Getting JVM heap size, used memory, total memory using Java Runtime

 
Java’€™s Runtime class provide lot of information about the resource details of JVM(Java Virtual Machine). The memory consumed by the JVM can be read by different methods in Runtime class.
 
In the following example, at start of program we get initial size of heap of JVM by calling freeMemory, totalMemory and maxMemory and then we create thousands of object which occupy space in heap which forces JVM to extend heap, now call to total memory, free memory will return different value based on current heap size but max memory will still return same. Suppose, when the object created requires large amount of memory, then JVM will begin to throw OutOfMemoryError.
 

import java.util.ArrayList;
public class TestMemoryUtil
{
private static final int MegaBytes = 1024 * 1024;

public static void main(String args[])
{

long totalMemory = Runtime.getRuntime().totalMemory() / MegaBytes;
long maxMemory = Runtime.getRuntime().maxMemory() / MegaBytes;
long freeMemory = Runtime.getRuntime().freeMemory() / MegaBytes;

System.out.println("**** Heap utilization Analysis [MB] ****");
System.out.println("JVM totalMemory also equals to initial heap size of JVM :"+ totalMemory);
System.out.println("JVM maxMemory also equals to maximum heap size of JVM: "+ maxMemory);
System.out.println("JVM freeMemory: " + freeMemory);

ArrayList objects = new ArrayList();

 for (int i = 0; i < 10000000; i++)
 {
	objects.add(("" + 10 * 2710));
 }

totalMemory = Runtime.getRuntime().totalMemory() / MegaBytes;
maxMemory = Runtime.getRuntime().maxMemory() / MegaBytes;
freeMemory = Runtime.getRuntime().freeMemory() / MegaBytes;

System.out.println("Used Memory in JVM: " + (maxMemory - freeMemory));
System.out.println("totalMemory in JVM shows current size of java heap:"+totalMemory);
System.out.println("maxMemory in JVM: " + maxMemory);
System.out.println("freeMemory in JVM: " + freeMemory);

}
}

Output:

**** Heap utilization Analysis [MB] ****
JVM totalMemory also equals to initial heap size of JVM : 15
JVM maxMemory also equals to maximum heap size of JVM: 247
JVM freeMemory: 15
Used Memory in JVM: 202
totalMemory in JVM shows current size of java heap : 118
maxMemory in JVM: 247
freeMemory in JVM: 45