Pete Freitag Pete Freitag

Determining the size of objects in memory

coldfusionjava

Someone asked for a method to find out how much memory their cached queries, and components are using on the cf-talk mailing list today. In CFMX and in java there are no build in methods for determining the size of an object. The actual size of objects are a lot less important to Java and ColdFusion developers because the JVM handles all of the memory management for us. In languages like C the developer has to do all this work.

To find the size of an object you simply find the size of each object that it contains. That may sound recursive, and it is, but the base case (the values we know) are the sizes of primitive data types (such as int, char, float, double, etc). Primitive data types are always the same size on any platform in java (unlike some other languages). So to find the size of a string (there aren't really any primitives in CF, many types are just strings) we can just take the length of it and multiply it by the number of bytes per char, 2. Granted that is an estimate, there could be other data members inside the String object, and that's not counting things like the sizes of memory references.

Here's the function I came up with to determine the size of a query in ColdFusion:

<cffunction name="sizeOfQuery" returntype="numeric">
    <cfargument name="query" type="query" required="true">
    <cfset chars = 0>
    <cfoutput query="arguments.query">
        <cfloop list="#arguments.query.columnlist#" index="column">
            <cfset chars = chars +
Len(Evaluate("arguments.query.#column#"))>
        </cfloop>
    </cfoutput>
    <!--- multiply by two since chars in java are double bytes --->
    <cfreturn chars * 2>
</cffunction>

Finding the size of a Component is a bit more complicated, I will leave that as an exercise for the reader...


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Determining the size of objects in memory was first published on December 10, 2003.

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Comments

deepblue, I should have made it more clear that this is just an estimate. The actual size of the object is pretty much always going to be bigger than the function returns. Most objects do store private meta data, and there is no way for us to determine that size.
by Pete Freitag on 01/02/2004 at 10:26:41 PM UTC
another thing to keep in mind is that Java strings are usually interned.

meaning that if the string "Java Programmer" appears in the query 100 times, the memory allocation for "Java Programmer" only occurs once, and there are 100 pointers to it (in a 32bit JVM that would be 4 bytes each).
by Jay on 03/01/2011 at 2:31:09 PM UTC
This function you've written for getting the size of a ColdFusion Query in bytes has been very helpful to me. Thanks Pete!
by Marty McGee on 10/12/2011 at 7:13:39 PM UTC