1. JAVA设计模式
1.1. 创建型模式
1.1.1. Abstract Factory—抽象工厂模式
追MM少不了请吃饭了，麦当劳的鸡翅和肯德基的鸡翅都是MM爱吃的东西，虽然口味有所不同，但不管你带MM去麦当劳或肯德基，只管向服务员说“来四个鸡翅”就行了。麦当劳和肯德基就是生产鸡翅的Factory
客户类和工厂类分开。消费者任何时候需要某种产品，只需向工厂请求即可。消费者无须修改就可以接纳新产品。缺点是当产品修改时，工厂类也要做相应的修改。如：如何创建及如何向客户端提供。
1.1.2. Builder—建造模式
MM最爱听的就是“我爱你”这句话了，见到不同地方的MM,要能够用她们的方言跟她说这句话哦，我有一个多种语言翻译机，上面每种语言都有一个按键，见到MM我只要按对应的键，它就能够用相应的语言说出“我爱你”这句话了，国外的MM也可以轻松搞掂，这就是我的“我爱你”builder。（这一定比美军在伊拉克用的翻译机好卖）
将产品的内部表象和产品的生成过程分割开来，从而使一个建造过程生成具有不同的内部表象的产品对象。建造模式使得产品内部表象可以独立的变化，客户不必知道产品内部组成的细节。建造模式可以强制实行一种分步骤进行的建造过程。
1.1.3. Factory Method—工厂方法模式
请MM去麦当劳吃汉堡，不同的MM有不同的口味，要每个都记住是一件烦人的事情，我一般采用Factory Method模式，带着MM到服务员那儿，说“要一个汉堡”，
具体要什么样的汉堡呢，让MM直接跟服务员说就行了。
核心工厂类不再负责所有产品的创建，而是将具体创建的工作交给子类去做，成为一个抽象工厂角色，仅负责给出具体工厂类必须实现的接口，而不接触哪一个产品类应当被实例化这种细节。
1.1.4. Prototype—原始模型模式
跟MM用QQ聊天，一定要说些深情的话语了，我搜集了好多肉麻的情话，需要时只要copy出来放到QQ里面就行了，这就是我的情话prototype了。（100块钱一份，你要不要）
通过给出一个原型对象来指明所要创建的对象的类型，然后用复制这个原型对象的方法创建出更多同类型的对象。原始模型模式允许动态的增加或减少产品类，产品类不需要非得有任何事先确定的等级结构，原始模型模式适用于任何的等级结构。缺点是每一个类都必须配备一个克隆方法。
1.1.5. Singleton—单例模式
俺有6个漂亮的老婆，她们的老公都是我，我就是我们家里的老公Sigleton，她们只要说道“老公”，都是指的同一个人，那就是我(刚才做了个梦啦，哪有这么好的事)
单例模式确保某一个类只有一个实例，而且自行实例化并向整个系统提供这个实例单例模式。单例模式只应在有真正的“单一实例”的需求时才可使用
1.2. 结构型模式
1.2.1. Adapter—适配器（变压器）模式
在朋友聚会上碰到了一个美女Sarah，从香港来的，可我不会说粤语，她不会说普通话，只好求助于我的朋友kent了，他作为我和Sarah之间的Adapter，让我和Sarah可以相互交谈了(也不知道他会不会耍我)
把一个类的接口变换成客户端所期待的另一种接口，从而使原本因接口原因不匹配而无法一起工作的两个类能够一起工作。适配类可以根据参数返还一个合适的实例给客户端。
1.2.2. Bridge—桥梁模式
早上碰到MM，要说早上好，晚上碰到MM，要说晚上好；碰到MM穿了件新衣服，要说你的衣服好漂亮哦，碰到MM新做的发型，要说你的头发好漂亮哦。不要问我“早上碰到MM新做了个发型怎么说”这种问题，自己用BRIDGE组合一下不就行了
将抽象化与实现化脱耦，使得二者可以独立的变化，也就是说将他们之间的强关联变成弱关联，也就是指在一个软件系统的抽象化和实现化之间使用组合/聚合关系而不是继承关系，从而使两者可以独立的变化。
1.2.3. Composite—合成模式
Mary今天过生日。
“我过生日，你要送我一件礼物。”
“嗯，好吧，去商店，你自己挑。”
