[java] How do I monitor the computer's CPU, memory, and disk usage in Java?

/* YOU CAN TRY THIS TOO */

import java.io.File;
 import java.lang.management.ManagementFactory;
// import java.lang.management.OperatingSystemMXBean;
 import java.lang.reflect.Method;
 import java.lang.reflect.Modifier;
 import java.lang.management.RuntimeMXBean;
 import java.io.*;
 import java.net.*;
 import java.util.*;
 import java.io.LineNumberReader;
 import java.lang.management.ManagementFactory;
import com.sun.management.OperatingSystemMXBean;
import java.lang.management.ManagementFactory;
import java.util.Random;



 public class Pragati
 {

     public static void printUsage(Runtime runtime)
     {
     long total, free, used;
     int mb = 1024*1024;

     total = runtime.totalMemory();
     free = runtime.freeMemory();
     used = total - free;
     System.out.println("\nTotal Memory: " + total / mb + "MB");
     System.out.println(" Memory Used: " + used / mb + "MB");
     System.out.println(" Memory Free: " + free / mb + "MB");
     System.out.println("Percent Used: " + ((double)used/(double)total)*100 + "%");
     System.out.println("Percent Free: " + ((double)free/(double)total)*100 + "%");
    }
    public static void log(Object message)
         {
            System.out.println(message);
         }

        public static int calcCPU(long cpuStartTime, long elapsedStartTime, int cpuCount)
        {
             long end = System.nanoTime();
             long totalAvailCPUTime = cpuCount * (end-elapsedStartTime);
             long totalUsedCPUTime = ManagementFactory.getThreadMXBean().getCurrentThreadCpuTime()-cpuStartTime;
             //log("Total CPU Time:" + totalUsedCPUTime + " ns.");
             //log("Total Avail CPU Time:" + totalAvailCPUTime + " ns.");
             float per = ((float)totalUsedCPUTime*100)/(float)totalAvailCPUTime;
             log( per);
             return (int)per;
        }

        static boolean isPrime(int n)
        {
     // 2 is the smallest prime
            if (n <= 2)
            {
                return n == 2;
            }
     // even numbers other than 2 are not prime
            if (n % 2 == 0)
            {
                return false;
            }
     // check odd divisors from 3
     // to the square root of n
         for (int i = 3, end = (int)Math.sqrt(n); i <= end; i += 2)
         {
            if (n % i == 0)
         {
         return false;
        }
        }
 return true;
}
    public static void main(String [] args)
    {
            int mb = 1024*1024;
            int gb = 1024*1024*1024;
             /* PHYSICAL MEMORY USAGE */
             System.out.println("\n**** Sizes in Mega Bytes ****\n");
            com.sun.management.OperatingSystemMXBean operatingSystemMXBean = (com.sun.management.OperatingSystemMXBean)ManagementFactory.getOperatingSystemMXBean();
            //RuntimeMXBean runtimeMXBean = ManagementFactory.getRuntimeMXBean();
            //operatingSystemMXBean = (com.sun.management.OperatingSystemMXBean) ManagementFactory.getOperatingSystemMXBean();
            com.sun.management.OperatingSystemMXBean os = (com.sun.management.OperatingSystemMXBean)
            java.lang.management.ManagementFactory.getOperatingSystemMXBean();
            long physicalMemorySize = os.getTotalPhysicalMemorySize();
            System.out.println("PHYSICAL MEMORY DETAILS \n");
            System.out.println("total physical memory : " + physicalMemorySize / mb + "MB ");
            long physicalfreeMemorySize = os.getFreePhysicalMemorySize();
            System.out.println("total free physical memory : " + physicalfreeMemorySize / mb + "MB");
            /* DISC SPACE DETAILS */
            File diskPartition = new File("C:");
            File diskPartition1 = new File("D:");
            File diskPartition2 = new File("E:");
            long totalCapacity = diskPartition.getTotalSpace() / gb;
            long totalCapacity1 = diskPartition1.getTotalSpace() / gb;
            double freePartitionSpace = diskPartition.getFreeSpace() / gb;
            double freePartitionSpace1 = diskPartition1.getFreeSpace() / gb;
            double freePartitionSpace2 = diskPartition2.getFreeSpace() / gb;
            double usablePatitionSpace = diskPartition.getUsableSpace() / gb;
            System.out.println("\n**** Sizes in Giga Bytes ****\n");
            System.out.println("DISC SPACE DETAILS \n");
            //System.out.println("Total C partition size : " + totalCapacity + "GB");
            //System.out.println("Usable Space : " + usablePatitionSpace + "GB");
            System.out.println("Free Space in drive C: : " + freePartitionSpace + "GB");
            System.out.println("Free Space in drive D:  : " + freePartitionSpace1 + "GB");
            System.out.println("Free Space in drive E: " + freePartitionSpace2 + "GB");
            if(freePartitionSpace <= totalCapacity%10 || freePartitionSpace1 <= totalCapacity1%10)
            {
                System.out.println(" !!!alert!!!!");
            }
            else
                System.out.println("no alert");

            Runtime runtime;
            byte[] bytes;
            System.out.println("\n \n**MEMORY DETAILS  ** \n");
            // Print initial memory usage.
            runtime = Runtime.getRuntime();
            printUsage(runtime);

            // Allocate a 1 Megabyte and print memory usage
            bytes = new byte[1024*1024];
            printUsage(runtime);

            bytes = null;
            // Invoke garbage collector to reclaim the allocated memory.
            runtime.gc();

            // Wait 5 seconds to give garbage collector a chance to run
            try {
            Thread.sleep(5000);
            } catch(InterruptedException e) {
            e.printStackTrace();
            return;
            }

            // Total memory will probably be the same as the second printUsage call,
            // but the free memory should be about 1 Megabyte larger if garbage
            // collection kicked in.
            printUsage(runtime);
            for(int i = 0; i < 30; i++)
                     {
                         long start = System.nanoTime();
                        // log(start);
                        //number of available processors;
                         int cpuCount = ManagementFactory.getOperatingSystemMXBean().getAvailableProcessors();
                         Random random = new Random(start);
                         int seed = Math.abs(random.nextInt());
                         log("\n \n CPU USAGE DETAILS \n\n");
                         log("Starting Test with " + cpuCount + " CPUs and random number:" + seed);
                         int primes = 10000;
                         //
                         long startCPUTime = ManagementFactory.getThreadMXBean().getCurrentThreadCpuTime();
                         start = System.nanoTime();
                         while(primes != 0)
                         {
                            if(isPrime(seed))
                            {
                                primes--;
                            }
                            seed++;

                        }
                         float cpuPercent = calcCPU(startCPUTime, start, cpuCount);
                         log("CPU USAGE : " + cpuPercent + " % ");


                         try
                         {
                             Thread.sleep(1000);
                         }
                         catch (InterruptedException e) {}
        }

            try
            {
                Thread.sleep(500);
            }`enter code here`
            catch (Exception ignored) { }
        }
    }