org.github.jamm

Class VM

java.lang.Object

org.github.jamm.VM

public final class VM
extends Object

Utility class for retrieving information from the JVM.

Method Summary

Modifier and Type	Method and Description
static int	contendedPaddingWidth() Returns the number of bytes used to pad the fields/classes annotated with Contended.
static boolean	enableContended() Checks if @Contended annotations are enabled.
static int	getObjectAlignmentInBytes() Retrieve the object alignment in bytes from the JVM.
static sun.misc.Unsafe	getUnsafe() Returns Unsafe if it is available.
static boolean	hasUnsafe() Checks if Unsafe is available.
static boolean	is32Bits() Checks if the JVM is a 32 bits one.
static boolean	isPreJava12JVM() Checks if the JVM is a pre-Java 12 version.
static void	printOffsets(Object obj) Utility method using Unsafe to print the field offset for debugging.
static boolean	restrictContended() Checks if the JVM restricts the use of @Contended to internal classes.
static boolean	useCompressedClassPointers() Checks if the JVM uses compressed class pointers.
static boolean	useCompressedOops() Checks if the JVM uses compressed reference.
static boolean	useEmptySlotsInSuper() Checks if the JVM uses more aggressive optimizations to avoid unused gaps in instances.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Method Detail

getObjectAlignmentInBytes

public static int getObjectAlignmentInBytes()

Retrieve the object alignment in bytes from the JVM. If the alignment cannot be retrieved the default value will be returned.

Returns:

the object alignment in bytes if it can be retrieved or the default value (8).

useCompressedOops

public static boolean useCompressedOops()

Checks if the JVM uses compressed reference.

Returns:

{true if the JVM use compressed references false otherwise.

useCompressedClassPointers

public static boolean useCompressedClassPointers()

Checks if the JVM uses compressed class pointers.

Returns:

{true if the JVM use compressed class pointers false otherwise.

useEmptySlotsInSuper

public static boolean useEmptySlotsInSuper()

Checks if the JVM uses more aggressive optimizations to avoid unused gaps in instances.

Returns:

{true if the JVM use empty slots in super class false otherwise.

restrictContended

public static boolean restrictContended()

Checks if the JVM restricts the use of @Contended to internal classes.

Returns:

{true if the JVM restricts the use of @Contended to internal classes, false otherwise.

enableContended

public static boolean enableContended()

Checks if @Contended annotations are enabled.

Returns:

{true if @Contended annotations are enabled, false otherwise.

contendedPaddingWidth

public static int contendedPaddingWidth()

Returns the number of bytes used to pad the fields/classes annotated with Contended.

The value will be between 0 and 8192 (inclusive) and will be a multiple of 8.

Returns:

the number of bytes used to pad the fields/classes annotated with Contended.

is32Bits

public static boolean is32Bits()

Checks if the JVM is a 32 bits one.

Returns:

true if the JVM is a 32 bits version, false otherwise.

isPreJava12JVM

public static boolean isPreJava12JVM()

Checks if the JVM is a pre-Java 12 version.

Returns:

true if the JVM is a pre-Java 12 version, false otherwise.

hasUnsafe

public static boolean hasUnsafe()

Checks if Unsafe is available.

Returns:

true if unsafe is available, false otherwise.

getUnsafe

public static sun.misc.Unsafe getUnsafe()

Returns Unsafe if it is available.

Returns:

Unsafe if it is available, null otherwise.

printOffsets

public static void printOffsets(Object obj)

Utility method using Unsafe to print the field offset for debugging.

Parameters:

obj - the object to analyze