Sample Page

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.Cite error: A <ref> tag is missing the closing </ref> (see the help page).

• Evaluate a string as if it were a source-code statement at runtime.^[1][2]
• 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.

Reflection may allow a user to create unexpected control flow paths through an application, potentially bypassing security measures. This may be exploited by attackers.^[3] 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.^[4]

Runtime reflection systems introduce a non-negligible runtime performance overhead. For instance, in Java, because reflective operations are resolved dynamically at execution time, many compiler and JVM optimizations—such as method inlining, static binding, and aggressive just-in-time specialization—cannot be fully applied. As a consequence, reflective calls are typically slower than their statically resolved counterparts. Microbenchmark and application-level studies on Java have shown that reflective operations can incur substantial runtime overhead, especially for method invocation and dynamic object creation, with slowdowns ranging from around 20–40% in moderate cases to more than 300× in heavily reflective dispatch scenarios.^[5] In sequential applications, reflection can significantly increase execution time and memory consumption when used in performance-critical code paths. In multithreaded applications, reflective implementations generally preserve scalability, but still exhibit noticeably higher absolute execution times—commonly between 1.5x and 10× slower—than equivalent non-reflective code.^[5]

In languages like C++ and Rust, reflection is performed at compile time. Compile-time reflection systems are weaker than runtime reflection systems like those of Java and C#, but incur no runtime overhead due to being performed during compilation.^[6]

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.

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)))

Reflection is not possible in C, though parts of reflection can be emulated.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct {
    // ...
} Foo;

typedef void (*Method)(void*);

// The method: Foo::printHello
void Foo_printHello([[maybe_unused]] void* this) {
    (void)this; // Instance ignored for a static method
    printf("Hello, world!\n");
}

// Simulated method table
typedef struct {
    const char* name;
    Method fn;
} MethodEntry;

MethodEntry fooMethods[] = {
    { "printHello", Foo_printHello },
    { NULL, NULL } // Sentinel to mark end
};

// Simulate reflective method lookup
[[nodiscard]]
Method findMethodByName(const char* name) {
    for (size_t i = 0; fooMethods[i].name; i++) {
        if (strcmp(fooMethods[i].name, name) == 0) {
            return fooMethods[i].fn;
        }
    }
    return NULL;
}

int main() {
    // Without reflection
    Foo fooInstance;
    Foo_printHello(&fooInstance);

    // With emulated reflection
    Foo* reflectedFoo = (Foo*)malloc(sizeof(Foo));
    if (!reflectedFoo) {
        fprintf(stderr, "Memory allocation failed\n");
        return EXIT_FAILURE;
    }

    const char* methodName = "printHello";
    Method m = findMethodByName(methodName);

    if (m) {
        m(reflectedFoo);
    } else {
        fprintf(stderr, "Method '%s' not found\n", methodName);
    }

    free(reflectedFoo);
    return EXIT_SUCCESS;
}

The following is an example in C++ (using reflection added in C++26).^[7]

import std;

using std::string_view;
using std::views::filter;
using std::meta::access_context;
using std::meta::exception;
using std::meta::info;

namespace meta = std::meta;

consteval bool isNonstaticMethod(info mem) noexcept {
    return meta::is_class_member(mem) &&
        !meta::is_static_member(mem) &&
        meta::is_function(mem);
}

consteval info findMethod(info type, string_view name) {
    for (info member : meta::members_of(type, access_context::current())
        | filter(isNonstaticMethod)) {
        if (meta::has_identifier(member) && meta::identifier_of(member) == name) {
            return member;
        }
    }
    // Note: this is std::meta::exception, not std::exception
    throw exception(std::format("Failed to retrieve method {} from type {}", name, meta::identifier_of(type)), ^^findMethod);
}

template <info T, const char* Name>
constexpr auto createInvokerImpl = []() -> auto {
    static constexpr info M = findMethod(T, Name);
    contract_assert(
        meta::parameters_of(M).size() == 0 &&
        meta::return_type_of(M) == ^^void
    );
    return []([:T:]& instance) -> void { instance.[:M:](); };
}();

consteval info createInvoker(info type, string_view name) {
    return meta::substitute(
        ^^createInvokerImpl,
        { meta::reflect_constant(type), meta::reflect_constant_string(name) }
    );
}

class Foo {
private:
    // ...
public:
    Foo() = default;

    void printHello() const {
        std::println("Hello, world!");
    }
};

int main(int argc, char* argv[]) {
    Foo foo;

    // Without reflection
    foo.printHello();

    // With reflection
    auto invokePrint = [:createInvoker(^^Foo, "printHello"):];
    invokePrint(foo);

    return 0;
}

The following is an example in C#:

namespace Wikipedia.Examples;

using System;
using System.Reflection;

class Foo
{
    // ...

    public void PrintHello()
    {
        Console.WriteLine("Hello, world!");
    }
}

public class InvokeFooExample
{
    static void Main(string[] args)
    {
        // Without reflection
        Foo foo = new();
        foo.PrintHello();

        // With reflection
        Object reflectedFoo = Activator.CreateInstance(typeof(Foo));
        MethodInfo method = reflectedFoo.GetType()
            .GetMethod("PrintHello");
        method.Invoke(foo, null);
    }
}

