Overview
Java was designed with the
principle that you shouldn't need to know the size of an object. There are
times when you really would like to know and want to avoid the guess work.
Measuring how much memory an object uses
There are three factors which make measuring how much an
object uses difficult.
- The TLAB
allocates blocks of memory to a thread. This means small amount of
memory don't appear to reduce the free memory. If you do this repeatedly,
you will see a block of free memory be used. The way around this is to
turn off the TLAB. -XX:-UseTLAB
- A GC can occur
while you are creating your object. This will result in more free memory
at the end than when you started. I ignore any negative sizes in this test
;)
- Other threads in
the system could use memory at the same time. I perform multiple test and
take the median, which removes any outliers.
Size of objects in a 32-bit JVM
Running this SizeofTest,
with on 32-bit Sun/Oracle Java 6 update 26, -XX:-UseTLAB I get
The average size of an int is 4.0 bytes
The average size of an Object is 8.0 bytes
The average size of an Integer is 16.0 bytes
The average size of a Long is 16.0 bytes
The average size of an AtomicReference is 16.0 bytes
The average size of an SimpleEntry(Map.Entry) is 16.0 bytes
The average size of a DateTime is 24.0 bytes
The average size of a Calendar is 424.0 bytes
The average size of an Exception is 400.0 bytes
The average size of a bit in a BitSet is 0.125 bytes
Looking a the size of Long confirms the size of header/Object being 8 bytes.
Size of objects with 32-bit references
Running this SizeofTest,
with 32-bit references On Sun/Oracle Java 6 update 26, -XX:+UseCompressedOops
-XX:-UseTLAB I get
The average size of an int is 4.0 bytes
The average size of an Object is 16.0 bytes
The average size of an Integer is 16.0 bytes
The average size of a Long is 24.0 bytes
The average size of an AtomicReference is 16.0 bytes
The average size of an SimpleEntry(Map.Entry) is 24.0 bytes
The average size of a DateTime is 24.0 bytes
The average size of a Calendar is 448.0 bytes
The average size of an Exception is 440.0 bytes
The average size of a bit in a BitSet is 0.125 bytes
Objects are 8-byte aligned on this JVM, and you could conclude from the size of
an Integer that the header is 12-bytes in size.
Size of objects with 64-bit references
Running the same test with 64-bit references. i.e. -XX:-UseCompressedOops
-XX:-UseTLAB
The average size of an int is 4.0 bytes
The average size of an Object is 16.0 bytes
The average size of an Integer is 24.0 bytes
The average size of a Long is 24.0 bytes
The average size of an AtomicReference is 24.0 bytes
The average size of an SimpleEntry(Map.Entry) is 32.0 bytes
The average size of a DateTime is 32.0 bytes
The average size of a Calendar is 544.0 bytes
The average size of an Exception is 648.0 bytes
The average size of a bit in a BitSet is 0.125 bytes
From looking at the size of a Long, confirms the size of
the header/Object is 16 bytes in length.
// sizeofUtil.java
package com.google.code.java.core.sizeof;
import java.util.Arrays;
public abstract class SizeofUtil {
public double averageBytes() {
int runs = runs();
double[] sizes = new double[runs];
int retries = runs / 2;
final Runtime runtime
= Runtime.getRuntime();
for (int i = 0; i <
runs; i++) {
Thread.yield();
long used1 = memoryUsed(runtime);
int number = create();
long used2 = memoryUsed(runtime);
double avgSize = (double) (used2 -
used1) / number;
// System.out.println(avgSize);
if (avgSize < 0) {
// GC was performed.
i--;
if (retries-- < 0)
throw new
RuntimeException("The eden space is not large enough to hold all the
objects.");
} else if (avgSize == 0)
{
throw new
RuntimeException("Object is not large enough to register, try turning off
the TLAB with -XX:-UseTLAB");
} else {
sizes[i] = avgSize;
}
}
Arrays.sort(sizes);
return sizes[runs /
2];
}
protected long memoryUsed(Runtime runtime) {
return
runtime.totalMemory() - runtime.freeMemory();
}
protected int runs() {
return 11;
}
protected abstract int create();
}
