In computer science, reflection is the ability of a computer program to examine, introspect, and modify its own structure and behavior at runtime.
Contents
Historical background
The earliest computers were programmed in their native assembly language, which were inherently reflective as these original architectures could be programmed by defining instructions as data and using self-modifying code. As programming moved to compiled higher-level languages such as Algol, Cobol, and Fortran (but also Pascal and C and many other languages), this reflective ability largely disappeared until programming languages with reflection built into their type systems appeared.
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 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.
In object-oriented programming languages such as Java, reflection allows inspection of classes, interfaces, fields and methods at runtime without knowing the names of the interfaces, fields, methods at compile time. It also allows instantiation of new objects and invocation of methods.
Reflection can be used to adapt a given program to different situations dynamically. Reflection-oriented programming almost always requires additional knowledge, framework, relational mapping, and object relevance in order to take advantage of more generic code execution.
Reflection is often used as part of software testing, such as for the runtime creation/instantiation of mock objects.
Reflection is also a key strategy for metaprogramming.
In some object-oriented programming languages, such as C# and Java, reflection can be used to override member accessibility rules. For example, reflection makes it possible to change the value of a field marked "private" in a third-party library's class.
Implementation
A language supporting reflection provides a number of features available at runtime that would otherwise be difficult to accomplish in a lower-level language. Some of these features are the abilities to:
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 code snippets create an instance foo of class Foo, and invoke its method hello. For each programming language, normal and reflection-based call sequences are shown.
Delphi
This Delphi example assumes a TFoo class has been declared in a unit called Unit1:
This is a notable example since Delphi is an unmanaged, fully natively compiled language, unlike most other languages that support reflection. Its language architecture inherits from strongly-typed Pascal, but with significant influence from SmallTalk. Compare with the other examples here, many of which are dynamic or script languages like Perl, Python or PHP, or languages with a runtime like Java or C#.
eC
The following is an example in eC:
ECMAScript
The following is an example in ECMAScript, and therefore also applies to JavaScript and ActionScript:
Go
The following is an example in Go:
Java
The following is an example in Java:
Objective-C
The following is an example in Objective-C—implying either the OpenStep or Foundation Kit framework is used:
Perl
The following is an example in Perl:
PHP
The following is an example in PHP:
Python
The following is an example in Python:
R
The following is an example in R:
Ruby
The following is an example in Ruby: