Reflective programming
In
Historical background
The earliest computers were programmed in their native
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
Reflection can be used for observing and modifying program execution at
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
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 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
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.[4] 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.[5]
Examples
The following code snippets create an
PrintHello
. For each programming languageCommon 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)))
(print-hello 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")))
(print-hello-method (find-method (symbol-function (read-from-string "print-hello"))
nil (list foo-class))))
(funcall (sb-mop:method-generic-function print-hello-method)
(make-instance foo-class)))
C#
The following is an example in C#:
// Without reflection
var 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, Object Pascal
This Delphi and Object Pascal 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);
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 {
Object foo = Foo.class.getDeclaredConstructor().newInstance();
Method m = foo.getClass().getDeclaredMethod("hello", new Class<?>[0]);
m.invoke(foo);
} catch (ReflectiveOperationException ignored) {}
JavaScript
The following is an example in JavaScript:
// Without reflection
const foo = new Foo()
foo.hello()
// With reflection
const foo = Reflect.construct(Foo)
const hello = Reflect.get(foo, 'hello')
Reflect.apply(hello, foo, [])
// With eval
eval('new Foo().hello()')
Julia
The following is an example in Julia:
julia> struct Point
x::Int
y
end
# Inspection with reflection
julia> fieldnames(Point)
(:x, :y)
julia> fieldtypes(Point)
(Int64, Any)
julia> p = Point(3,4)
# 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:
# 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:[6]
// 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
class_name <- "foo"
generic_having_foo_method <- "hello"
obj <- do.call(class_name, list())
do.call(generic_having_foo_method, alist(obj))
Ruby
The following is an example in Ruby:
# Without reflection
obj = Foo.new
obj.hello
# With reflection
obj = Object.const_get("Foo").new
obj.send :hello
# 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
See also
- List of reflective programming languages and platforms
- Mirror (programming)
- Programming paradigms
- Self-hosting (compilers)
- Self-modifying code
- Type introspection
- typeof
References
Citations
- ^ 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
- ^ 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 Archived 2015-12-13 at the Wayback Machine. Technical Report MIT-LCS-TR-272, Massachusetts Institute of Technology, Cambridge, Massachusetts, January 1982.
- ^ Barros, Paulo; Just, René; Millstein, Suzanne; Vines, Paul; Dietl, Werner; d'Amorim, Marcelo; Ernst, Michael D. (August 2015). Static Analysis of Implicit Control Flow: Resolving Java Reflection and Android Intents (PDF) (Report). University of Washington. UW-CSE-15-08-01. Retrieved October 7, 2021.
- ^ Eauvidoum, Ieu; disk noise (October 5, 2021). "Twenty years of Escaping the Java Sandbox". Phrack. Vol. 10, no. 46. Retrieved October 7, 2021.
- ^ "PHP: ReflectionClass - Manual". www.php.net.
Sources
- Jonathan M. Sobel and Daniel P. Friedman. An Introduction to Reflection-Oriented Programming (1996), Indiana University.
- Anti-Reflection technique using C# and C++/CLI wrapper to prevent code thief
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