The Client Server Framework in Practice in Java

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The Client Server Framework in Practice
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Kill the lion of Nemea Kill the nine-headed Hydra Capture the Ceryneian Hind Kill the wild boar of Erymanthus Clean the stables of King Augeas Kill the birds of Stymphalis Capture the wild bull of Crete Capture the man-eating mares of Diomedes Obtain the girdle of Hippolyta, Queen of the Amazons Capture the oxen of Geryon Take golden apples from the garden of Hesperides Bring Cerberus, the three-headed dog of the underworld, to the surface The Twelve Labors of Hercules
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This chapter works through the code for an example client and server to illustrate the main points of the Symbian OS client server architecture, which I discussed in detail in 11 This chapter will be of particular interest if you plan to implement your own server, or if you want to know more about how a client s request to a server is transferred and handled The code examines the typical features of a client server implementation using a transient server, which is started by its rst client connection and terminates when its last outstanding client session closes, to save system resources I ll take the main elements of client server code in turn and discuss the most important sections of each You can nd the entire set of sample code on the Symbian Press website (wwwsymbiancom/books) The bootstrap code used by a client to start a server can be quite complex and you may nd it helps to download this example and step through the code to follow how it works As in the previous chapter, I discuss the client server model only for Symbian OS releases up to and including v70s (the code samples in this chapter use the client server APIs from Symbian OS v70) Some of
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THE CLIENT SERVER FRAMEWORK IN PRACTICE
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the APIs have changed from Symbian OS 80 onwards and, although the concepts are generally the same, I ve decided to concentrate solely on the releases available at the time of going to press, rather than confuse matters Client server code can be notoriously complicated, so I have kept the services provided by the server as simple as possible This comes, perhaps, at the expense of making the example code rather contrived; my client server example provides a software representation of the twelve labors of the Greek hero Hercules1 For each labor, I ve exported a function from a client-side implementation class, RHerculesSession These functions send a request to the server to perform the necessary heroic activity The client-side implementation is delivered as a shared library DLL (clientdll) callers wishing to use the functionality provided by server should link to this The server code itself is built into a separate EPOCEXE component (shared library DLLs and targettype EPOCEXE are discussed in 13)
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Client Server Request Codes
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A set of enumerated values is used to identify which service the client requests from the server These values are quite straightforward and are de ned as follows:
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enum THerculeanLabors { ESlayNemeanLion, ESlayHydra, ECaptureCeryneianHind, ESlayErymanthianBoar, ECleanAugeanStables, ESlayStymphalianBirds, ECaptureCretanBull, ECaptureMaresOfDiomedes, EObtainGirdleOfHippolyta, ECaptureOxenOfGeryon, ETakeGoldenApplesOfHesperides, ECaptureCerberus, ECancelCleanAugeanStables, ECancelSlayStymphalianBirds };
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Later in the chapter you ll see these shared request opcodes passed to an overload of RSessionBase::SendReceive() on the client side and stored in the corresponding server-side RMessage object (which represents the client request) to identify which service the client has requested
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1 If nothing else, this chapter will prepare you well for a pub quiz question about the Herculean labors
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CLIENT BOILERPLATE CODE
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Much of the client-side implementation is made up of the API used by callers to submit requests to the server This API is exported by RHerculesSession, which derives from RSessionBase Each of the request methods passes the associated opcode, and any parameter data, to the server via a call to the base class method RSessionBase::SendReceive(), using the synchronous or asynchronous overload as appropriate Here is the de nition of the main client-side class (I ve shown only six of the Herculean labor request methods):
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// Forward declarations the actual class declarations must be // accessible to both client and server code class CHerculesData; struct THydraData; class RHerculesSession : public RSessionBase { public: IMPORT_C TInt Connect(); public: IMPORT_C TInt SlayNemeanLion(const TDesC8& aDes, TInt aVal); IMPORT_C TInt SlayHydra(THydraData& aData); IMPORT_C TInt CaptureCeryneianHind(TInt& aCaptureCount); IMPORT_C TInt SlayErymanthianBoar(const CHerculesData& aData); IMPORT_C void CleanAugeanStables(TRequestStatus& aStatus); IMPORT_C void CancelCleanAugeanStables(); IMPORT_C void SlayStymphalianBirds(TInt aCount, TDes8& aData, TRequestStatus& aStatus); IMPORT_C void CancelSlayStymphalianBirds(); };
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I ve included a range of parameter input, and implemented synchronous and asynchronous functions for illustration purposes Here are the implementations of the request submission methods:
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EXPORT_C TInt RHerculesSession::SlayNemeanLion(const TDesC8& aDes, TInt aVal) { const TAny* p[KMaxMessageArguments]; p[0]=&aDes; p[1]=(TAny*)aVal; return (SendReceive(ESlayNemeanLion,p)); } EXPORT_C TInt RHerculesSession::SlayHydra(THydraData& aData) { const TAny* p[KMaxMessageArguments]; TPckg<THydraData> data(aData); p[0]=&data; return (SendReceive(ESlayHydra,p));
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