Question

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In Java, what is the best way to determine the size of an object?

Suppose you have a data structure which allows you to add data to it, say a list:

List<KeyValuePair>

As client of the system actually adds content … at some point, JVM will run out of memory, it is only a question of when this happens. It would be nice to periodically save the object that contains this list to disk and gc the rest.

Using Java, is it possible to estimate how much memory an object is taking now?

Answer 1

You are looking for java.lang.instrument.Instrumentation.getObjectSize(Object object)

Documentation:

Returns an implementation-specific approximation of the amount of
storage consumed by the specified object. The result may include some
or all of the object’s overhead, and thus is useful for comparison
within an implementation but not between implementations. The estimate
may change during a single invocation of the JVM.

http://docs.oracle.com/javase/1.5.0/docs/api/java/lang/instrument/Instrumentation.html#getObjectSize%28java.lang.Object%29

But in order to be able to use it:

The only way to access an instance of the Instrumentation interface is
for the JVM to be launched in a way that indicates the agent class –
see the package specification. The Instrumentation instance is passed
to the premain method of the agent class. Once an agent acquires the
Instrumentation instance, the agent may call methods on the instance
at any time.

Good implementation sample:
http://www.javalobby.org/java/forums/t19309.html