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Compressed oops in the Hotspot JVM

What's an oop?

An "oop", or "ordinary object pointer" in HotSpot parlance is a managed pointer to an object.  It is normally the same size as a native machine pointer.  A managed pointer is carefully tracked by the Java application and GC subsystem, so that storage for unused objects can be reclaimed.  This process can also involve relocating (copying) objects which are in use, so that storage can be compacted.

The term "oop" is traditional to certain VMs that derive from Smalltalk and Self, including the following:

• Self (an prototype-based relative of Smalltalk) https://github.com/russellallen/self/blob/master/vm/src/any/objects/oop.hh^{Image Removed}
• Strongtalk (a Smalltalk implementation) http://code.google.com/p/strongtalk/wiki/VMTypesForSmalltalkObjects^{Image Removed}
• Hotspot  http://hg.openjdk.java.net/hsx/hotspot-main/hotspot/file/0/src/share/vm/oops/oop.hpp^{Image Removed}
• V8 http://code.google.com/p/v8/source/browse/trunk/src/objects.h^{Image Removed} (mentions "smi" but not "oop")

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