Process vs threads

The distinction between processes and threads is important.

• Process: A process runs independently and isolated of other processes. It cannot directly access shared data in other processes. The resources of the process are allocated to it via the operating system, e.g. memory and CPU time.
  
• Threads: threads are so called lightweight processes which have their own call stack but an access shared data. Every thread has its own memory cache. If a thread reads shared data it stores this data in its own memory cache. A thread can re-read the shared data, when this happens in Java will be explained in Java memory model part of this article.
  

Within a Java application you work with several threads to archive parallel processing or asynchronously behavior.

How to run external programs by using Java ProcessBuilder class?

The class java.lang.ProcessBuilder, in Java 1.5, is used to create operating system processes. Each process builder manages these process attributes : (See Java API Document)

• a command, a list of strings which signifies the external program file to be invoked and its arguments, if any. Which string lists represent a valid operating system command is system-dependent. For example, it is common for each conceptual argument to be an element in this list, but there are operating systems where programs are expected to tokenize command line strings themselves - on such a system a Java implementation might require commands to contain exactly two elements.
• an environment, which is a system-dependent mapping from variables to values. The initial value is a copy of the environment of the current process (see System.getenv()).
• a working directory. The default value is the current working directory of the current process, usually the directory named by the system property user.dir.
• a redirectErrorStream property. Initially, this property is false, meaning that the standard output and error output of a subprocess are sent to two separate streams, which can be accessed using the Process.getInputStream() and Process.getErrorStream() methods. If the value is set to true, the standard error is merged with the standard output. This makes it easier to correlate error messages with the corresponding output. In this case, the merged data can be read from the stream returned by Process.getInputStream(), while reading from the stream returned by Process.getErrorStream() will get an immediate end of file.

The java.lang.ProcessBuilder and java.lang.Process classes are available for executing and communicating with external programs. With an instance of the java.lang.ProcessBuilder class, it can execute an external program and return an instance of a subclass of java.lang.Process. The class Process provides methods for performing input from the process, performing output to the process, waiting for the process to complete, checking the exit status of the process, and destroying (killing) the process.

//running a 
String[] command = {"CMD", "/C", "dir"};
ProcessBuilder probuilder = new ProcessBuilder( command );

//You can set up your work directory
probuilder.directory(new File("c:\\vaanidemo"));

Process process = probuilder.start();

//Read out dir output
InputStream is = process.getInputStream();
InputStreamReader isr = new InputStreamReader(is);
BufferedReader br = new BufferedReader(isr);
String line;
System.out.printf("Output of running %s is:\n",
        Arrays.toString(command));
while ((line = br.readLine()) != null) {
    System.out.println(line);
}

//Wait to get exit value
try {
    int exitValue = process.waitFor();
    System.out.println("\n\nExit Value is " + exitValue);
} catch (InterruptedException e) {
    e.printStackTrace();
}