Reflection (computer programming)

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[2][3] introduced the notion of computational reflection in procedural programming languages and the notion of the meta-circular interpreter as a component of 3-Lisp.

Uses

Reflection helps programmers make generic software libraries to display data, process different formats of data, perform serialization or 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 (e.g. C) have to use auxiliary compilers, e.g. for 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 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 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 (.Net-assemblies and Java-archives).

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:

  • 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 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 PrintHello. For each programming language, normal and reflection-based call sequences are shown.

C#

The following is an example in C#:

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

// With reflection
Object foo = Activator.CreateInstance("complete.classpath.and.Foo");
MethodInfo method = foo.GetType().GetMethod("PrintHello");
method.Invoke(foo, null);

Delphi

This Delphi example assumes that a TFoo class has been declared in a unit called Unit1:

uses RTTI, Unit1;

procedure WithoutReflection;
var
  Foo: TFoo;
begin
  Foo := TFoo.Create;
  try
    Foo.Hello;
  finally
    Foo.Free;
  end;
end;

procedure WithReflection;
var
  RttiContext: TRttiContext;
  RttiType: TRttiInstanceType;
  Foo: TObject;
begin
  RttiType := RttiContext.FindType('Unit1.TFoo') as TRttiInstanceType;
  Foo := RttiType.GetMethod('Create').Invoke(RttiType.MetaclassType, []).AsObject;
  try
    RttiType.GetMethod('Hello').Invoke(Foo, []);
  finally
    Foo.Free;
  end;
end;

eC

The following is an example in eC:

// Without reflection
Foo foo { };
foo.hello();

// With reflection
Class fooClass = eSystem_FindClass(__thisModule, "Foo");
Instance foo = eInstance_New(fooClass);
Method m = eClass_FindMethod(fooClass, "hello", fooClass.module);
((void (*)())(void *)m.function)(foo);

ECMAScript

The following is an example in ECMAScript, and therefore also applies to 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']()

// or simply
eval('new Foo().hello()')

// Using ECMAScript 2015's new Reflect class:
Reflect.construct(Foo, [])['hello']()

Go

The following is an example in Go:

import "reflect"

// Without reflection
f := Foo{}
f.Hello()

// With reflection
fT := reflect.TypeOf(Foo{})
fV := reflect.New(fT)

m := fV.MethodByName("Hello")
if m.IsValid() {
    m.Call(nil)
}

Java

The following is an example in Java:

import java.lang.reflect.Method;

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

// With reflection
try {
    // Alternatively: Object foo = Foo.class.newInstance();
    Object foo = Class.forName("complete.classpath.and.Foo").newInstance();

    Method m = foo.getClass().getDeclaredMethod("hello", new Class<?>[0]);
    m.invoke(foo);
} catch (Exception e) {
    // Catching ClassNotFoundException, NoSuchMethodException
    // InstantiationException, IllegalAccessException
}

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:

# Without reflection
my $foo = Foo->new;
$foo->hello;

# or
Foo->new->hello;

# With reflection
my $class = "Foo"
my $constructor = "new";
my $method = "hello";

my $f = $class->$constructor;
$f->$method;

# or
$class->$constructor->$method;

# with eval
eval "new Foo->hello;";

PHP

The following is an example in PHP:

// 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:

# Without reflection
obj = Foo()
obj.hello()

# With reflection
obj = globals()['Foo']()
getattr(obj, 'hello')()

# With eval
eval('Foo().hello()')

R

The following is an example in R:

# Without reflection, assuming foo() returns an S3-type object that has method "hello"
obj <- foo()
hello(obj)

# With reflection
the.class <- "foo"
the.method <- "hello"
obj <- do.call(the.class, list())
do.call(the.method, alist(obj))

Ruby

The following is an example in Ruby:

# Without reflection
obj = Foo.new
obj.hello

# With reflection
class_name = "Foo"
method_name = :hello
obj = Object.const_get(class_name).new
obj.send method_name

# With eval
eval "Foo.new.hello"

Xojo

The following is an example using Xojo:

' Without reflection
Dim fooInstance As New Foo
fooInstance.PrintHello

' With reflection
Dim classInfo As Introspection.Typeinfo = GetTypeInfo(Foo)
Dim constructors() As Introspection.ConstructorInfo = classInfo.GetConstructors
Dim fooInstance As Foo = constructors(0).Invoke
Dim methods() As Introspection.MethodInfo = classInfo.GetMethods
For Each m As Introspection.MethodInfo In methods
  If m.Name = "PrintHello" Then
    m.Invoke(fooInstance)
  End If
Next
gollark: sorry, no, I got confused with JS.
gollark: Lua does.
gollark: Also, just using `==` to compare ~~a password and hash~~ secret values of some kind is actually somewhat unsafe because timing channel attacks.
gollark: 2 xor 1024?
gollark: Password hashing algorithms generally let you pass the salt as a separate parameter.

See also

References

Citations
  1. A Tutorial on Behavioral Reflection and its Implementation by Jacques Malenfant et al. (PDF), unknown, archived from the original (PDF) on 21 August 2017, retrieved 23 June 2019
  2. Brian Cantwell Smith, Procedural Reflection in Programming Languages, Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, PhD dissertation, 1982.
  3. Brian C. Smith. Reflection and semantics in a procedural language Archived 2015-12-13 at the Wayback Machine. Technical Report MIT-LCS-TR-272, Massachusetts Institute of Technology, Cambridge, Massachusetts, January 1982.

Sources

Further reading
  • Ira R. Forman and Nate Forman, Java Reflection in Action (2005), ISBN 1-932394-18-4
  • Ira R. Forman and Scott Danforth, Putting Metaclasses to Work (1999), ISBN 0-201-43305-2
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