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{{Short description|Ability of a process to examine and modify itself}}{{distinguish|Reflection (computer graphics)}}In computer science, reflective programming or reflection is the ability of a process to examine, introspect, and modify its own structure and behavior.{{Citation | title = A Tutorial on Behavioral Reflection and its Implementation by Jacques Malenfant et al. | publisher = unknown | url =weblink | access-date = 23 June 2019 | archive-url =weblink" title="web.archive.org/web/20170821214626weblink">weblink | archive-date = 21 August 2017 }}

Historical background

The earliest computers were programmed in their native assembly languages, which were inherently reflective, as these original architectures could be programmed by defining instructions as data and using self-modifying code. As the bulk of programming moved to higher-level compiled languages such as Algol, Cobol, Fortran, Pascal, and C, this reflective ability largely disappeared until new programming languages with reflection built into their type systems appeared.{{Citation needed|date=July 2015}}Brian Cantwell Smith's 1982 doctoral dissertation introduced the notion of computational reflection in procedural programming languages and the notion of the meta-circular interpreter as a component of 3-Lisp.Brian Cantwell Smith, Procedural Reflection in Programming Languages, Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, PhD dissertation, 1982.Brian C. Smith. Reflection and semantics in a procedural language {{Webarchive|url=https://web.archive.org/web/20151213034343weblink |date=2015-12-13 }}. Technical Report MIT-LCS-TR-272, Massachusetts Institute of Technology, Cambridge, Massachusetts, January 1982.

Uses

Reflection helps programmers make generic software libraries to display data, process different formats of data, perform serialization and deserialization of data for communication, or do bundling and unbundling of data for containers or bursts of communication.Effective use of reflection almost always requires a plan: A design framework, encoding description, object library, a map of a database or entity relations.Reflection makes a language more suited to network-oriented code. For example, it assists languages such as Java to operate well in networks by enabling libraries for serialization, bundling and varying data formats. Languages without reflection such as C are required to use auxiliary compilers for tasks like Abstract Syntax Notation to produce code for serialization and bundling.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 of 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 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 bypass member accessibility rules. For C#-properties this can be achieved by writing directly onto the (usually invisible) backing field of a non-public property. It is also possible to find non-public methods of classes and types and manually invoke them. This works for project-internal files as well as external libraries such as .NET's assemblies and Java's archives.

Implementation

{{Unreferenced section|date=January 2008}}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:

• Discover and modify source-code constructions (such as code blocks, classes, methods, protocols, etc.) as first-class objects 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()[0] (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 without built-in reflection by using a program transformation system to define automated source-code changes.

Security considerations

Reflection may allow a user to create unexpected control flow paths through an application, potentially bypassing security measures. This may be exploited by attackers.REPORT, Paulo, Barros, René, Just, Suzanne, Millstein, Paul, Vines, Werner, Dietl, Marcelo, d'Amorim, Michael D., Ernst, August 2015, Static Analysis of Implicit Control Flow: Resolving Java Reflection and Android Intents,weblink University of Washington, UW-CSE-15-08-01, October 7, 2021, Historical vulnerabilities in Java caused by unsafe reflection allowed code retrieved from potentially untrusted remote machines to break out of the Java sandbox security mechanism. A large scale study of 120 Java vulnerabilities in 2013 concluded that unsafe reflection is the most common vulnerability in Java, though not the most exploited.MAGAZINE, Eauvidoum, Ieu, disk noise, October 5, 2021, Twenty years of Escaping the Java Sandbox,weblink Phrack, 10, 46, October 7, 2021,

Examples

The following code snippets create an instance {{code|foo}} of class {{code|Foo}} and invoke its method {{code|PrintHello}}. For each programming language, normal and reflection-based call sequences are shown.

Common Lisp

The following is an example in Common Lisp using the Common Lisp Object System:(defclass foo () ())(defmethod print-hello ((f foo)) (format T "Hello from ~S~%" f))

Normal, without reflection

(let ((foo (make-instance 'foo)))

With reflection to look up the class named "foo" and the method

named "print-hello" that specializes on "foo".

(let* ((foo-class (find-class (read-from-string "foo")))

C#

The following is an example in C#:// Without reflectionvar foo = new Foo();foo.PrintHello();// With reflectionObject foo = Activator.CreateInstance("complete.classpath.and.Foo");MethodInfo method = foo.GetType().GetMethod("PrintHello");method.Invoke(foo, null);

Delphi, Object Pascal

This Delphi and Object Pascal example assumes that a {{mono|TFoo}} class has been declared in a unit called {{mono|Unit1}}:uses RTTI, Unit1;procedure WithoutReflection;var begin end;procedure WithReflection;var begin end;

eC

The following is an example in eC:// Without reflectionFoo foo { };foo.hello();// With reflectionClass fooClass = eSystem_FindClass(__thisModule, "Foo");Instance foo = eInstance_New(fooClass);Method m = eClass_FindMethod(fooClass, "hello", fooClass.module);((void (*)())(void *)m.function)(foo);

Go

The following is an example in Go:import "reflect"// Without reflectionf := Foo{}f.Hello()// With reflectionfT := reflect.TypeOf(Foo{})fV := reflect.New(fT)m := fV.MethodByName("Hello")if m.IsValid() { }

Java

The following is an example in Java:import java.lang.reflect.Method;// Without reflectionFoo foo = new Foo();foo.hello();// With reflectiontry {

} catch (ReflectiveOperationException ignored) {}

JavaScript

The following is an example in JavaScript:// Without reflectionconst foo = new Foo()foo.hello()// With reflectionconst foo = Reflect.construct(Foo)const hello = Reflect.get(foo, 'hello')Reflect.apply(hello, foo, [])// With evaleval('new Foo().hello()')

Julia

The following is an example in Julia:julia> struct Point

1. Inspection with reflection

julia> fieldnames(Point)(:x, :y)julia> fieldtypes(Point)(Int64, Any)julia> p = Point(3,4)

1. Access with reflection

julia> getfield(p, :x)3

Objective-C

The following is an example in Objective-C, implying either the OpenStep or Foundation Kit framework is used:// Foo class.@interface Foo : NSObject- (void)hello;@end// Sending "hello" to a Foo instance without reflection.Foo *obj = Foo alloc] init];[obj hello];// Sending "hello" to a Foo instance with reflection.id obj = NSClassFromString(@"Foo") alloc] init];[obj performSelector: @selector(hello)];

Perl

The following is an example in Perl:

1. Without reflection

my $foo = Foo->new;$foo->hello;

1. or

Foo->new->hello;

1. With reflection

my $class = "Foo"my $constructor = "new";my $method = "hello";my $f = $class->$constructor;$f->$method;

1. or

$class->$constructor->$method;

1. with eval

eval "new Foo->hello;";

PHP

The following is an example in PHP:WEB, PHP: ReflectionClass - Manual,weblink www.php.net, // Without reflection$foo = new Foo();$foo->hello();// With reflection, using Reflections API$reflector = new ReflectionClass("Foo");$foo = $reflector->newInstance();$hello = $reflector->getMethod("hello");$hello->invoke($foo);

Python

The following is an example in Python:

1. Without reflection

obj = Foo()obj.hello()

1. With reflection

obj = globals()["Foo"]()getattr(obj, "hello")()

1. With eval

eval("Foo().hello()")

R

The following is an example in R:

1. Without reflection, assuming foo() returns an S3-type object that has method "hello"

obj