How to get CPU usage in Linux from Java

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The easiest method to get the CPU usage from a Java class in Linux is to parse the output of the top command. Check out the following code.

import java.io.*;
public class SystemData
{
    static String cmdTop = "top -n 2 -b -d 0.2";
    // returns user cpu usage 
    public static double getCpu()
    {
        double cpu = -1;
        try
        {
            // start up the command in child process
            String cmd = cmdTop;
            Process child = Runtime.getRuntime().exec(cmd);

            // hook up child process output to parent
            InputStream lsOut = child.getInputStream();
            InputStreamReader r = new InputStreamReader(lsOut);
            BufferedReader in = new BufferedReader(r);

            // read the child process' output
            String line;
            int emptyLines = 0;
            while(emptyLines<3)
            {
                line = in.readLine();
                if (line.length()<1) emptyLines++;
            }
            in.readLine();
            in.readLine();
            line = in.readLine();
            System.out.println("Parsing line "+ line);
            String delims = "%";
            String[] parts = line.split(delims);
            System.out.println("Parsing fragment " + parts[0]);
            delims =" ";

            parts = parts[0].split(delims);
            cpu = Double.parseDouble(parts[parts.length-1]);
        }
        catch (Exception e)
        { // exception thrown
            System.out.println("Command failed!");
        }
        return cpu;
    }
}

The parameters from the top command line mean: -b = batch mode, -d 0.2 = take the readings at 0.2 seconds intervals, -n 2 = take 2 readings (the first reading is not correct, from my experience, so we take two).
This code fragment only gets the user CPU percentage. To get the whole CPU percentage, you have to add this value with the system CPU. These values are easy to obtain by changing a few numeric values in the above code. This is left as an exercise for the user. Note that although in general the system CPU and nice CPU percentages are very small, this is not always the case and it is safer to add these values together than to approximate the CPU usage as user CPU only.