com.twitter.common.objectsize
Class ObjectSizeCalculator

java.lang.Object
  extended by com.twitter.common.objectsize.ObjectSizeCalculator

public class ObjectSizeCalculator
extends Object

Contains utility methods for calculating the memory usage of objects. It only works on the HotSpot JVM, and infers the actual memory layout (32 bit vs. 64 bit word size, compressed object pointers vs. uncompressed) from best available indicators. It can reliably detect a 32 bit vs. 64 bit JVM. It can only make an educated guess at whether compressed OOPs are used, though; specifically, it knows what the JVM's default choice of OOP compression would be based on HotSpot version and maximum heap sizes, but if the choice is explicitly overridden with the -XX:{+|-}UseCompressedOops command line switch, it can not detect this fact and will report incorrect sizes, as it will presume the default JVM behavior.


Nested Class Summary
static interface ObjectSizeCalculator.MemoryLayoutSpecification
          Describes constant memory overheads for various constructs in a JVM implementation.
 
Constructor Summary
ObjectSizeCalculator(ObjectSizeCalculator.MemoryLayoutSpecification memoryLayoutSpecification)
          Creates an object size calculator that can calculate object sizes for a given memoryLayoutSpecification.
 
Method Summary
 long calculateObjectSize(Object obj)
          Given an object, returns the total allocated size, in bytes, of the object and all other objects reachable from it.
static long getObjectSize(Object obj)
          Given an object, returns the total allocated size, in bytes, of the object and all other objects reachable from it.
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Constructor Detail

ObjectSizeCalculator

public ObjectSizeCalculator(ObjectSizeCalculator.MemoryLayoutSpecification memoryLayoutSpecification)
Creates an object size calculator that can calculate object sizes for a given memoryLayoutSpecification.

Parameters:
memoryLayoutSpecification - a description of the JVM memory layout.
Method Detail

getObjectSize

public static long getObjectSize(Object obj)
                          throws UnsupportedOperationException
Given an object, returns the total allocated size, in bytes, of the object and all other objects reachable from it. Attempts to to detect the current JVM memory layout, but may fail with UnsupportedOperationException;

Parameters:
obj - the object; can be null. Passing in a Class object doesn't do anything special, it measures the size of all objects reachable through it (which will include its class loader, and by extension, all other Class objects loaded by the same loader, and all the parent class loaders). It doesn't provide the size of the static fields in the JVM class that the Class object represents.
Returns:
the total allocated size of the object and all other objects it retains.
Throws:
UnsupportedOperationException - if the current vm memory layout cannot be detected.

calculateObjectSize

public long calculateObjectSize(Object obj)
Given an object, returns the total allocated size, in bytes, of the object and all other objects reachable from it.

Parameters:
obj - the object; can be null. Passing in a Class object doesn't do anything special, it measures the size of all objects reachable through it (which will include its class loader, and by extension, all other Class objects loaded by the same loader, and all the parent class loaders). It doesn't provide the size of the static fields in the JVM class that the Class object represents.
Returns:
the total allocated size of the object and all other objects it retains.