org.mmbase.util

Class SizeOf

java.lang.Object

org.mmbase.util.SizeOf

public class SizeOf
extends Object

Implementation of 'sizeof'. It is very hard to get reasonable estimations of how much memory your structures take, this class it trying it any way. Often it's quite a heavy operation, so don't use it too much.

A count of the byte size of an object is done recursively and for every count a SizeOf instance must be instantiated. The static getByteSize(Object) does that for you.

The core of the SizeOf object is then sizeof(Object) plus a (private) 'countedObjects' collection. The sizeof method returns the size the given Object would increase the size of this countedObjects administration. This means that it returns 0 if the Object was already measured by the SizeOf instance.

This means that it can be tricky to interpret the results of sizeof. The basic rule is that you should take the size of a lot of similar objects with the same SizeOf instance, and take the average.

A good example is MMObjectNode. The first one counted will also count a MMObjectBuilder object - because that is linked to it, so its memory is also taken (indirectly) - and will therefor give an unexpectedly large value like 20 kb or so. The second MMObjectNode, of the same type, that you'd count would however give a much better estimation of the memory used by one Node in MMBase. The MMObjectBuilder is not counted any more in this second Object, because it was already counted because of the first one.

For every individual entity there are several strategies for guessing the size.

• For atomic types (boolean, byte, char, short, int, long, float, double) a constant value is returned
• If the entity implements SizeMeasurable it uses SizeMeasurable.getByteSize(SizeOf)
• For a limited number of known classes, a reasonable guess is done. E.g. if it is a Collection it will simply sum the results of sizeof of its elements, and for a String it will return getBytes().length().
• If all that fails, reflection will be used to sum the results of sizeof of all readable members.

Don't forget to dereference or clear the SizeOf after use, otherwise it itself is a memory leak.

Since:

MMBase-1.6

Version:

$Id$

Author:

Michiel Meeuwissen

To Do:

We need to know how well this actually works...

Field Summary

Fields

Modifier and Type	Field and Description
static int	SZ_REF

Constructor Summary

Constructors

Constructor and Description
SizeOf()

Method Summary

Modifier and Type	Method and Description
void	clear() Makes this SizeOf object ready for reuse.
static int	getByteSize(Object obj)
static void	main(String[] argv)
static int	sizeof(boolean b)
static int	sizeof(byte b)
static int	sizeof(char c)
static int	sizeof(double d)
static int	sizeof(float f)
static int	sizeof(int i)
static int	sizeof(long l)
int	sizeof(Object obj)
static int	sizeof(short s)

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

SZ_REF

public static final int SZ_REF

See Also:

Constant Field Values

Constructor Detail

SizeOf

public SizeOf()

Method Detail

sizeof

public static int sizeof(boolean b)

sizeof

public static int sizeof(byte b)

sizeof

public static int sizeof(char c)

sizeof

public static int sizeof(short s)

sizeof

public static int sizeof(int i)

sizeof

public static int sizeof(long l)

sizeof

public static int sizeof(float f)

sizeof

public static int sizeof(double d)

getByteSize

public static int getByteSize(Object obj)

clear

public void clear()

Makes this SizeOf object ready for reuse.

Since:

MMBase-1.8

sizeof

public int sizeof(Object obj)

Returns:

The size in bytes obj structure will take, or 0 if the object was already counted by this SizeOf object.

main

public static void main(String[] argv)
                 throws InterruptedException

Throws:

InterruptedException