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4.12. Processes

Earlier in the chapter, we saw how easy it is to create and manipulate multiple threads of execution running within the same Java interpreter. Java also has a java.lang.Process class that represents a program running externally to the interpreter. A Java program can communicate with an external process using streams in the same way that it might communicate with a server running on some other computer on the network. Using a Process is always platform-dependent and is rarely portable, but it is sometimes a useful thing to do:

// Maximize portability by looking up the name of the command to execute
// in a configuration file. 
java.util.Properties config;  
String cmd = config.getProperty("sysloadcmd");
if (cmd != null) {
  // Execute the command; Process p represents the running command
  Process p = Runtime.getRuntime().exec(cmd);         // Start the command
  InputStream pin = p.getInputStream();               // Read bytes from it
  InputStreamReader cin = new InputStreamReader(pin); // Convert them to chars
  BufferedReader in = new BufferedReader(cin);        // Read lines of chars
  String load = in.readLine();                        // Get the command output
  in.close();                                         // Close the stream

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