The DataLogger Class in Java

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The DataLogger Class
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We ll start by exploring the main class of the application: DataLogger DataLogger implements the network (TCP/IP) server and accepts and manages inbound connections from remote clients A skeleton of the DataLogger class including its constructor is shown in Listing 71 The DataLogger class extends Thread and overrides the run method, making it the server s main loop The primordial thread is allowed to die after successful initialization of the application During construction of DataLogger an instance of HumidityLogger is created specifying the sample count and delay time in seconds between samples After the logger thread is started, the DataLogger thread is not concerned with the operation of the logger or even what kind of data it is collecting It maintains a reference to the logger object that is used to satisfy log requests for inbound client connections We ll cover the data collection classes in detail in the next section Listing 71 DataLogger
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import javaio*; import javanet*; class DataLogger extends Thread { static final int SERVER_PORT = 5588; HumidityLogger logger; DataLogger(int samples, int delay) throws LoggingException { // Create and start the logging daemon logger = new HumidityLogger(samples, delay); loggerstart(); } }
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requires the number of data readings to be maintained and the delay in seconds between each reading to be speci ed on the command line DataLogger s main method is shown in Listing 72
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DataLogger
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Listing 72
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DataLogger s main method
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public static void main(String[] args) { Systemoutprintln("Starting DataLogger "); if (argslength != 2) { Systemoutprintln("Usage: java DataLogger samples delay"); Systemexit(1);
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} int samples = IntegerparseInt(args[0]); int delay = IntegerparseInt(args[1]); try { (new DataLogger(samples, delay))start(); } catch (Exception e) { Systemoutprintln("Error creating data logger"); eprintStackTrace(); // In case any non-daemon threads have been started // Systemexit(1); } }
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After extracting the samples and count values from the command line, the method creates a new DataLogger object, which also creates a new thread of execution After constructing the new instance of DataLogger, the start method is invoked to kick off the server The server spends eternity in the run method, processing network connections DataLogger s run method along with the inner class LogWorker is shown in Listing 73 It starts by creating a ServerSocket object to listen for inbound connections from remote clients The SERVER_PORT number used in creating the ServerSocket object is simply chosen as a large magic number Anything that is comfortably above the range of well-known port 2 numbers will do As implemented here, DataLogger uses a port number of 5588 The application could easily be modi ed to use a port number speci ed on the command line
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main
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Listing 73
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DataLogger s run method
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public void run() { ServerSocket ss = null; try { ss = new ServerSocket(SERVER_PORT); } catch (Exception e) { eprintStackTrace(); // Abort if we can t create ServerSocket instance return; } while (true) { Socket s = null; try { // Wait for client connections over PPP or Ethernet s = ssaccept(); } catch (IOException ioe) { // Shut down the logging daemon loggerstopLogging(); Systemoutprintln("Fatal problem with server socket");
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2 The well-known ports are listed in RFC 1700
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The DataLogger Class
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ioeprintStackTrace(); // Fall out of run method break; } // Create a new thread to handle this connection (new LogWorker(s))start(); } } private class LogWorker extends Thread { private Socket s; private LogWorker(Socket s) { thiss = s; } public void run() { DataOutputStream dout = null; try { dout = new DataOutputStream( new BufferedOutputStream(sgetOutputStream())); loggerwriteLog(dout); doutflush(); } catch (IOException ioe) { Systemoutprintln("I/O error writing log data"); ioeprintStackTrace(); } finally { try { sclose(); doutclose(); } catch (IOException e) {} } } }
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After the ServerSocket object is created, the run method enters an in nite loop that accepts and processes inbound client connections After a new instance of Socket is returned from the ServerSocket object s accept method, a new thread (an instance of LogWorker) is created to manage the connection The socket s getOutputStream method is invoked to obtain the lowest-level output stream (an instance of SocketOutputStream) for writing data to the underlying connection This stream is used in constructing an instance of BufferedOutputStream, and the resulting buffered output stream is wrapped in a DataOutputStream The idea of using a buffered DataOutputStream for writing the log data is that the data-collecting daemon will write all of the samples it has collected to the output stream in an iterative fashion If the output stream were not buffered, every write method invoked on the output stream would perform a write to the lowlevel SocketOutputStream This forces a write to the native socket layer Writing