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wikipedia Reflection

In computer science, reflection is the ability of a computer program to examine, introspect, and modify its own structure and behavior at runtime.[1]

NOTE : 关于reflection和introspect之间的差异参见introspect

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.[citation needed]

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.

NOTE : 反射帮助程序员制作通用软件库以显示数据，处理不同格式的数据，执行数据的序列化或反序列化以进行通信，或者为容器或突发通信进行数据的捆绑和分拆。

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 override member accessibility rules. For example, reflection makes it possible to change the value of a field marked "private" in a third-party library's class.

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:

1、Discover and modify source-code constructions (such as code blocks, classes, methods, protocols, etc.) as first-class objects at runtime.

2、Convert a string matching the symbolic name of a class or function into a reference to or invocation of that class or function.

3、Evaluate a string as if it were a source-code statement at runtime.

NOTE: 看到此处，想到了python的eval()

4、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()[1] (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.

Python

The following is an example in Python:

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

# with reflection
class_name = "Foo"
method = "hello"
obj = globals()[class_name]()
getattr(obj, method)()

# with eval
eval("Foo().hello()")