“这件T恤挺漂亮，买，这条裙子好看，买，这个包也不错，买。”
“喂，买了三件了呀，我只答应送一件礼物的哦。”
“什么呀，T恤加裙子加包包，正好配成一套呀，小姐，麻烦你包起来。”
“„„”，
MM都会用Composite模式了，你会了没有？
合成模式将对象组织到树结构中，可以用来描述整体与部分的关系。合成模式就是一个处理对象的树结构的模式。合成模式把部分与整体的关系用树结构表示出来。合成模式使得客户端把一个个单独的成分对象和由他们复合而成的合成对象同等看待。
1.2.4. Decorator—装饰模式
Mary过完轮到Sarly过生日，还是不要叫她自己挑了，不然这个月伙食费肯定玩完，拿出我去年在华山顶上照的照片，在背面写上“最好的的礼物，就是爱你的Fita”，再到街上礼品店买了个像框（卖礼品的MM也很漂亮哦），再找隔壁搞美术设计的Mike
设计了一个漂亮的盒子装起来„„，我们都是Decorator，最终都在修饰我这个人呀，怎么样，看懂了吗？
装饰模式以对客户端透明的方式扩展对象的功能，是继承关系的一个替代方案，提供比继承更多的灵活性。动态给一个对象增加功能，这些功能可以再动态的撤消。增加由一些基本功能的排列组合而产生的非常大量的功能。
1.2.5. Facade—门面模式
我有一个专业的Nikon相机，我就喜欢自己手动调光圈、快门，这样照出来的照片才专业，但MM可不懂这些，教了半天也不会。幸好相机有Facade设计模式，把相机调整到自动档，只要对准目标按快门就行了，一切由相机自动调整，这样MM也可以用这个相机给我拍张照片了。
外部与一个子系统的通信必须通过一个统一的门面对象进行。门面模式提供一个高层次的接口，使得子系统更易于使用。每一个子系统只有一个门面类，而且此门面类只有一个实例，也就是说它是一个单例模式。但整个系统可以有多个门面类。
1.2.6. Flyweight—享元模式
每天跟MM发短信，手指都累死了，最近买了个新手机，可以把一些常用的句子存在手机里，要用的时候，直接拿出来，在前面加上MM的名字就可以发送了，再不用一个字一个字敲了。共享的句子就是Flyweight，MM的名字就是提取出来的外部特征，根据上下文情况使用。
FLYWEIGHT在拳击比赛中指最轻量级。享元模式以共享的方式高效的支持大量的细粒度对象。享元模式能做到共享的关键是区分内蕴状态和外蕴状态。内蕴状态存储在享元内部，不会随环境的改变而有所不同。外蕴状态是随环境的改变而改变的。外蕴状态不能影响内蕴状态，它们是相互独立的。将可以共享的状态和不可以共享的状态从常规类中区分开来，将不可以共享的状态从类里剔除出去。客户端不可以直接创建被共享的对象，而应当使用一个工厂对象负责创建被共享的对象。享元模式大幅度的降低内存中对象的数量。
1.2.7. Proxy—代理模式
跟MM在网上聊天，一开头总是“hi,你好”,“你从哪儿来呀？”“你多大了？”“身高多少呀？”这些话，真烦人，写个程序做为我的Proxy吧，凡是接收到这些话都设置好了自动的回答，接收到其他的话时再通知我回答，怎么样，酷吧。
代理模式给某一个对象提供一个代理对象，并由代理对象控制对源对象的引用。代理就是一个人或一个机构代表另一个人或者一个机构采取行动。某些情况下，客户不想或者不能够直接引用一个对象，代理对象可以在客户和目标对象直接起到中介的作用。客户端分辨不出代理主题对象与真实主题对象。代理模式可以并不知道真正的被代理对象，而仅仅持有一个被代理对象的接口，这时候代理对象不能够创建被代理
对象，被代理对象必须有系统的其他角色代为创建并传入。
1.3. 行为型模式
1.3.1. Chain Of Responsibility—责任链模式
晚上去上英语课，为了好开溜坐到了最后一排，哇，前面坐了好几个漂亮的MM哎，找张纸条，写上“Hi,可以做我的女朋友吗？如果不愿意请向前传”，纸条就一个接一个的传上去了，糟糕，传到第一排的MM把纸条传给老师了，听说是个老处女呀，快跑!
