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UML（统一建模语言）是软件工程中用于建模系统的一种标准语言，它提供了一种图形化的表示方式，帮助分析和设计系统，理解系统的行为。UML经历了多个版本的迭代发展，最新规范UML 2.0对早期版本进行了重要的改进，以增强其对模型驱动架构（MDA）的支持，改善了在大型应用中的可用性，并且使得图表更加易于阅读。 UML 2.0的引入主要是为了使UML模型能够承载MDA的理念，这意味着UML必须进化成为一个更加以模型为中心的符号体系。同时，OMG（对象管理组织）也改善了UML 1.x的符号集，以提高大型应用项目的适用性。UML 1.x中用于建模逻辑流程的设计往往复杂且有时无法实现。在UML 2中对序列图符号集的改变，大大改进了在序列图中建模逻辑的能力。序列图是UML中的一个重要图示，主要用于展示对象之间交互的时序关系，即交互按照什么顺序发生。与类图一样，序列图是设计者在进行系统设计和分析时经常考虑的图之一。序列图能够展示参与交互的对象以及这些对象之间随时间变化的消息传递。在面向对象的程序设计中，序列图提供了一种可视化的手段来理解对象如何在特定场景或用例中合作，从而帮助设计者构建更为合理和高效的系统。 OMG采纳的UML 2.0规范仍然是草案状态，但这意味着规范已经足够稳定，以至于被这样一个不轻易接纳新标准的组织所采用。在UML 2完全被采纳之前，规范仍会有一些小的变动，但预计这些变动会是微小的。主要的变更将集中在UML的内部，涉及那些通常被软件公司用来实现UML工具的特征。序列图的主要作用是展示对象间按顺序发生的交互。通过对象间消息的传递，序列图可以清晰地描绘出在特定场景下系统的动态行为。这些消息包括调用操作、返回结果、创建和销毁对象等。序列图是分析和设计过程中不可或缺的一部分，特别是在处理涉及多个对象协作完成任务的复杂系统时。开发人员通常会利用序列图来梳理系统中各个对象之间的交互逻辑，尤其是对于那些并发处理或事务处理的复杂场景。通过序列图，设计者可以理解在特定操作序列中，对象之间是如何进行沟通的，哪些对象是主动的参与者，哪些是被动的响应者，以及对象间消息传递的具体时机。 UML 2.0的引入，特别是对于序列图的改进，增强了UML在模型化逻辑流程方面的能力。序列图通过清晰地展示对象之间的交互，帮助设计者验证和改进系统的动态行为。此外，UML 2.0还改进了其表达能力和灵活性，使得建模者可以更精确地描述复杂的交互过程。值得注意的是，虽然UML是一个国际标准，但它并不是唯一的建模语言。还有其他一些建模语言，如SysML和Petri网等，它们在特定的领域或对特定问题类型提供了特定的支持和优势。然而，UML由于其普遍性和标准化，它仍然是软件工程和系统分析领域内最广泛使用的建模语言之一。

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(784条消息) UML 基础: 序列图 _Everything is possible--博客的内容只是作为⼀种备忘-CSDN博客

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It's February, and by now you've probably read about, or heard people talk about, making the change to

UML 2.0--the new specification for UML that contains a number of improvements. Given the importance

of the new spec, we are changing the basis of this article series, too, shifting our attention from OMG's

UML 1.4 Specification to OMG's Adopted 2.0 Draft Specification of UML (a.k.a. UML 2). I hate to change

emphasis from 1.4 to 2.0 in the middle of a series of articles, but the UML 2.0 Draft Specification is an

important step forward, and I feel the need to spread the word.

There were a couple of reasons that the OMG improved UML. The main reason was that they wanted

UML models to be capable of delivering Model Driven Architecture (MDA), which meant that the UML

had to function as a more model driven notation. Also, the UML 1.x notation set was at times difficult to

apply to larger applications. Furthermore, the notation elements needed to be improved in order to make

diagrams more readable. (For example, modeling logical flow in UML 1.x was complicated and at times

impossible. Changes to the sequence diagram's notation set in UML 2 have made vast improvements in

modeling logic in sequences.)

Notice the wording in my statement above: "Adopted 2.0 Draft Specification of UML." It is true that the

specification is still in draft status, but the key is that the Draft Specification has been adopted by OMG, a

consortium that does not adopt new standards until they become pretty solid. There will be some

changes to the specification before UML 2 is completely adopted, but these changes should be minimal.

The main changes will be in the internals of UML--involving features typically used by software

companies who implement UML tools.

The main purpose of this article is to continue our focus on the essential UML diagrams; this month, we

take a close look at the sequence diagram. Please note, again, that the examples provided below are

based on the new UML 2 specification.

The diagram's purpose

The sequence diagram is used primarily to show the interactions between objects in the sequential order

that those interactions occur. Much like the class diagram, developers typically think sequence diagrams

were meant exclusively for them. However, an organization's business staff can find sequence diagrams

useful to communicate how the business currently works by showing how various business objects

interact. Besides documenting an organization's current affairs, a business-level sequence diagram can

be used as a requirements document to communicate requirements for a future system implementation.

During the requirements phase of a project, analysts can take use cases to the next level by providing a

more formal level of refinement. When that occurs, use cases are often refined into one or more

sequence diagrams.

