En un exemple court et simple :

package com.memorynotfound;

import java.lang.management.ManagementFactory;
import java.lang.management.RuntimeMXBean;

public class GetUpTime {

    public static void main(String... args) throws InterruptedException {
        RuntimeMXBean rb = ManagementFactory.getRuntimeMXBean();
        System.out.println("Up time: " + rb.getUptime() + " ms");
    }

}