在责任链模式中，很多对象由每一个对象对其下家的引用而接起来形成一条链。请求在这个链上传递，直到链上的某一个对象决定处理此请求。客户并不知道链上的哪一个对象最终处理这个请求，系统可以在不影响客户端的情况下动态的重新组织链和分配责任。处理者有两个选择：承担责任或者把责任推给下家。一个请求可以最终不被任何接收端对象所接受。
1.3.2. Command—命令模式
俺有一个MM家里管得特别严，没法见面，只好借助于她弟弟在我们俩之间传送信息，她对我有什么指示，就写一张纸条让她弟弟带给我。这不，她弟弟又传送过来一个COMMAND，为了感谢他，我请他吃了碗杂酱面，哪知道他说：“我同时给我姐姐三个男朋友送COMMAND，就数你最小气，才请我吃面。”，
命令模式把一个请求或者操作封装到一个对象中。命令模式把发出命令的责任
和执行命令的责任分割开，委派给不同的对象。命令模式允许请求的一方和发送的一方独立开来，使得请求的一方不必知道接收请求的一方的接口，更不必知道请求是怎么被接收，以及操作是否执行，何时被执行以及是怎么被执行的。系统支持命令的撤消。
1.3.3. Interpreter—解释器模式
俺有一个《泡MM真经》，上面有各种泡MM的攻略，比如说去吃西餐的步骤、去看电影的方法等等，跟MM约会时，只要做一个Interpreter，照着上面的脚本执行就可以了。
给定一个语言后，解释器模式可以定义出其文法的一种表示，并同时提供一个解释器。客户端可以使用这个解释器来解释这个语言中的句子。解释器模式将描述怎样在有了一个简单的文法后，使用模式设计解释这些语句。在解释器模式里面提到的语言是指任何解释器对象能够解释的任何组合。在解释器模式中需要定义一个代表文法的命令类的等级结构，也就是一系列的组合规则。每一个命令对象都有一个解释方法，代表对命令对象的解释。命令对象的等级结构中的对象的任何排列组合都是一个语言。
1.3.4. Iterator—迭代子模式
我爱上了Mary，不顾一切的向她求婚。
Mary：“想要我跟你结婚，得答应我的条件”
我：“什么条件我都答应，你说吧”
Mary：“我看上了那个一克拉的钻石”
我：“我买，我买，还有吗？”
Mary：“我看上了湖边的那栋别墅”
我：“我买，我买，还有吗？”
Mary：“我看上那辆法拉利跑车”
我脑袋嗡的一声，坐在椅子上，一咬牙：“我买，我买，还有吗？”
迭代子模式可以顺序访问一个聚集中的元素而不必暴露聚集的内部表象。多个对象聚在一起形成的总体称之为聚集，聚集对象是能够包容一组对象的容器对象。迭代子模式将迭代逻辑封装到一个独立的子对象中，从而与聚集本身隔开。迭代子模式简化了聚集的界面。每一个聚集对象都可以有一个或一个以上的迭代子对象，每一个迭代子的迭代状态可以是彼此独立的。迭代算法可以独立于聚集角色变化。
1.3.5. Mediator—调停者模式
四个MM打麻将，相互之间谁应该给谁多少钱算不清楚了，幸亏当时我在旁边，按照各自的筹码数算钱，赚了钱的从我这里拿，赔了钱的也付给我，一切就OK啦，俺得到了四个MM的电话。
调停者模式包装了一系列对象相互作用的方式，使得这些对象不必相互明显作用。从而使他们可以松散偶合。当某些对象之间的作用发生改变时，不会立即影响其他的一些对象之间的作用。保证这些作用可以彼此独立的变化。调停者模式将多对多的相互作用转化为一对多的相互作用。调停者模式将对象的行为和协作抽象化，把对象在小尺度的行为上与其他对象的相互作用分开处理。
1.3.6. Memento—备忘录模式
同时跟几个MM聊天时，一定要记清楚刚才跟MM说了些什么话，不然MM发现了会不高兴的哦，幸亏我有个备忘录，刚才与哪个MM说了什么话我都拷贝一份放到备忘录里面保存，这样可以随时察看以前的记录啦。
备忘录对象是一个用来存储另外一个对象内部状态的快照的对象。备忘录模式的用意是在不破坏封装的条件下，将一个对象的状态捉住，并外部化，存储起来，从而可以在将来合适的时候把这个对象还原到存储起来的状态。
1.3.7. Observer—观察者模式
想知道咱们公司最新MM情报吗？加入公司的MM情报邮件组就行了，tom负责搜集情报，他发现的新情报不用一个一个通知我们，直接发布给邮件组，我们作为订阅者（观察者）就可以及时收到情报啦
观察者模式定义了一种一队多的依赖关系，让多个观察者对象同时监听某一个主题对象。这个主题对象在状态上发生变化时，会通知所有观察者对象，使他们能够自动更新自己。
1.3.8. State—状态模式
跟MM交往时，一定要注意她的状态哦，在不同的状态时她的行为会有不同，比如你约她今天晚上去看电影，对你没兴趣的MM就会说“有事情啦”，对你不讨厌但还没喜欢上的MM就会说“好啊，不过可以带上我同事么？”，已经喜欢上你的MM就会说“几点钟？看完电影再去泡吧怎么样？”，当然你看电影过程中表现良好的话，
也可以把MM的状态从不讨厌不喜欢变成喜欢哦。
状态模式允许一个对象在其内部状态改变的时候改变行为。这个对象看上去象是改变了它的类一样。状态模式把所研究的对象的行为包装在不同的状态对象里，每一个状态对象都属于一个抽象状态类的一个子类。状态模式的意图是让一个对象在其内部状态改变的时候，其行为也随之改变。状态模式需要对每一个系统可能取得的状态创立一个状态类的子类。当系统的状态变化时，系统便改变所选的子类。
1.3.9. Strategy—策略模式
跟不同类型的MM约会，要用不同的策略，有的请电影比较好，有的则去吃小吃效果不错，有的去海边浪漫最合适，单目的都是为了得到MM的芳心，我的追MM锦囊中有好多Strategy哦。
策略模式针对一组算法，将每一个算法封装到具有共同接口的独立的类中，从而使得它们可以相互替换。策略模式使得算法可以在不影响到客户端的情况下发生变化。策略模式把行为和环境分开。环境类负责维持和查询行为类，各种算法在具体的策略类中提供。由于算法和环境独立开来，算法的增减，修改都不会影响到环境和客户端。
1.3.10. Template Method—模板方法模式
看过《如何说服女生上床》这部经典文章吗？女生从认识到上床的不变的步骤分为巧遇、打破僵局、展开追求、接吻、前戏、动手、爱抚、进去八大步骤(Template method)，但每个步骤针对不同的情况，都有不一样的做法，这就要看你随机应变啦(具体实现)；
模板方法模式准备一个抽象类，将部分逻辑以具体方法以及具体构造子的形式实现，然后声明一些抽象方法来迫使子类实现剩余的逻辑。不同的子类可以以不同的方式实现这些抽象方法，从而对剩余的逻辑有不同的实现。先制定一个顶级逻辑框架，而将逻辑的细节留给具体的子类去实现。
1.3.11. Visitor—访问者模式
情人节到了，要给每个MM送一束鲜花和一张卡片，可是每个MM送的花都要针对她个人的特点，每张卡片也要根据个人的特点来挑，我一个人哪搞得清楚，还是找花店老板和礼品店老板做一下Visitor，让花店老板根据MM的特点选一束花，让礼品店老板也根据每个人特点选一张卡，这样就轻松多了；
访问者模式的目的是封装一些施加于某种数据结构元素之上的操作。一旦这些
操作需要修改的话，接受这个操作的数据结构可以保持不变。访问者模式适用于数据结构相对未定的系统，它把数据结构和作用于结构上的操作之间的耦合解脱开，使得操作集合可以相对自由的演化。访问者模式使得增加新的操作变的很容易，就是增加一个新的访问者类。访问者模式将有关的行为集中到一个访问者对象中，而不是分散到一个个的节点类中。当使用访问者模式时，要将尽可能多的对象浏览逻辑放在访问者类中，而不是放到它的子类中。访问者模式可以跨过几个类的等级结构访问属于不同的等级结构的成员类。
2. J2EE设计模式
2.1. 表示层模式
2.1.1. Intercepting Filter—拦截过滤器模式
Context
The presentation-tier request handling mechanism receives many different types of requests, which require varied types of processing. Some requests are simply forwarded to the appropriate handler component, while other requests
must be modified, audited, or uncompressed before being further processed.
Problem
