STARTING THE SERVER AND CONNECTING TO IT FROM THE CLIENT in Java

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STARTING THE SERVER AND CONNECTING TO IT FROM THE CLIENT
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it is required by the system If a system server panics for some reason, Symbian OS is forced to reboot and restart the server because it cannot function without it Client connection to a system server is straightforward because the server is already running In effect, a Connect() method simply needs to call RSessionBase::CreateSession() However, as I ve described, the Hercules example server discussed in this chapter is not an essential system server; instead it is implemented as a transient server If it is not already running when a client calls CreateSession(), that is if no other client is connected to it, the Connect() method of the client-side implementation must start the server process The server runs in a separate process on phone hardware and Symbian OS processes run in separate threads within the single process of the Windows emulator Here is the implementation of Connect() for the Hercules server:
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// The server s identity within the client server framework _LIT(KServerName,"HerculesServer"); EXPORT_C TInt RHerculesSession::Connect() { TInt retry=2; // A maximum of two iterations of the loop are required for (;;) { // Uses system-pool message slots TInt r=CreateSession(KServerName,TVersion(1,0,0)); if ( (KErrNotFound!=r) && (KErrServerTerminated!=r) ) return (r); if (--retry==0) return (r); r=StartTheServer(); if ( (KErrNone!=r) && (KErrAlreadyExists!=r) ) return (r); } }
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The method calls RSessionBase::CreateSession(), passing in the name of the server required If the server is already running and can create a new client session, this call returns KErrNone and the function returns successfully, having connected a new session However, if the server is not running, CreateSession() returns KErrNotFound The Connect() method must check for this result and, under these circumstances, attempt to start the server You ll notice that the method also checks to see if KErrServerTerminated was returned instead of KErrNotFound, which indicates that the client connection request was submitted just as the server was shutting down For either value,4 the server should be started (or restarted) by calling
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4 If any other error value besides KErrNotFound or KErrTerminated is returned, Connect() returns it to the caller
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THE CLIENT SERVER FRAMEWORK IN PRACTICE
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StartTheServer() I ll discuss this function shortly if it returns KErrNone, the server has started successfully The next iteration of the for loop submits another request to create a client session by calling RSessionBase::CreateSession() If some other client managed to start the server between this client s failed attempt to create a session on it and the call to StartTheServer(), it returns KErrAlreadyExists Under the circumstances, this is not an error,5 because the server is now running, even if the client didn t start it So, again, the next iteration of the loop can submit a request to create a new client session If the server fails to start or the client connection fails, an error is returned and the code breaks out of the loop, returning the error value to indicate that session creation failed StartTheServer() launches the server in a new thread on the Windows emulator, or in a new, separate process on hardware running Symbian OS Because the server runs differently depending on whether it is running on the emulator or a phone, the client-side server startup code is quite complex The code is implemented as follows (I ve included a number of comments to make it clear what is going on and I ll discuss the most important features below):
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// Runs client-side and starts the separate server process/thread static TInt StartTheServer() { TRequestStatus start; TServerStart starter(start); // HerculesServerexe or HerculesServerdll _LIT(KServerBinaryName,"HerculesServer"); const TUid KServerUid3={0x01010101}; // Temporary UID const TUidType serverUid(KNullUid, KNullUid, KServerUid3); #ifdef __WINS__ // On the Windows emulator the server is a DLL RLibrary lib; TInt r=libLoad(KServerBinaryName, serverUid); if (r!=KErrNone) return (r); // The entry point returns a TInt representing the thread // function for the server TLibraryFunction export1 = libLookup(1); TThreadFunction threadFunction = reinterpret_cast<TThreadFunction>(export1()); TName name(KServerName); // Defined previously // Randomness ensures a unique thread name nameAppendNum(Math::Random(), EHex);
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5 The KErrAlreadyExists error code is returned because there must only be one copy of the server running on the system Simultaneous attempts to launch two copies of the server process will be detected by the kernel when the second attempt is made to create a CServer object of the same name It is this which fails with KErrAlreadyExists
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