Reflective programming

In computer science, reflection is the process by which a computer program can observe and modify its own structure and behavior at runtime.

In many computer architectures, program instructions are stored as data - hence the distinction between instruction and data is merely a matter of how the information is treated by the computer and programming language. Normally, instructions are executed and data is processed; however, in some languages, programs can also treat instructions as data and therefore make reflective modifications. Reflection is most commonly used in high-level virtual machine programming languages like Smalltalk and scripting languages, and less commonly used in manifestly typed and/or statically typed programming languages such as Java, C, ML or Haskell.

Historical background

Brian Cantwell Smith's 1982 doctoral dissertation introduced the notion of computational reflection in programming languages, and the notion of the meta-circular interpreter as a component of 3-Lisp.

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.

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.
• Create a new interpreter for the language's bytecode to give a new meaning or purpose for a programming construct.

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.

Examples

The following examples show an instance foo of a class Foo being created, and a method hello (or Hello) of the instance being called. For each language, two versions are shown; the first being a call sequence without reflection and the second using reflection to access the class and the method.

C#

Here is an example in C#:

//Without reflection
Foo foo = new Foo();
foo.Hello();

//With reflection
object foo = Activator.CreateInstance(null, "Foo");
foo.GetType().GetMethod("Hello").Invoke(foo, null);

ECMAScript

Here is an equivalent example in ECMAScript, and therefore works in JavaScript and ActionScript:

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

Java

The following is an example in Java using the Java package java.lang.reflect:

// Without reflection
new Foo().hello();

// With reflection
Class cls = Class.forName("Foo");
cls.getMethod("hello", null).invoke(cls.newInstance(), null);

Qt (framework)/C++

Qt framework extends C++ with its meta-language and provides reflection ability of member/method reference and query by name for Qt objects with QMetaObject class, which contains meta-information about the Qt objects.

Lua

The following is an example in Lua

-- without reflection
Foo.hello()
 
-- with reflection
_G['Foo']['hello']()

Objective-C

The following is an example in Objective-C

// Without reflection
Foo *foo = [[Foo alloc] init];
[foo hello];

// With reflection
Class cls = NSClassFromString(@"Foo");
id foo = [[cls alloc] init];
SEL selector = NSSelectorFromString(@"hello");
[foo performSelector:selector withObject:nil];

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();

PHP

Here is an equivalent example in PHP:

// without reflection
$Foo = new Foo();
$Foo->hello();

// with reflection
$class_name = "Foo";
$f = new $class_name();
$method = "hello";
$f->$method();

Python

Here is an equivalent example in Python:

# without reflection
Foo().hello()

# with reflection
getattr(globals()['Foo'](), 'hello')()

Ruby

Here is an equivalent example in Ruby:

# without reflection
Foo.new.hello

# with reflection
Object.const_get(:Foo).send(:new).send(:hello)

Smalltalk

Here is an equivalent example in Smalltalk:

"Without reflection"
Foo new hello

"With reflection"
((Compiler evaluate: 'Foo') perform: #new) perform: #hello

Io

Here is an equivalent example in Io:

Foo := Object clone do(
    hello := method(
        "Hello" println
    )
)

#Without reflection
Foo hello

#With reflection
getSlot("Foo") getSlot("hello") call

ActionScript 3.0

Here is an equivalent example in Actionscript:

//Without reflection
var foo:Foo = new Foo();
foo.hello();

//With reflection
var cls:Object = getDefinitionByName("Foo");
var foo:Object = new cls();
foo.hello();

See also

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

References

Notes

Documents

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

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

External links

• Reflection in logic, functional and object-oriented programming: a short comparative study
• An Introduction to Reflection-Oriented Programming
• Brian Foote's pages on Reflection in Smalltalk
• Java Reflection Tutorial from Sun Microsystems