Calculate total, free & used memory of JVM heap store

Given a java program, where we are allocating memory in heap store.
- We will allocate array (array of long primitives).
We will analyze (or calculate) the memory consumption of JVM heap store.
- We will use java runtime to get JVM total memory, free memory and used memory.

1. Program flow to analyze total memory, free & used memory.

Calculate total memory, used memory & free memory using java runtime.
Allocate array of longs.
- We have allocate big enough buffer, so that we can easily make out the difference in memory consumption.
Again calculate total memory, used memory & free memory using java runtime.
- Used memory should be increased.
Allocate array of integers.
Perform step 1 again.
- Used memory will increase.
Clean up the both arrays (Step 2 & Step 4) and request GC to collect the memory.
Perform Step 1,

2. Get total memory, free memory & used memory in java (JVM heap store).

package org.learn;
 
public class JVMHeapStore {
 
    public static void main(String[] args) {
        long totalMemory ;
        long freeMemory;
        long usedMemory;
        int MB = 1024 *1024 ;
 
        Runtime runtime = Runtime.getRuntime();
        runtime.gc();
        totalMemory = runtime.totalMemory() / MB;
        freeMemory = runtime.freeMemory() / MB;
        usedMemory = totalMemory - freeMemory;
 
        System.out.printf("1. Total memory=%dMB, Free memory=%dMB, Used memory=%dMB\n",
                                totalMemory,freeMemory,usedMemory);
 
        System.out.println("2. Allocating space for 1000000 longs");
        long[] arrLong = new long[1000000];
        for(int index = 0; index < arrLong.length;index++) {
            arrLong[index] = index;
        }
        totalMemory = runtime.totalMemory() / MB;
        freeMemory = runtime.freeMemory() / MB;
        usedMemory = totalMemory - freeMemory;
 
        System.out.printf("3. Total memory=%dMB, Free memory=%dMB, Used memory=%dMB",
                totalMemory,freeMemory,usedMemory);
 
        System.out.println("\n4. Allocating space for another 1000000 longs");
        long[] anotherLongArray = new long[1000000];
        for(int index = 0; index < anotherLongArray.length;index++) {
            anotherLongArray[index] = index;
        }
 
        totalMemory = runtime.totalMemory() / MB;
        freeMemory = runtime.freeMemory() / MB;
        usedMemory = totalMemory - freeMemory;
 
        System.out.printf("5. Total memory=%dMB, Free memory=%dMB, Used memory=%dMB",
                totalMemory,freeMemory,usedMemory);
 
 
        System.out.println("\n6. Invalidate array's memory & request GC to collect");
        //clean up both arrays
        anotherLongArray = null;
        arrLong = null;
 
        //Run gc collect
        runtime.gc();
 
        totalMemory = runtime.totalMemory() / MB;
        freeMemory = runtime.freeMemory() / MB;
        usedMemory = totalMemory - freeMemory;
 
        System.out.printf("7. Total memory=%dMB, Free memory=%dMB, Used memory=%dMB",
                totalMemory,freeMemory,usedMemory);
    }
}

Output: total memory, free & used memory of java application (JVM).

Total memory=241MB, Free memory=241MB, Used memory=0MB
Allocating space for 1000000 longs
Total memory=241MB, Free memory=229MB, Used memory=12MB
Allocating space for another 1000000 longs
Total memory=241MB, Free memory=222MB, Used memory=19MB
Invalidate array's memory & request GC to collect
Total memory=241MB, Free memory=241MB, Used memory=0MB