how to do multiprocessing in java, and what speed gains to expect?

Question

I am a newbie using Java to do some data processing on csv files. For that I use the multithreading capabilities of Java (pools of threads) to batch-import the csv files into Ja

Answer 1

There are several ways to start a new process in Java:

1. ProcessBuilder.start()
2. Runtime.exec() works around ProcessBuilder
3. Apache Commons Exec that works around Runtime.exec()

With ProcessBuilder:

ProcessBuilder pb =
new ProcessBuilder("myCommand", "myArg1", "myArg2");
Map env = pb.environment();
env.put("VAR1", "myValue");
env.remove("OTHERVAR");
env.put("VAR2", env.get("VAR1") + "suffix");
pb.directory(new File("myDir"));
File log = new File("log");
pb.redirectErrorStream(true);
pb.redirectOutput(Redirect.appendTo(log));
Process p = pb.start();
assert pb.redirectInput() == Redirect.PIPE;
assert pb.redirectOutput().file() == log;
assert p.getInputStream().read() == -1;

With Runtime:

Runtime r = Runtime.getRuntime();
Process p = r.exec("firefox");
p.waitFor(10, TimeUnit.SECONDS);
p.destroy();

With Apache Commons Exec:

String line = "AcroRd32.exe /p /h " + file.getAbsolutePath();
CommandLine cmdLine = CommandLine.parse(line);
DefaultExecutor executor = new DefaultExecutor();
int exitValue = executor.execute(cmdLine);

Key differences between Multiprocessing and Multithreading from this:

• The key difference between multiprocessing and multithreading is that multiprocessing allows a system to have more than two CPUs added to the system whereas multithreading lets a process generate multiple threads to increase the computing speed of a system.
• Multiprocessing system executes multiple processes simultaneously whereas, the multithreading system let execute multiple threads of a process simultaneously.
• Creating a process can consume time and even exhaust the system resources. However creating threads is economical as threads belonging to the same process share the belongings of that process.
• Multiprocessing can be classified into symmetric multiprocessing and asymmetric multiprocessing whereas, multithreading is not classified further.

Additional links:

• https://alvinalexander.com/java/java-exec-processbuilder-process-1
• https://alvinalexander.com/java/java-exec-processbuilder-process-2
• https://alvinalexander.com/java/java-exec-processbuilder-process-3