paradigms, specify that there is an exact predetermined sequence of operations with which to process data. The reflection-oriented programming paradigm, however, adds that program instructions can be modified dynamically at runtime and invoked in their modified state. That is, the program architecture itself can be decided at runtime based upon the data, services, and specific operations that are applicable at runtime.

Programming sequences can be classified in one of two ways, atomic or compound. Atomic operations are those that can be viewed as completing in a single, logical step, such as the addition of two numbers. Compound operations are those that require a series of multiple atomic operations.

A compound statement, in classic procedural or object-oriented programming, can lose its structure once it is compiled. The reflective programming paradigm introduces the concept of meta-information, which keeps knowledge of program structure. Meta-information stores information such as the name of the contained methods, the name of the class, the name of parent classes, and/or what the compound statement is supposed to do. Using this stored information, as an object is consumed (processed), it can be reflected upon to find out the operations that it supports. The operation that issues in the required state via the desired state transition can be chosen at run-time without hard-coding it.

[edit] Uses

Reflection can be used for observing and/or modifying program execution at runtime. A reflection-oriented program component can monitor the execution of an enclosure of code and can modify itself according to a desired goal related to that enclosure. This is typically accomplished by dynamically assigning program code at runtime.

Reflection can also be used to adapt a given program to different situations dynamically. For example, consider an application that uses two different classes X and Y interchangeably to perform similar operations. Without reflection-oriented programming, the application might be hard-coded to call method names of class X and class Y. However, using the reflection-oriented programming paradigm, the application could be designed and written to utilize reflection in order to invoke methods in classes X and Y without hard-coding method names. Reflection-oriented programming almost always requires additional knowledge, framework, relational mapping, and object relevance in order to take advantage of more generic code execution. Hard-coding can be avoided to the extent that reflection-oriented programming is used.

Reflection is also a key strategy for metaprogramming.

[edit] Implementation

A language supporting reflection provides a number of features available at runtime that would otherwise be very obscure or impossible to accomplish in a lower-level language. Some of these features are the abilities to:

• Discover and modify source code constructions (such as code blocks, classes, methods, protocols, etc.) as a first-class object at runtime.
• Convert a string matching the symbolic name of a class or function into a reference to or invocation of that class or function.
• Evaluate a string as if it were a source code statement at runtime.

These features can be implemented in different ways. In MOO, reflection forms a natural part of everyday programming idiom. When verbs (methods) are called, various variables such as verb (the name of the verb being called) and this (the object on which the verb is called) are populated to give the context of the call. Security is typically managed by accessing the caller stack programmatically: Since callers() is a list of the methods by which the current verb was eventually called, performing tests on callers()[1] (the command invoked by the original user) allows the verb to protect itself against unauthorised use.

Compiled languages rely on their runtime system to provide information about the source code. A compiled Objective-C executable, for example, records the names of all methods in a block of the executable, providing a table to correspond these with the underlying methods (or selectors for these methods) compiled into the program. In a compiled language that supports runtime creation of functions, such as Common Lisp, the runtime environment must include a compiler or an interpreter.

Reflection can be implemented for languages not having built-in reflection facilities by using a program transformation system to define automated source code changes..

[edit] Examples

[edit] ActionScript

Here is an equivalent example in ActionScript:

// Without reflection
var foo:Foo = new Foo();
foo.hello();
 
// With reflection
var ClassReference:Class = flash.utils.getDefinitionByName("Foo") as Class;
var instance:Object = new ClassReference();
instance.hello();

Even with an import statement on “Foo”, the method call to “getDefinitionByName” will break without an internal reference to the class. This is because runtime compilation of source is not currently supported. To get around this, you have to have at least one instantiation of the class type in your code for the above to work. So in your class definition you declare a variable of the custom type you want to use:

var customType : Foo;

So for the idea of dynamically creating a set of views in Flex 2 using these methods in conjunction with an xml file that may hold the names of the views you want to use, in order for that to work, you will have to instantiate a variable of each type of view that you want to utilize.

[edit] C#

Here is an equivalent example in C#:

//Without reflection
Foo foo = new Foo();
foo.Hello();
 
//With reflection
Type t = Assembly.GetCallingAssembly().GetType("FooNamespace.Foo");
t.InvokeMember("Hello", BindingFlags.InvokeMethod, null, Activator.CreateInstance(t), null);

The following example, written in C#, demonstrates the use of advanced features of reflection. It loads an assembly (which can be thought of as a class library) dynamically and uses reflection to find the methods that take no parameters and figure out whether it was recently modified or not. To decide whether a method was recently modified or not, it uses a custom attribute RecentlyModified which the developer tags the methods with. Presence of the attribute indicates it was recently modified. An attribute is itself implemented as a class, that derives from the Attribute class.

