Poplar：Java的演化组件集成扩展

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"这篇PDF文献是‘Poplar: A Java Extension for Evolvable Component Integration’，由Johan Nyström-Persson和Shinichi Honiden共同撰写，来自东京大学和日本信息科学研究所。该文探讨了Java编程语言在组件基础软件开发中的优势，如接口、可见性级别和封装支持，但同时也指出组件演进过程中常出现的破坏性变化问题，主要由于组件间连接的刚性，如显式方法调用和字段访问。作者提出了一种名为Poplar的Java扩展，旨在解决这个问题。" 在Poplar这个Java扩展中，组件间的依赖关系通过声明性查询来表达，而非传统的显式方法调用和字段访问。编译时，Poplar会使用规划算法生成具体的链接代码来替代这些查询。此外，Poplar还包含一个基于类型状态协议和标签的最小规格说明语言，以及一个轻量级的影响系统。这个影响系统确保自动生成的代码与手写的代码之间不会发生未预期的干扰。文献通过多个实例展示了Poplar如何实现组件的全自动整合，强调了其在组件演进过程中的适应性和灵活性。Poplar的创新之处在于它允许组件之间的交互更加灵活，减少了因组件更新而导致的兼容性问题，从而促进了组件的可进化性。通过使用Poplar，开发者可以更好地管理和控制组件间的依赖关系，减少由于版本升级或功能修改而引发的系统不稳定性。同时，Poplar的规划算法生成的链接代码能够动态适应组件的变化，降低了维护成本，提高了软件的可维护性和可扩展性。 Poplar是一个针对Java的创新性扩展，它通过引入声明性查询和自动代码生成技术，解决了组件化软件开发中的一大难题——组件演进的兼容性问题。这种技术对于大型复杂系统的持续集成和持续交付（CI/CD）流程尤其有价值，有助于构建更健壮、更易于演化的软件系统。

Cl a ss Calen d a r i m p l e m e nts Tim e A n d D a t e {

lab e ls ( int ) ho urMar ker , minute M a rker ,

secondM a r k e r ;

lab e ls d e f a u l tT i m e Z on e ;

fi n al int HO U R _ O F _ D A Y + ho u r M a r k e r = 1 1;

/* etc . */

Calend a r ()

res u lt : + d e f a ul t T i m eZ o n e { ... }

int get ( int selector )

th is : de f a u l t TimeZone ,

( se l e c t o r : hou r Marke r ,

res u lt : + nowHour )? ,

( se l e c t o r : minut e M arker ,

res u lt : + nowMinute )? { ... }

}

Figure 2. TimeAndDate annotations for Java 1.5. Here the

+ sign indicates an invariant.

need to have a Date object with the currentTime label. We

repeat the search and ﬁnd that there is a constructor that takes

no arguments and that produces such an object. This yields

a complete solution, and thus, after the code has been gener-

ated and the substitution has taken place, the client class will

look like the following:

cl a ss TimeUtils implements TimeA n d D a t e {

vo id pr i n t H o u r () {

Da te v1 = new Date () ;

int h our = v1 . getHou r ();

Sys t em . out . println (" The curr e n t hour is : " +

ho ur );

}

In addition, the solver will remove non-Java elements

from the code, such as the label declarations from the various

classes, so that the result is valid Java source code.

2.2 Upgrading to Java 1.5

Let us now consider how we could adapt this client to the

Java 1.5 version. In this case, the service component would

resemble the one shown in Figure 2.

In this case, all the values are accessed through one

method, which takes a selector argument. We have given

the ﬁeld HOUR_OF_DAY an explicit label hourMarker, which

links it to its possible use as an argument for the get(int)

method. We group invariants and postconditions using the

(a, b, ...)? syntax, which makes the pre- and postcon-

ditions inside the group optional. This is mainly a syntactic

shorthand to avoid having to repeat the defaultTimeZone

requirement for the this object. In this case, however, clearly

minuteMarker, hourMarker and so on will be mutually ex-

clusive labels. The plan search now takes the same query as

a starting point, but the APIs supplied as input are different

(and perhaps the 1.5 API is marked as taking precedence

over the 1.4 one if both are available) and after substituting

a solution for the query we end up with:

new Calendar()

get(int)

(Calendar,

defaultTimeZone)

(int, nowHour)

Start

Finish

Calendar.HOUR_OF_DAY

(int, hourMarker)

new Date()

getHour()

(Date, currentTime)

(int, nowHour)

Start

Finish

Java 1.4 Java 1.5

Figure 3. Visual representations of the plans generated in

the case of producing the current hour of the day in Java 1.4

and 1.5, respectively.

cl a ss TimeUtils implements TimeA n d D a t e {

vo id pr i n t H o u r () {

Da te v1 = new C a l e n d a r () ;

int v2 = C alendar . HO U R_OF _ D A Y ;

int h our = v1 . get (v2 );

Sys t em . out . println (" The curr e n t hour is : " +

ho ur );

}

We show the plans visually in Figure 3. The resulting

code is extracted directly from the plans. Rounded boxes are

pre- and postconditions (the existence of a variable with a

given label), and square boxes are actions such as method

invocation and ﬁeld access. Dashed lines represent sequen-

tial constraints, which impose an ordering on the actions.

These constraints will be at least as strong as the dataﬂow

dependencies of the solution, and possibly stronger due to

possible conﬂicts.

These examples demonstrate how clients can automati-

cally be reconﬁgured to use a new version of an API, or

even a different API, given that the necessary annotations

are present on both the client and the service side. In this

case it would simply be a matter of re-running the integra-

tion tool with the newest service components added to the

classpath.

4 2011/6/10

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