Preprocessing and post-processing of a client Web request and response are required.
When a request enters a Web application, it often must pass several entrance tests prior to the main processing stage. For example,
Has the client been authenticated?
Does the client have a valid session?
Is the client's IP address from a trusted network?
Does the request path violate any constraints?
What encoding does the client use to send the data?
Do we support the browser type of the client?
Some of these checks are tests, resulting in a yes or no answer that determines whether processing will continue. Other checks manipulate the incoming data stream into a form suitable for processing.
The classic solution consists of a series of conditional checks, with any failed check aborting the request. Nested if/else statements are a standard strategy, but this solution leads to code fragility and a copy-and-paste style of programming, because the
flow of the filtering and the action of the filters is compiled into the application.
The key to solving this problem in a flexible and unobtrusive manner is to have a simple mechanism for adding and removing processing components, in which each component completes a specific filtering action.
Forces
Common processing, such as checking the data-encoding scheme or logging information about each request, completes per request.
Centralization of common logic is desired.
Services should be easy to add or remove unobtrusively without affecting existing components, so that they can be used in a variety of combinations, such as
Logging and authentication
Debugging and transformation of output for a specific client
Uncompressing and converting encoding scheme of input
Solution
Create pluggable filters to process common services in a standard manner without requiring changes to core request processing code. The filters intercept incoming requests and outgoing responses, allowing preprocessing and post-processing.
We are able to add and remove these filters unobtrusively, without requiring changes to our existing code.
We are able, in effect, to decorate our main processing with a variety of common services, such as security, logging, debugging, and so forth. These filters are components
that are independent of the main application code, and they may be added or removed declaratively. For example, a deployment configuration file may be modified to set up a chain of filters. The same configuration file might include a mapping of specific URLs
to this filter chain. When a client requests a resource that matches this configured URL mapping, the filters in the chain are each processed in order before the requested target resource is invoked.
2.1.2. Front Controller—前端控制器模式
Context
The presentation-tier request handling mechanism must control and coordinate processing of each user across multiple requests. Such control mechanisms may be managed in either a centralized or decentralized manner.
Problem
The system requires a centralized access point for presentation-tier request handling to support the integration of system services, content retrieval, view management, and navigation. When the user accesses the view directly without
going through a centralized mechanism, two problems may occur:
Each view is required to provide its own system services, often resulting in duplicate code.
View navigation is left to the views. This may result in commingled view content and view navigation.
Additionally, distributed control is more difficult to maintain, since changes will often need to be made in numerous places.
