In computer science, reflection is the ability of a program to examine and possibly modify its high level structure at runtime. It is most common in high-level virtual machine programming languages like Smalltalk, and less common in low-level programming languages like C.

When program source code is compiled, information about the structure of the program is normally lost as lower level code (typically assembly language code) is produced. If a system supports reflection, the structure is preserved as metadata with the emitted code.

Known platforms supporting reflection are:

• Forth
• Lisp, and lisp variants (eg, Scheme) (fully reflective with no distinction between compile-time and run-time)
• The Java Virtual Machine (JVM), but only to a limited extent
• .NET and the Common Language Runtime (CLR)
• Objective-C
• Python
• PHP, since version 5
• Ruby
• The Maude system of rewriting logic
• Smalltalk (fully reflective with no distinction between compile-time and run-time)

More generally, reflection is an activity in computation that reasons about its own computation. The programming paradigm driven by reflection is called reflective programming.

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 include:

• The ability to discover and modify source-code constructions (such as code blocks , classes, methods, protocols, etc.) as first-class objects at runtime.
• The ability to convert a string matching the symbolic name of a class or function into an invocation of that class or function.
• The ability to evaluate a string as if it were a source-code statement at runtime.

These features can be implemented in different ways. Interpreted programming languages, such as Ruby and PHP, are ideally suited to reflection, since their source code is never lost in the process of translation to machine language— the interpreter has the source readily available. Complied 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.

Example

The following is an example in Java. Consider two pieces of code

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

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

The both code creates the instance of a class 'Foo' and calls its method 'hello'. The difference is that in the first piece, the name of the class and the method is hard-coded; it is not possible to use a class of another name. In the second piece, the names of the class and the method can vary at runtime.

(See java.lang.reflect for more of this feature.)

References

1. Reflection in logic, functional and object-oriented programming: a Short Comparative Study (Citeseer page).