The program loads the assembly dynamically at runtime, and checks that the assembly supports the RecentlyModified attribute. This check is facilitated by using another custom attribute on the entire assembly: SupportsRecentlyModified. If it is supported, names of all the methods are then retrieved. For each method, objects representing all its parameters as well as the objects representing the RecentlyAttribute are retrieved. If the attribute retrieval succeeds and the parameter retrieval fails (indicating presence of the attribute but absence of any parameters), a match has been found.

Definition of the custom attributes

//RecentlyModified: Applicable only to methods. 
//SupportsRecentlyModified: Applicable to the entire assembly.
 
[AttributeUsage(AttributeTargets.Method, AllowMultiple=false)]
class RecentlyModifiedAttribute : Attribute
{
    public RecentlyModifiedAttribute() { }
}
 
[AttributeUsage(AttributeTargets.Assembly, AllowMultiple=false)]
class SupportsRecentlyModifiedAttribute : Attribute
{
    public SupportsRecentlyModifiedAttribute() { }
}

Implementation of the method filter

static void Main(string[] args)
{
    //Load the assembly dynamically and make sure it supports the Recently Modified Attribute
    Assembly loadedAssembly = Assembly.LoadAssembly(Console.ReadLine());
    if (Attribute.GetCustomAttribute(
           loadedAssembly, typeof(SupportsRecentlyModifiedAttribute)) != null)
        //For each class in the assembly, get all the methods. Iterate through the methods
        //and find the ones that have both the RecentlyModified attribute set as well as do
        //not require any arguments.
        foreach (Type t in loadedAssembly.GetTypes())
        {
            if (t.IsClass)
                foreach (MethodInfo method in t.GetMethods())
                {
                    //Try to retrieve the RecentlyModified attribute
                    object[] rmAttribute = method.GetCustomAttributes(
                        typeof(RecentlyModifiedAttribute), false);
 
                    //Try to retrieve the parameters
                    ParameterInfo[] parames = method.GetParameters();
 
                    if (rmAttribute.Length == 1 && parames.Length == 0)
                        Console.WriteLine("{0}", method.Name);
                }
        }
}

[edit] C++

Although the language itself does not provide any support for reflection, there are some attempts based on templates, RTTI information, using debug information provided by the compiler, or even patching the GNU compiler to provide extra information.

[edit] Common Lisp

Here is an equivalent example in Common Lisp:

;;Without reflection
 (hello)
 
 ;;With reflection
 (funcall (read-from-string "hello"))
 
 ;;or
 (funcall (symbol-function (intern "hello")))

However, this works only for symbols in topmost lexical environment (or dynamic one). A better example, using CLOS, is:

;;Method hello is called on instance of class foo.
 (hello (make-instance 'foo))
 
 ;;or
 (hello (make-instance (find-class 'foo))

This code is equivalent to Java's one. Common Lisp, like Scheme, is able to transform lists into functions or procedures. This is the source of the Lisp macro ability to change the code behavior (including from other macros) during (and after) compilation.

[edit] Curl

Here is an equivalent example in Curl:

 || Without reflection
 let foo1:Foo = {Foo}
 {foo1.hello}
 
 || Using runtime 'any' calls
 let foo2:any = {Foo}
 {foo2.hello}
 
 || Using reflection
 let foo3:any = {Foo}
 {type-switch {type-of foo3}
  case class:ClassType do
    {if-non-null method = {class.get-method "hello"} then
       {method.invoke foo3}
    }
 }    

All three of these examples instantiate an instance of the 'Foo' class and invoke its 'hello' method. The first, resolves the lookup at compile-time and will generate the most efficient code for invoking the method at runtime. The second, invokes the method through an 'any' variable which will do both the lookup and invocation at runtime, which is much less efficient. The third, uses the reflection interface to lookup the method in the 'Foo' class and invoke it, which is also much less efficient than the first example.

[edit] ECMAScript (JavaScript)

Here is an equivalent example in ECMAScript:

// Without reflection
new Foo().hello()
 
// With reflection
 
// assuming that Foo resides in this
new this['Foo']()['hello']()
 
// or without assumption
new (eval('Foo'))()['hello']()

[edit] Io

Here is an equivalent example in Io:

// Without reflection
hello

// With reflection
doString("hello")

[edit] Java

The following is an example in Java using the Java package java.lang.reflect. Consider two pieces of code

// Without reflection
Foo foo = new Foo();
foo.hello();
 
// With reflection
Class cls = Class.forName("Foo");
Method method = cls.getMethod("hello", null);
method.invoke(cls.newInstance(), null);

Both code fragments create an instance of a class Foo and call its hello() method. The difference is that, in the first fragment, the names of the class and method are hard-coded; it is not possible to use a class of another name. In the second fragment, the names of the class and method can easily be made to vary at runtime. The downside is that the second version is harder to read, and is not protected by compile-time syntax and semantic checking. For example, if no class Foo exists, an error will be generated at compile time for the first version. The equivalent error will only be generated at run time for the second version.