Forces
Common system services processing completes per request. For example, the security service completes authentication and authorization checks.
Logic that is best handled in one central location is instead replicated within numerous views.
Decision points exist with respect to the retrieval and manipulation of data.
Multiple views are used to respond to similar business requests.
A centralized point of contact for handling a request may be useful, for example, to control and log a user's progress through the site.
System services and view management logic are relatively sophisticated.
Solution
Use a controller as the initial point of contact for handling a request. The controller manages the handling of the request, including invoking security services such as authentication and authorization, delegating business processing,
managing the choice of an appropriate view, handling errors, and managing the selection of content creation strategies.
The controller provides a centralized entry point that controls and manages Web request handling. By centralizing decision points and controls, the controller also helps reduce the amount of Java code, called scriptlets, embedded in the JavaServer Pages (JSP)
page.
Centralizing control in the controller and reducing business logic in the view promotes code reuse across requests. It is a preferable approach to the alternative-embedding code in multiple views-because that approach may lead to a more error-prone, reuse-by-copy-
and-paste environment.
Typically, a controller coordinates with a dispatcher component. Dispatchers are responsible for view management and navigation. Thus, a dispatcher chooses the next view for the user and vectors control to the resource. Dispatchers may be encapsulated within
the controller directly or can be extracted into a separate component.
While the Front Controller pattern suggests centralizing the handling of all requests, it does not limit the number of handlers in the system, as does a Singleton. An application may use multiple controllers in a system, each mapping to a set of distinct services.
2.1.3. View Helper—视图助手模式
Context
The system creates presentation content, which requires processing of dynamic business data.
Problem
Presentation tier changes occur often and are difficult to develop and maintain when business data access logic and presentation formatting logic are interwoven. This makes the system less flexible, less reusable, and generally
less resilient to change.
Intermingling the business and systems logic with the view processing reduces modularity and also provides a poor separation of roles among Web production and software development teams.
Forces
Business data assimilation requirements are nontrivial.
Embedding business logic in the view promotes a copy-and-paste type of reuse. This causes maintenance problems and bugs because a piece of logic is reused in the same or different view by simply duplicating it in the new location.
It is desirable to promote a clean separation of labor by having different individuals fulfill the roles of software developer and Web production team member.
One view is commonly used to respond to a particular business request.
Solution
A view contains formatting code, delegating its processing responsibilities to its helper classes, implemented as JavaBeans or custom tags. Helpers also store the view's intermediate data model and serve as business data adapters.
There are multiple strategies for implementing the view component. The JSP View Strategy suggests using a JSP as the view component. This is the preferred strategy, and it is the one most commonly used. The other principal strategy is the Servlet View Strategy,
which utilizes a servlet as the view (see the section "Strategies" for more information).
Encapsulating business logic in a helper instead of a view makes our application more modular and facilitates component reuse. Multiple clients, such as controllers and views, may leverage the same helper to retrieve and adapt similar model state for presentation
in multiple ways. The only way to reuse logic embedded in a view is by copying and pasting it elsewhere. Furthermore, copy-and-paste duplication makes a system harder to maintain, since the same bug potentially needs to be corrected in multiple places.
A signal that one may need to apply this pattern to existing code is when scriptlet code dominates the JSP view. The overriding goal when applying this pattern, then, is the partitioning of business logic outside of the view. While some logic is best encapsulated
within helper objects, other logic is better placed in a centralized component that sits in front of the views and the helpers-this might include logic that is common across multiple requests, such as authentication checks or logging services, for example.
Refer to the "Intercepting Filter" on page 4 and "Front Controller" on page 21 for more information on these issues.
If a separate controller is not employed in the architecture, or is not used to handle all requests, then the view component becomes the initial contact point for handling some requests. For certain requests, particularly those involving minimal processing,
this scenario works fine. Typically, this situation occurs for pages that are based on static information, such as the first of a set of pages that will be served to a user to gather some information (see "Dispatcher View" on page 232). Additionally, this
scenario occurs in some cases when a mechanism is employed to create composite pages (see "Composite View" on page 203).
The View Helper pattern focuses on recommending ways to partition your application responsibilities. For related discussions about issues dealing with directing client requests directly to a view, please refer to the section "Dispatcher View" on page 232.
2.1.4. Composite View—复合视图模式
Context
Sophisticated Web pages present content from numerous data sources, using multiple subviews that comprise a single display page. Additionally, a variety of individuals with different skill sets contribute to the development and
maintenance of these Web pages.
Problem
Instead of providing a mechanism to combine modular, atomic portions of a view into a composite whole, pages are built by embedding formatting code directly within each view.
Modification to the layout of multiple views is difficult and error prone, due to the duplication of code.
Forces
Atomic portions of view content change frequently.
Multiple composite views use similar subviews, such as a customer inventory table. These atomic portions are decorated with different surrounding template text, or they appear in a different location within the page.
Layout changes are more difficult to manage and code harder to maintain when subviews are directly embedded and duplicated in multiple views.