An organization's technical staff can find sequence diagrams useful in documenting how a future system

should behave. During the design phase, architects and developers can use the diagram to force out the

2021/1/8

(784条消息) UML 基础: 序列图 _Everything is possible--博客的内容只是作为⼀种备忘-CSDN博客

https://blog.csdn.net/iteye_34/article/details/82554730 2/33

system's object interactions, thus fleshing out overall system design.

One of the primary uses of sequence diagrams is in the transition from requirements expressed as use

cases to the next and more formal level of refinement. Use cases are often refined into one or more

sequence diagrams. In addition to their use in designing new systems, sequence diagrams can be used

to document how objects in an existing (call it "legacy") system currently interact. This documentation is

very useful when transitioning a system to another person or organization.

The notation

Since this is the first article in my UML diagram series that is based on UML 2, we need to first discuss

an addition to the notation in UML 2 diagrams, namely a notation element called a frame. The frame

element is used as a basis for many other diagram elements in UML 2, but the first place most people will

encounter a frame element is as the graphical boundary of a diagram. A frame element provides a

consistent place for a diagram's label, while providing a graphical boundary for the diagram. The frame

element is optional in UML diagrams; as you can see in Figures 1 and 2, the diagram's label is placed in

the top left corner in what I'll call the frame's "namebox," a sort of dog-eared rectangle, and the actual

UML diagram is defined within the body of the larger enclosing rectangle.

Figure 1: An empty UML 2 frame element

In addition to providing a visual border, the frame element also has an important functional use in

diagrams depicting interactions, such as the sequence diagram. On sequence diagrams incoming and

outgoing messages (a.k.a. interactions) for a sequence can be modeled by connecting the messages to

the border of the frame element (as seen in Figure 2). This will be covered in more detail in the "Beyond

the basics" section below.

2021/1/8

(784条消息) UML 基础: 序列图 _Everything is possible--博客的内容只是作为⼀种备忘-CSDN博客

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modeled.

Lifelines are drawn as a box with a dashed line descending from the center of the bottom

edge (Figure 3). The lifeline's name is placed inside the box.

Figure 3: An example of the Student class used in a lifeline whose instance name is freshman

The UML standard for naming a lifeline follows the format of:

In the example shown in Figure 3, the lifeline represents an instance of the class Student, whose

instance name is freshman. Note that, here, the lifeline name is underlined. When an underline is used, it

means that the lifeline represents a specific instance of a class in a sequence diagram, and not a

particular kind of instance (i.e., a role). In a future article we'll look at structure modeling. For now, just

observe that sequence diagrams may include roles (such as buyer and seller) without specifying who

plays those roles (such as Billand Fred). This allows diagram reuse in different contexts. Simply put,

instance names in sequence diagrams are underlined; roles names are not.

Our example lifeline in Figure 3 is a named object, but not all lifelines represent named objects. Instead a

lifeline can be used to represent an anonymous or unnamed instance. When modeling an unnamed

instance on a sequence diagram, the lifeline's name follows the same pattern as a named instance; but

instead of providing an instance name, that portion of the lifeline's name is left blank. Again referring to

Figure 3, if the lifeline is representing an anonymous instance of the Student class, the lifeline would be: "

Student." Also, because sequence diagrams are used during the design phase of projects, it is

completely legitimate to have an object whose type is unspecified: for example, "freshman."

Messages

The first message of a sequence diagram always starts at the top and is typically located on the left side

of the diagram for readability. Subsequent messages are then added to the diagram slightly lower then

the previous message.

To show an object (i.e., lifeline) sending a message to another object, you draw a line to the receiving

object with a solid arrowhead (if a synchronous call operation) or with a stick arrowhead (if an

Instance Name : Class Name

2021/1/8

(784条消息) UML 基础: 序列图 _Everything is possible--博客的内容只是作为⼀种备忘-CSDN博客

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asynchronous signal). The message/method name is placed above the arrowed line. The message that

is being sent to the receiving object represents an operation/method that the receiving object's class

implements. In the example in Figure 4, the analyst object makes a call to the system object which is an

instance of the ReportingSystem class. The analyst object is calling the system object's

getAvailableReports method. The system object then calls the getSecurityClearance method with the

argument of userId on the secSystem object, which is of the class type SecuritySystem.

Figure 4: An example of messages being sent between objects

Besides just showing message calls on the sequence diagram, the Figure 4 diagram includes return

messages. These return messages are optional; a return message is drawn as a dotted line with an open

arrowhead back to the originating lifeline, and above this dotted line you place the return value from the

operation. In Figure 4 the secSystem object returns userClearance to the system object when the

getSecurityClearance method is called. The system object returns availableReports when the

getAvailableReports method is called.

Again, the return messages are an optional part of a sequence diagram. The use of return messages

depends on the level of detail/abstraction that is being modeled. Return messages are useful if finer

detail is required; otherwise, the invocation message is sufficient. I personally like to include return

messages whenever a value will be returned, because I find the extra details make a sequence diagram

easier to read.

When modeling a sequence diagram, there will be times that an object will need to send a message to

itself. When does an object call itself? A purist would argue that an object should never send a message

to itself. However, modeling an object sending a message to itself can be useful in some cases. For

example, Figure 5 is an improved version of Figure 4. The Figure 5 version shows the system object

calling its determineAvailableReports method. By showing the system sending itself the message

"determineAvailableReports," the model draws attention to the fact that this processing takes place in the

system object.

To draw an object calling itself, you draw a message as you would normally, but instead of connecting it

to another object, you connect the message back to the object itself.

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