Why does Ehcache double the size of Strings it stores in memory? - ehcache

Question

While trying to cache some objects of few MB I observed that Ehcache doubles their size while keeping them cached.
Why does this happen? Is it an optimization? Can it be cancelled?
The following code:
public class Main {
public static void main(String[] args) {
CacheManager manager = CacheManager.newInstance();
Cache oneCache = manager.getCache("OneCache");
String oneMbString = generateDummyString(1024 * 1024);
Element bigElement = new Element("key", oneMbString);
oneCache.put(bigElement);
System.out.println("size: "+ oneCache.getSize());
System.out.println("inMemorySize: " + oneCache.calculateInMemorySize());
System.out.println("size of string: " + oneMbString.getBytes().length);
}
/**
* Generate a dummy string
*
* #param size the size of the string in bytes.
* #return
*/
private static String generateDummyString(int size) {
StringBuilder sb = new StringBuilder();
for (int i = 0; i < size; i++) {
sb.append("a");
}
return sb.toString();
}
}
Will output:
size: 1
inMemorySize: 2097384
size of string: 1048576
PS: The ehcache.xml file:
<?xml version="1.0" encoding="UTF-8"?>
<ehcache xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="ehcache.xsd"
updateCheck="false" monitoring="autodetect" maxBytesLocalHeap="512M">
<cache name="OneCache"
eternal="false"
overflowToDisk="false"
diskPersistent="false"
memoryStoreEvictionPolicy="LFU">
<sizeOfPolicy maxDepth="10000" maxDepthExceededBehavior="abort"/>
</cache>
</ehcache>

Answer 1

Strings in Java use 2-byte characters. Ehcache is not doubling the size. When you call toBytes(), you get the encoded bytes (in this case the default UTF-8 encoding is used). This is why you see the difference.