Embedding frequently changing portions of template text directly into views also potentially affects the availability and administration of the system. The server may need to be restarted before clients see the modifications or updates to these template components.
Solution
Use composite views that are composed of multiple atomic subviews. Each component of the template may be included dynamically into the whole and the layout of the page may be managed independently of the content.
This solution provides for the creation of a composite view based on the inclusion and substitution of modular dynamic and static template fragments. It promotes the reuse of atomic portions of the view by encouraging modular design. It is appropriate to use
a composite view to generate pages containing display components that may be combined in a variety of ways. This scenario occurs, for example, with portal sites that include numerous independent subviews, such as news feeds, weather information, and stock
quotes on a single page. The layout of the page is managed and modified independent of the subview content.
Another benefit of this pattern is that Web designers can prototype the layout of a site, plugging static content into each of the template regions. As site development progresses, the actual content is substituted for these placeholders.
This pattern is not without its drawbacks. There is a runtime overhead associated with it, a tradeoff for the increased flexibility that it provides. Also, the use of a more sophisticated layout mechanism brings with it some manageability and development issues,
since there are more artifacts to maintain and a level of implementation indirection to
understand.
2.1.5. Service to Worker—工作者服务模式
Context
The system controls flow of execution and access to business data, from which it creates presentation content.
Note
The Service to Worker pattern, like the Dispatcher View pattern, describes a common combination of other patterns from the catalog. Both of these macro patterns describe the combination of a controller and dispatcher with views
and helpers. While describing this common structure, they emphasize related but different usage patterns.
Problem
The problem is a combination of the problems solved by the Front Controller and View Helper patterns in the presentation tier. There is no centralized component for managing access control, content retrieval, or view management,
and there is duplicate control code scattered throughout various views. Additionally, business logic and presentation formatting logic are intermingled within these views, making the system less flexible, less reusable, and generally less resilient to change.
Intermingling business logic with view processing also reduces modularity and provides a poor separation of roles among Web production and software development teams.
Forces
Authentication and authorization checks are completed per request.
Scriptlet code within views should be minimized.
Business logic should be encapsulated in components other than the view.
Control flow is relatively complex and based on values from dynamic content.
View management logic is relatively sophisticated, with multiple views potentially mapping to the same request.
Solution
Combine a controller and dispatcher with views and helpers (see "Front Controller" on page 172 and "View Helper" on page 186) to handle client requests and prepare a dynamic presentation as the response. Controllers delegate content
retrieval to helpers,
which manage the population of the intermediate model for the view. A dispatcher is responsible for view management and navigation and can be encapsulated either within a controller or a separate component.
Service to Worker describes the combination of the Front Controller and View Helper patterns with a dispatcher component.
While this pattern and the Dispatcher View pattern describe a similar structure, the two patterns suggest a different division of labor among the components. In Service to Worker, the controller and the dispatcher have more responsibilities.
Since the Service to Worker and Dispatcher View patterns represent a common combination of other patterns from the catalog, each warrants its own name to promote efficient communication among developers. Unlike the Service to Worker pattern, the Dispatcher
View pattern suggests deferring content retrieval to the time of view processing.
In the Dispatcher View pattern, the dispatcher typically plays a limited to moderate role in view management. In the Service to Worker pattern, the dispatcher typically plays a moderate to large role in view management.
A limited role for the dispatcher occurs when no outside resources are utilized in order to choose the view. The information encapsulated in the request is sufficient to determine the view to dispatch the request. For example,
http://some.server.com/servlet/Controller?next=login.jsp
The sole responsibility of the dispatcher component in this case is to dispatch to the view login.jsp.
An example of the dispatcher playing a moderate role is the case where the client submits a request directly to a controller with a query parameter that describes an action to be completed:
http://some.server.com/servlet/Controller?action=login
The responsibility of the dispatcher component here is to translate the logical name login into the resource name of an appropriate view, such as login.jsp, and dispatch to that view. To accomplish this translation, the dispatcher
may access resources such as an XML configuration file that specifies the appropriate view to display.
On the other hand, in the Service to Worker pattern, the dispatcher might be more sophisticated. The dispatcher may invoke a business service to determine the appropriate view to display.
The shared structure of Service to Worker and Dispatcher View consists of a controller working with a dispatcher, views, and helpers.
2.1.6. Dispatcher View—分发者视图模式
Context
System controls flow of execution and access to presentation processing, which is responsible for generating dynamic content.
Note
The Dispatcher View pattern, like the Service to Worker pattern, describes a common combination of other patterns from the catalog. Both of these macro patterns describe the combination of a controller and dispatcher with views
and helpers. While describing this common structure, they emphasize related but different usage patterns.
Problem
The problem is a combination of the problems solved by the Front Controller and View Helper patterns in the presentation tier. There is no centralized component for managing access control, content retrieval or view management,
and there is duplicate control code scattered throughout various views. Additionally, business logic and presentation formatting logic are intermingled within these views, making the system less flexible, less reusable, and generally less resilient to change.