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;

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);

The following is an example in Go:

import (
    "fmt"
    "reflect"
)

type Foo struct{}

func (f Foo) Hello() {
    fmt.Println("Hello, world!")
}

func main() {
    // Without reflection
    var f Foo
    f.Hello()

    // With reflection
    var fT reflect.Type = reflect.TypeOf(Foo{})
    var fV reflect.Value = reflect.New(fT)

    var m reflect.Value = fV.MethodByName("Hello")

    if m.IsValid() {
        m.Call(nil)
    } else {
        fmt.Println("Method not found")
    }
}

The following is an example in Java:

package org.wikipedia.examples;

import java.lang.reflect.Method;

class Foo {
    // ...

    public void printHello() {
       System.out.println("Hello, world!");
    }
}

public class InvokeFooExample {
    public static void main(String[] args) {
        // Without reflection
        Foo foo = new Foo();
        foo.printHello();

        // With reflection
        try {
            Foo reflectedFoo = Foo.class
                .getDeclaredConstructor()
                .newInstance();

            Method m = reflectedFoo.getClass()
                .getDeclaredMethod("printHello", new Class<?>[0]);
            m.invoke(reflectedFoo);
        } catch (ReflectiveOperationException e) {
            System.err.printf("An error occurred: %s%n", e.getMessage());
        }
    }
}

Java also provides an internal class (not officially in the Java Class Library) in module jdk.unsupported, sun.reflect.Reflection which is used by sun.misc.Unsafe. It contains one method, static Class<?> getCallerClass(int depth) for obtaining the class making a call at a specified depth.^[8] This is now superseded by using the class java.lang.StackWalker.StackFrame and its method Class<?> getDeclaringClass().

The following is an example in JavaScript:

import 'reflect-metadata';

// 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()');

The following is the same example in TypeScript:

import 'reflect-metadata';

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

// With reflection
const foo: Foo = Reflect.construct(Foo);
const hello: (this: Foo) => void = Reflect.get(foo, 'hello') as (this: Foo) => void;
Reflect.apply(hello, foo, []);

// With eval
eval('new Foo().hello()');

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

Using Java reflection:

package org.wikipedia.examples

import java.lang.reflect.Method

class Foo {
    // ...
    constructor()

    fun printHello() {
        println("Hello, world!")
    }
}

fun main(args: Array<String>) {
    // Without reflection
    val foo = Foo()
    foo.printHello()

    // With reflection
    try {
        // Foo::class.java retrieves a java.lang.Class<Foo>
        val reflectedFoo = Foo::class.java
            .getDeclaredConstructor()
            .newInstance()

        val m: Method = reflectedFoo.javaClass
            .getDeclaredMethod("printHello")

        m.invoke(reflectedFoo)
    } catch (e: ReflectiveOperationException) {
        System.err.printf("An error occurred: %s%n", e.message)
    }
}

Using pure Kotlin:

package org.wikipedia.examples

import kotlin.reflect.full.createInstance
import kotlin.reflect.full.functions

class Foo {
    // ...
    fun printHello() {
        println("Hello, world!")
    }
}

fun main(args: Array<String>) {
    // Without reflection
    val foo = Foo()
    foo.printHello()

    // With reflection
    try {
        val kClass = Foo::class
        val reflectedFoo = kClass.createInstance()
        val function = kClass.functions.first { it.name == "printHello" }
        function.call(reflectedFoo)
    } catch (e: Exception) {
        System.err.printf("An error occurred: %s%n", e.message)
    }
}

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)];

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;";

The following is an example in PHP:^[9]

// 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);

The following is an example in Python:

from typing import Any

class Foo:
    # ...
    def print_hello(self) -> None:
        print("Hello, world!")

if __name__ == "__main__":
    # Without reflection
    obj: Foo = Foo()
    obj.print_hello()

    # With reflection
    obj: Foo = globals()["Foo"]()
    _: Any = getattr(obj, "print_hello")()

    # With eval
    eval("Foo().print_hello()")

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))

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"

The following is an example in Rust (using procedural macros).

// A basic macro that registers a method name string to an actual method call
macro_rules! invoke_method {
    ($instance:expr, $method_name:expr) => {
        match $method_name {
            "print_hello" => $instance.print_hello(),
            _ => eprintln!("An error occurred: Method not found."),
        }
    };
}

struct Foo;

impl Foo {
    fn print_hello(&self) {
        println!("Hello, world!");
    }
}

fn main() {
    let foo = Foo;

    // Normal call sequence
    foo.print_hello();

    // Reflection-style sequence via compile-time mapping
    let method_to_call = "print_hello";
    invoke_method!(foo, method_to_call);
}

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

• List of reflective programming languages and platforms
• Mirror (programming)
• Programming paradigms
• Self-hosting (compilers)
• Self-modifying code
• Type introspection
• typeof

• 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

• Reflection in logic, functional and object-oriented programming: a short comparative study
• An Introduction to Reflection-Oriented Programming
• Brian Foote’s pages on Reflection in Smalltalk
• Java Reflection API Tutorial from Oracle