軟件構(gòu)造的實(shí)質(zhì)：Transformation between different views(不同視圖之間的轉(zhuǎn)換)

Multi-dimensional software views

按階段分：build and run-time views
按動(dòng)態(tài)性分：moment and period views
按構(gòu)造對(duì)象的層次分：code and component views

Transformation between different views

Nothing -> Code
Code -> Component
Build-time -> Run-time
Moment -> Period

5個(gè)重要的質(zhì)量目標(biāo)

• Easy to understand
  elegant and beautiful code (understandability)
• Ready for change
  maintainability and adaptability
• Cheap for develop
  design for/with reuse: reusability
• Safe from bugs
  robustness
• Efficient to run
  performance

軟件多視圖的宏觀視角

構(gòu)建時(shí)期(Build-time views)

AST可以認(rèn)為是代碼的另一種等價(jià)視圖，對(duì)AST進(jìn)行修改就相當(dāng)于對(duì)源代碼進(jìn)行修改，對(duì)源代碼進(jìn)行修改就相當(dāng)于對(duì)AST進(jìn)行修改。
Semantics-oriented program structure(語(yǔ)義導(dǎo)向的程序結(jié)構(gòu))：用源代碼對(duì)現(xiàn)實(shí)世界進(jìn)行刻畫，例如使用UML圖描述接口、類、屬性、方法及其關(guān)系。先用人類語(yǔ)言表達(dá)需求和設(shè)計(jì)思想，再轉(zhuǎn)化成代碼。
版本控制。

運(yùn)行時(shí)期(Run-time views)

Executable programs：純粹的機(jī)器語(yǔ)言
Cibrary：運(yùn)行時(shí)庫(kù)，需要鏈接
Configuration and data files：配置文件，提供程序運(yùn)行必要的參數(shù)
Distributed programs：分布式程序，例如一臺(tái)服務(wù)器+若干臺(tái)客戶機(jī)
兩種可執(zhí)行文件：編譯成的機(jī)器碼(Native Machine Code)；完全解釋運(yùn)行(Fully Interpretation)；解釋型字節(jié)碼(Interpreted Byte Code)
動(dòng)態(tài)鏈接：庫(kù)文件不會(huì)在build階段被加入可執(zhí)行文件，只做出標(biāo)記，在程序運(yùn)行時(shí)，庫(kù)被按照標(biāo)記裝入內(nèi)存。

Transformation between views

• Nothing => Code
  Programming / Coding (ADT/OOP)
  Review, static analysis/checking
• Code => Component
  Design (ADT/OOP;Reusability;Maintainability)
  Build: compile, static link, package, install, etc
• Build-time => Run-time
  Install / deploy
  Debug, unit testing (Robustness and Correctness)
• Moment => Period
  Verson control
  Loading, dynamic linking, execution (dumping, profiling, logging)
  Concurrent threads

Quality properties 質(zhì)量指標(biāo)

External quality factors: 外部質(zhì)量因素，影響用戶，如運(yùn)行速度、易用性，包括correctness, robustness, extendibility, reusability, compatibility, efficiency, portability, ease of use, functionality, timeliness
Internal quality factors: 內(nèi)部質(zhì)量因素，影響軟件本身和它的開(kāi)發(fā)者，如模塊化程度、代碼可讀性

對(duì)于發(fā)布后的軟件而言，only external factors matter
且外部質(zhì)量取決于內(nèi)部質(zhì)量，External quality results from internal quality.

External quality factors

• Correctness
  Correctness is the ability of software products to perform their exact tasks, as defined by their specification.
  Correctness is the prime quality.
  設(shè)法保證正確性：Testing and debugging; Defensive programming such as typing and assertions(通過(guò)斷言在寫程序的時(shí)候就保證正確性而不是在調(diào)試時(shí)發(fā)現(xiàn)錯(cuò)誤); 通過(guò)形式化驗(yàn)證發(fā)現(xiàn)問(wèn)題
• Robustness
  Robustness is the ability of software systems to react appropriately to abnormal conditions.
  健壯性是對(duì)正確性的補(bǔ)充
• Extendibility
  規(guī)模越大，擴(kuò)展起來(lái)越困難
• Reusability
  一次開(kāi)發(fā)，多次使用。找到軟件系統(tǒng)中共性的部分，將它們抽取出來(lái)。
• Compatibility(兼容性)
  不同軟件系統(tǒng)之間互相集成是否容易。軟件系統(tǒng)都不是孤立存在的，需要和其他系統(tǒng)進(jìn)行交互。難點(diǎn)：不同軟件有不同的設(shè)定，例如，不同操作系統(tǒng)中有著互相不兼容的文件格式。解決方法：標(biāo)準(zhǔn)化。
• Efficiency
  包括時(shí)間效率和空間效率。
  Efficiency does not matter unless the software is correct.
  正確性是性能的前提。
  過(guò)度的優(yōu)化可能導(dǎo)致軟件不再適應(yīng)變化和復(fù)用。
• Portability(可移植性)
  軟件可方便的在不同的技術(shù)環(huán)境間移植。
• Ease of use(易用性)
  容易學(xué)、安裝、操作、監(jiān)控
• Functionality(功能性)
  Functionality is the extent of possibilities provided by a system.
  每增加一點(diǎn)點(diǎn)小功能，都要確保其他質(zhì)量指標(biāo)不受到損失。
• Timeliness(及時(shí)性)
  Timeliness is the ability of a software system to be released when or before its users want it.
  軟件產(chǎn)品出現(xiàn)的太晚，可能會(huì)錯(cuò)過(guò)它的受眾群體。
• Others: Verifiability(可驗(yàn)證性), Integrity(完整性), Repairability(可修復(fù)性), Economy(經(jīng)濟(jì)性)

Internal quality factors

• Readability
• Understandability
• Clearness
• Size

Trade off between quality properties (折中)

如果對(duì)efficiency有極致的要求，那么這將手具體機(jī)器型號(hào)的約束，軟件的portability就會(huì)受影響。
經(jīng)濟(jì)性(economy)和reusability也有沖突。
timeliness和extendibility也有沖突。
正確性(Correctness) 絕不能與其他質(zhì)量指標(biāo)沖突。

Summary

描述軟件系統(tǒng)的三個(gè)維度：
按階段分：build-time & run-time views
按動(dòng)態(tài)性分：moment & period views
按層次分：code & component views

軟件構(gòu)造的本質(zhì)是視圖之間的轉(zhuǎn)化：
nothing => Code
Code => Component
Build-time => Run-time
Moment => Period

質(zhì)量指標(biāo)：
external & internal quality factors
important external quality factors
tradeoff

五個(gè)質(zhì)量目標(biāo)：
easy to understand
ready for change
cheap for develop
safe from bugs
efficient to run

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