Memory usage for a specific list of object

Question

i have a list of simples pojos (a User class) with about 15 simple fields & 1 arrayList. Those represent users & maybe 100 or 1000 of them will be store in memory in order to avoid to retrieve them from external system every time. (i'm using Ehcache)

I would like to know with a junit test how much memory is used by a list of K of those users. I have the intuition that simple pojo like those one even for a 1000 ones are not threatening in any way (in other words less than 100 Ko)

Thanks in advance for your anwser. i really appreciate your help.

Answer 1

You can calculate the memory used by the JRE before and after you create your object, in order to approximate how many bytes are being used by your object.

System.gc();
System.runFinalization();
Thread.sleep(1000);
long before = Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory();

//build object here

System.gc();
System.runFinalization();
Thread.sleep(1000);
long after = Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory();

long objectSize = after - before;