Smalltalk and Object Orientation: an Introduction - Free

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It is also possible to move objects from PermSpace into OldSpace. This can be done using the “Perm Undo As... ” option ag ain from the “ File” menu option in the VisualLauncher. This will move all objects from PermSpace into OldSpace. This will obviously have an impact on the performance of the garbage collector, and is not recommended unless you intend to remove a large numbe r of objects which were previously in PermSpace. You should then save the image again using the “ Perm Save As...” option. 260
31. Concurrency in Smalltalk 31.1 Introduction This chapter presents and explains a short example of how concurrency can be accomplished wit hin Smalltalk. It also considers lazy evaluations (i.e. you only perform the evaluation if you absolutely have to), futures (you start an evaluation now which you will need later) and persistence. The remainder of the document is structured in the followin g manner: Section two introduces the concept of concurrency, Section three briefly discusses processes within Smalltalk. Section four introduces the concepts used in the time slicing scheduler and Section five explains how future and lazy evaluators may be implemented in Smalltalk. 31.2 Concurrent processes The concepts behind object oriented programming lend themselves particularly well to the concepts associated with concurrency. For example, a system can be described as a set of discrete obj ects communicating with one another when necessary. In most Smalltalk implementations, only one object may be executing at any one moment in time. However, conceptually at least, there is no reason why this restriction should be enforced. For example, the basic concepts behind object orientation still hold, even if each object executes within a separate independent process. Traditionally, a message send is treated just like a procedural call, in which the calling object’s execution is blocked until a respo nse is returned. However, we could extend this model quite simply to view each object as a concurrently executable program module, with activity starting when the object is created and continuing even when a message is sent to another object (unless the re sponse is required for further processing). In this model, there may be very many (concurrent) objects all executing at the same time. Of course this introduces issues associated with resource allocation etc. but no more so than in any concurrent system. One implication of this concurrent object model is that objects are necessarily larger than in the traditional single execution thread approach. This is because the overhead of having each object as a process. A process scheduler for handling these process es and resource allocation mechanisms mean that it is not feasible to have integers or characters etc. as separate processes. (Note in Smalltalk integers etc. are objects. In such a system there might be a separate type of object which is a concurrent obje ct which is assumed to be a large grained object). Smalltalk has limited built -in support for concurrency. However, it does support Processes and Semaphores as primitive types. Other conventional inter -process communication and protection mechanisms such as SharedQueues and critical regions, are implemented in terms of these primitives. However, the processor scheduler (part of the virtual machine) implements a naive non -preemptive scheduling policy, with limited support for re-scheduling within a particular priority level. Further, once a high -priority process is running, no lower priority process will run until the high priority process suspends or terminates. For these reasons, the basic Smalltalk system contains only a few processes and typical applications using concurrency do not create more than a few tens of processes. However, as the source code is available to the developer it is possible to extend the scheduler such that a pre-emptive policy is introduced. This is described in Section 3 following on from a discussion of the facilities provided in Smalltalk which enable the construction of such a scheduler. 261
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Smalltalk and Object Orientation: A
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Chapter 7: Smalltalk Constructs Thi
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The user interface construction fac
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Preface 1. INTRODUCTION TO OBJECT O
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12.5 A CLASS SIDE METHOD...........
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27.3 VISUALWORKS WINDOW PAINTING TO
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Part One Introduction to Object Ori
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interfaces between different parts
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een to ease the transition from one
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encapsulation. Attempting to introd
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is a controversial subject which is
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will become second nature to many o
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or return something. It has since b
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defining the behavior of different
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In time most developers start to ac
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2.4.3.4 Smalltalk environments are
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3. Constructing an Object Oriented
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Wash wipe switch Wiper motor Relay
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Point 2 above is more complicated.
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services working? working? working?
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Part Two The Smalltalk Language 42
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other systems. As the browsers, ins
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VisualAge (a version of Smalltalk w
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4.5.2 The VisualWorks Launcher and
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Class categories. These are group s
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5. A Little Smalltalk 5.1 Introduct
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Notifiers can be displayed under a
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These concepts of messages, receive
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Select the “Debug” option. This
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6. Smalltalk Building Blocks 6.1 In
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6.3.1 Class definitions In Smalltal
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Object Class1 Class 2 method search
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“Now calculate y “ y := x * 23.
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7. Smalltalk Constructs 7.1 Introdu
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They are more efficient for storage
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7.5.3 true, false and nil variables
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anArray at: 1 max: 10. This will tr
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Figure 8.1: Defining a new class 8.
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this convention at the moment, howe
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9. Control and Iteration 9.1 Introd
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9.3.3 Block temporary variables Blo
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The brackets round the 1 to: 10 are
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OrderedCollection class (respective
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holding other objects. Typically, c
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| temp | temp := Set new. temp add:
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Next we will define a message proto
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11. Further Collection Classes 11.1
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for (i = 0; i
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x inspect. This has some similariti
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11.9.3 The reject: message The reje
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• asSortedCollection. This create
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Figure 12.2: The Organizer class de
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12.6 The private-updating protocol
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do: [:item | Transcript show: item
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13. Streams and Files 13.1 Introduc
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Notice that in this example we used
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to reconstruct the data into object
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1. The ability to save the current
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14.3 The class Date Class Date, a s
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aCharacter isAlphabetic. aCharacter
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Breakpoints may be placed in a me t
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If the file does not exist you will
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Figure 15.4: Creating a project To
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Part Three Object Oriented Design 1
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16. Object Oriented Analysis and De
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devolved amongst the objects). This
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driving force behind the whole of t
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17. The Unified Modeling Language 1
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Deployment diagrams . Deployment di
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Window abstract +size : Area = (10,
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Micro Computer 1.. * Monitor System
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connectedTo * Lamp * Weight Sensor
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18. UML: Dynamic Modeling And Deplo
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edisplay() Controller window A Wind
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18.2.3.1 A start point A start poin
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etween nodes and their stereotype.
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OMT is a prescriptive method, it pr
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confirming the users’ needs, thei
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19.3.2.2 Prepare a data dictionary
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car, truck and bus under a supercla
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20.2.1.3 Prepare sequence and colla
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20.2.2.5 Specify optimization crite
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are therefore concurrent and can be
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20.3.2.3 Design optimization The an
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21. Frameworks and Patterns for Obj
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As the framework takes shape you sh
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about a set of classes, than to att
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instanceVariableNames: '' classVari
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21.5.5 Running the editor You have
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Part Four Testing and Style 181
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made worse, because the symbol held
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22.3.2 Incremental testing Incremen
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control the execution of the method
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22.7.2 The Perform: message The per
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From the scenario that a person mig
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23.4 Self testing An approach, sugg
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24. Smalltalk Style Guidelines 24.1
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24.2.4 Adding variables to system c
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should use a dictionary to hold the
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• A method should only send messa
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Table 24.2: Class message categorie
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25. The Perform and Dependency Mech
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• Inheritance (class to class rel
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Page 247 and 248: DrawingView model SmallDraw drawing
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Page 267 and 268: 31.4.3 Acknowledgments Acknowledgme
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Page 273 and 274: ehavior, but from the point of view
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Page 280 and 281: solutions are needed which suit the
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Page 284 and 285: 34. Appendix: The Smalltalk Languag
Page 286 and 287: A capitalization convention is used
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Page 292 and 293: [Coleman et al 1994] D. Coleman, P.
Page 294 and 295: OOPSLA/ECOOP’90, Joint Conference
Page 296 and 297: detect.............................
Page 298: True...............................
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