[edit] Lua

Here's an equivalent example in Lua:

-- Without reflection
hello()

-- With reflection
loadstring("hello()")()

The function loadstring compiles the chunk and returns it as a parameterless function.

If hello is a global function, it can be accessed by using the table _G:

-- Using the table _G, which holds all global variables
_G["hello"]()

[edit] MOO

Here is an equivalent example in MOO:

"without reflection";
foo:hello();

"with partial reflection";
foo:("hello")();

[edit] Objective-C

Here is an equivalent example in Objective-C (using Cocoa runtime):

// Without reflection
[[[Foo alloc] init] hello];
 
// With reflection
Class aClass = NSClassFromString(@"Foo");
SEL aSelector = NSSelectorFromString(@"hello"); // or @selector(hello) if the method
                                                // name is known at compile time
[[[aClass alloc] init] performSelector:aSelector];

[edit] Perl

Here is an equivalent example in Perl:

# without reflection
my $foo = Foo->new();
$foo->hello();
 
# with reflection
my $class  = "Foo";
my $method = "hello";
my $object = $class->new();
$object->$method();

[edit] PHP

Here is an equivalent example in PHP:

# without reflection
$Foo = new Foo();
$Foo->hello();
 
# with reflection
$class = "Foo";
$method = "hello";
$object = new $class();
$object->$method();

[edit] Python

Here is an equivalent example from the Python shell with two ways of doing reflection:

>>> # Class definition
>>> class Foo(object):
...     def Hello(self):
...         print "Hi"
... 
>>> # Instantiation
>>> foo = Foo()
>>> # Normal call
>>> foo.Hello()
Hi
>>> # Evaluate a string in the context of the global namespace
>>> eval("foo.Hello()", globals())
Hi
>>> # Interpret distinct instance & method names from strings
>>> instance = 'foo'
>>> method = 'Hello'
>>> getattr(globals()[instance], method)()
Hi

[edit] REBOL

Here is an example in REBOL:

; Without reflection
hello

; With reflection
do [hello]

This works because the language is fundamentally reflective, processing via symbols, not strings.

To illustrate that the above example is reflective (not simply evaluation of a code block) you can write:

if 'hello = first [hello] [print "this is true"]

[edit] Ruby

Here is an equivalent example in Ruby:

# without reflection
Foo.new.hello
 
# with reflection
Object.const_get(:Foo).new.send(:hello)

[edit] Scheme

Here is an equivalent example in Scheme:

; Without reflection
 (hello)
 
 ; With reflection
 (eval '(hello))
 
 ; With reflection via string using string I/O ports
 (eval (read (open-input-string "(hello)")))

[edit] Smalltalk

Here is an equivalent example in Smalltalk:

"Without reflection"
Foo new hello
 
"With reflection"
(Compiler evaluate: 'Foo') new perform: #hello

The class name and the method name will often be stored in variables and in practice runtime checks need to be made to ensure that it is safe to perform the operations:

"With reflection"
 
| x y className methodName |
 
className := 'Foo'.
methodName := 'hello'.
 
x := (Compiler evaluate: className).
 
(x isKindOf: Class) ifTrue: [
    y := x new.
 
    (y respondsTo: methodName asSymbol) ifTrue: [
        y perform: methodName asSymbol
    ]
]

Smalltalk also makes use of blocks of compiled code that are passed around as objects. This means that generalised frameworks can be given variations in behavior for them to execute. Blocks allow delayed and conditional execution. They can be parameterised. (Blocks are implemented by the class BlockClosure).

X := [ :op | 99 perform: op with: 1].
   ".....then later we can execute either of:"
X value: #+   "which gives 100, or"
X value: #-   "which gives 98."

[edit] Windows PowerShell

Here is an equivalent example in Windows PowerShell:

# without reflection
$foo = new-object Foo
$foo.hello()

# with reflection
$class  = 'Foo'
$method = 'hello'
$object = new-object $class
$object.$method.Invoke()

[edit] See also

• Type introspection
• Self-modifying code
• Programming paradigms
• List of reflective programming languages and platforms

[edit] References

• Jonathan M. Sobel and Daniel P. Friedman. An Introduction to Reflection-Oriented Programming (1996), Indiana University.

[edit] Further reading

• Ira R. Forman and Nate Forman, Java Reflection in Action (2005), ISBN 1932394184
• Ira R. Forman and Scott Danforth, Putting Metaclasses to Work (1999), ISBN 0-201-43305-2

[edit] External links

• Reflection in logic, functional and object-oriented programming: a Short Comparative Study (Citeseer page).
• An Introduction to Reflection-Oriented Programming
• Reflection in C++/CLI for .Net
• Reflection for C++
• Aspects of Reflection in C++
• LibReflection: A reflection library for C++.
• A library to provide full reflection for C++ through template metaprogramming techniques.
• A C++ reflection-based data dictionary