Intermingling business logic with view processing also reduces modularity and provides a poor separation of roles among Web production and software development teams.
Forces
Authentication and authorization checks are completed per request.
Scriptlet code within views should be minimized.
Business logic should be encapsulated in components other than the view.
Control flow is relatively simple and is typically based on values encapsulated with the request.
View management logic is limited in complexity.
Solution
Combine a controller and dispatcher with views and helpers (see "Front Controller" on page 172 and "View Helper" on page 186) to handle client requests and prepare a dynamic presentation as the response. Controllers do not delegate
content retrieval to helpers, because these activities are deferred to the time of view processing. A dispatcher is responsible for view management and navigation and can be encapsulated either within a controller, a view, or a separate component.
Dispatcher View describes the combination of the Front Controller and View Helper patterns with a dispatcher component. While this pattern and the Service to Worker pattern describe a similar structure, the two patterns suggest a different division of labor
among the components. The controller and the dispatcher typically have limited responsibilities, as compared to the Service to Worker pattern, since the upfront processing and view management logic are basic. Furthermore, if centralized control of the underlying
resources is considered unnecessary, then the controller is removed and the dispatcher may be moved into a view.
Since the Service to Worker and Dispatcher View patterns represent a common combination of other patterns from the catalog, each warrants its own name to promote
efficient communication among developers. Unlike the Service to Worker pattern, the Dispatcher View pattern suggests deferring content retrieval to the time of view processing.
In the Dispatcher View pattern, the dispatcher typically plays a limited to moderate role in view management. In the Service to Worker pattern, the dispatcher typically plays a moderate to large role in view management.
A limited role for the dispatcher occurs when no outside resources are utilized in order to choose the view. The information encapsulated in the request is sufficient to determine the view to dispatch the request. For example:
http://some.server.com/servlet/Controller?next=login.jsp
The sole responsibility of the dispatcher component in this case is to dispatch to the view login.jsp.
An example of the dispatcher playing a moderate role is the case where the client submits a request directly to a controller with a query parameter that describes an action to be completed:
http://some.server.com/servlet/Controller?action=login
The responsibility of the dispatcher component here is to translate the logical name login into the resource name of an appropriate view, such as login.jsp, and dispatch to that view. To accomplish this translation, the dispatcher
may access resources such as an XML configuration file that specifies the appropriate view to display.
On the other hand, in the Service to Worker pattern, the dispatcher might be more sophisticated. The dispatcher may invoke a business service to determine the appropriate view to display.
The shared structure of these two patterns, as mentioned above, consists of a controller working with a dispatcher, views, and helpers.
2.2. 业务层模式
2.2.1. Business Delegate—业务委托模式
Context
A multi-tiered, distributed system requires remote method invocations to send and receive data across tiers. Clients are exposed to the complexity of dealing with distributed components.
Problem
Presentation-tier components interact directly with business services. This direct interaction exposes the underlying implementation details of the business service application program interface (API) to the presentation tier.
As a result, the presentation-tier components are vulnerable to changes in the implementation of the business services: When the implementation of the business services change, the exposed implementation code in the presentation tier must change too.
Additionally, there may be a detrimental impact on network performance because presentation-tier components that use the business service API make too many invocations over the network. This happens when presentation-tier components use the underlying API directly,
with no client-side caching mechanism or aggregating service.
Lastly, exposing the service APIs directly to the client forces the client to deal with the networking issues associated with the distributed nature of Enterprise JavaBeans (EJB) technology.
Forces
Presentation-tier clients need access to business services.
Different clients, such as devices, Web clients, and thick clients, need access to business service.
Business services APIs may change as business requirements evolve.
It is desirable to minimize coupling between presentation-tier clients and the business service, thus hiding the underlying implementation details of the service, such as lookup and access.
Clients may need to implement caching mechanisms for business service information.
It is desirable to reduce network traffic between client and business services.
Solution
Use a Business Delegate to reduce coupling between presentation-tier clients and business services. The Business Delegate hides the underlying implementation details of the business service, such as lookup and access details of
the EJB architecture.
The Business Delegate acts as a client-side business abstraction; it provides an abstraction for, and thus hides, the implementation of the business services. Using a Business Delegate reduces the coupling between presentation-tier clients and the system's
business services. Depending on the implementation strategy, the Business Delegate may shield clients from possible volatility in the implementation of the business service API. Potentially, this reduces the number of changes that must be made to the presentation-tier
client code when the business service API or its underlying implementation changes.
However, interface methods in the Business Delegate may still require modification if the underlying business service API changes. Admittedly, though, it is more likely that changes will be made to the business service rather than to the Business Delegate.
