Information on memory usage of objects

The section on Java memory usage now contains the following additional articles:

• information on how to calculate the memory usage of a Java object in general, considering the memory used for "housekeeping" by the JVM
• calculating the memory usage of Strings, which can often the type of object to use up the biggest proportion of space in a Java application: this section actually considers the memory use of string-related objects such as StringBuffers and StringBuilders

A section on reducing the memory taken up by Strings looks at string canonicalisation, a fairly standard approach (but one which requires certain caveats), plus introduces the example of a CompactCharSequence class, that stores strings as 1 byte per character, thus taking up around half othe memory taken up by a regular Java String (at the expense of not supporting Unicode).

Comments on these articles welcome as usual.

Beta: Classmexer agent

The beta version of a simple instrumentation tool is available for download from the Javamex site. The Classmexer jar provides various calls for querying the memory usage of Java objects. Via the provided MemoryUtil class.deepMemoryUsageOf() method, it is possible to get an estimate from the JVM of the number of bytes taken up by an object and its "subobjects" (objects referred to by a non-public reference, or by references with other visibility criteria). The memory usage of subobjects is combined recursively (so subobjects of subobjects are considered etc), but without counting the same object more than once.

A variant of the call is also provided which gives the total memory usage of several objects at a time, without counting as duplicates objects referenced by more than one of the objects.