Reflection (computer science)

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)

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.

Example

// 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