Often, developers are skeptical when a design goal such as abstracting the business layer causes additional upfront work in return for future gains. However, using this pattern or its strategies results in only a small amount of additional upfront work and
provides considerable benefits. The main benefit is hiding the details of the underlying service. For example, the client can become transparent to naming and lookup services. The Business Delegate also handles the exceptions from the business services, such
as java.rmi.Remote exceptions, Java Messages Service (JMS) exceptions and so on. The Business Delegate may
intercept such service level exceptions and generate application level exceptions instead. Application level exceptions are easier to handle by the clients, and may be user friendly. The Business Delegate may also transparently perform any retry or recovery
operations necessary in the event of a service failure without exposing the client to the problem until it is determined that the problem is not resolvable. These gains present a compelling reason to use the pattern.
Another benefit is that the delegate may cache results and references to remote business services. Caching can significantly improve performance, because it limits unnecessary and potentially costly round trips over the network.
A Business Delegate uses a component called the Lookup Service. The Lookup Service is responsible for hiding the underlying implementation details of the business service lookup code. The Lookup Service may be written as part of the Delegate, but we recommend
that it be implemented as a separate component, as outlined in the Service Locator pattern (See "Service Locator" on page 368.)
When the Business Delegate is used with a Session Facade, typically there is a one-to-one relationship between the two. This one-to-one relationship exists because logic that might have been encapsulated in a Business Delegate relating to its interaction with
multiple business services (creating a one-to-many relationship) will often be factored back into a Session Facade.
Finally, it should be noted that this pattern could be used to reduce coupling between other tiers, not simply the presentation and the business tiers.
2.2.2. Transfer Object—传输对象模式
Context
Application clients need to exchange data with enterprise beans.
Problem
Java 2 Platform, Enterprise Edition (J2EE) applications implement server-side business components as session beans and entity beans. Some methods exposed by the business components return data to the client. Often, the client
invokes a business object's get methods multiple times until it obtains all the attribute values.
Session beans represent the business services and are not shared between users. A session bean provides coarse-grained service methods when implemented per the Session Facade pattern.
Entity beans, on the other hand, are multiuser, transactional objects representing
persistent data. An entity bean exposes the values of attributes by providing an accessor method (also referred to as a getter or get method) for each attribute it wishes to expose.
Every method call made to the business service object, be it an entity bean or a session bean, is potentially remote. Thus, in an Enterprise JavaBeans (EJB) application such remote invocations use the network layer regardless of the proximity of the client
to the bean, creating a network overhead. Enterprise bean method calls may permeate the network layers of the system even if the client and the EJB container holding the entity bean are both running in the same JVM, OS, or physical machine. Some vendors may
implement mechanisms to reduce this overhead by using a more direct access approach and bypassing the network.
As the usage of these remote methods increases, application performance can significantly degrade. Therefore, using multiple calls to get methods that return single attribute values is inefficient for obtaining data values from an enterprise bean.
Forces
All access to an enterprise bean is performed via remote interfaces to the bean. Every call to an enterprise bean is potentially a remote method call with network overhead.
Typically, applications have a greater frequency of read transactions than update transactions. The client requires the data from the business tier for presentation, display, and other read-only types of processing. The client updates the data in the business
tier much less frequently than it reads the data.
The client usually requires values for more than one attribute or dependent object from an enterprise bean. Thus, the client may invoke multiple remote calls to obtain the required data.
The number of calls made by the client to the enterprise bean impacts network performance. Chattier applications-those with increased traffic between client and server tiers-often degrade network performance.
Solution
Use a Transfer Object to encapsulate the business data. A single method call is used to send and retrieve the Transfer Object. When the client requests the enterprise bean for the business data, the enterprise bean can construct
the Transfer Object, populate it with its attribute values, and pass it by value to the client.
Clients usually require more than one value from an enterprise bean. To reduce the number of remote calls and to avoid the associated overhead, it is best to use Transfer Objects to transport the data from the enterprise bean to its client.
When an enterprise bean uses a Transfer Object, the client makes a single remote method invocation to the enterprise bean to request the Transfer Object instead of numerous remote method calls to get individual attribute values. The enterprise bean then constructs
a new Transfer Object instance, copies values into the object and returns it to the client. The client receives the Transfer Object and can then invoke accessor (or getter) methods on the Transfer Object to get the individual attribute values from the Transfer
Object. Or, the implementation of the Transfer Object may be such that it makes all attributes public. Because the Transfer Object is passed by value to the client, all calls to the Transfer Object instance are local calls instead of remote method invocations.
2.2.3. Session Facade—会话门面模式
Context
Enterprise beans encapsulate business logic and business data and expose their interfaces, and thus the complexity of the distributed services, to the client tier.
Problem
In a multitiered Java 2 Platform, Enterprise Edition (J2EE) application environment, the following problems arise:
Tight coupling, which leads to direct dependence between clients and business objects;
Too many method invocations between client and server, leading to network performance problems;
Lack of a uniform client access strategy, exposing business objects to misuse.
A multitiered J2EE application has numerous server-side objects that are implemented as enterprise beans. In addition, some other arbitrary objects may provide services, data, or both. These objects are collectively referred to as business objects, since they
encapsulate business da