What is Java Compressed Class Pointers: -XX:-UseCompressedClassPointers?

Usage:

• -XX:-UseCompressedClassPointers
• -XX:+UseCompressedClassPointers

Description:

The -XX:-UseCompressedClassPointers flag disables the use of compressed
class pointers. It is only available or relevant on 64-bit JVMs.

This feature is a variant of -XX:-UseCompressedOops except that it relates to
class pointers rather than object pointers. Every Java object has a pointer to
a native data structure outside the Java heap, known as the Klass structure.
Using compressed pointers allows this pointer to the Klass structure to be stored
by each object as a 32-bit number rather than a 64-bit number.

See -XX:-UseCompressedOops for more information on compressed pointers.

The -XX:-UseCompressedClassPointers flag turns off the compressed class
pointers feature. There is little reason ever to do this except if it is suspected to
be the cause of a bug or JVM crash.

The -XX:+UseCompressedClassPointers flag re-enables the use of Compressed Pointers. Where both -XX:-UseCompressedClassPointers and

-XX:+UseCompressedClassPointers have been used on a command line, the
last one takes precedence.

Default Value:

Compressed Class Pointers are enabled by default on Java 1.7 and above when
the maximum heap size (as specified by -Xmx) does not exceed 32 GB.
When compressed object pointers are disabled, either with -XX:-UseCompressedOops
or because the Java heap exceeds 32 GB, compressed class pointers are also
disabled (i.e. as if -XX:-UseCompressedClassPointers had been specified).

Errors:

None.

Arguments Related to -XX:-UseCompressedClassPointers:

• TODO: link to -Xmx
• TODO: link to -XX:-UseCompressedOops
• TODO: link to -XX:ObjectAlignmentInBytes

Related Posts:

• Compressed OOPs in the JVM
• What is Compressed Class Space?

NOTE:
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