org.github.jamm.strategies

Class UnsafeStrategy

java.lang.Object

org.github.jamm.strategies.MemoryLayoutBasedStrategy

org.github.jamm.strategies.UnsafeStrategy

All Implemented Interfaces:

MemoryMeterStrategy

public final class UnsafeStrategy
extends MemoryLayoutBasedStrategy

MemoryMeterStrategy relying on Unsafe to measure object sizes for Java versions ≥ 15.

In Java 15, the way the JVM layout fields across the hierarchy changed. Prior to Java 15 superclass field always came first. Therefore that strategy could only look at the current class to find the greatest offsets. From Java 15 onward it is not the case anymore. The JVM optimizes ensure minimal memory usage by packing the fields in the best possible way across the hierarchy (https://bugs.openjdk.org/browse/JDK-8237767).

Another important change that came in Java 15 is the introduction of hidden classes (https://openjdk.org/jeps/371) and the use of hidden class for lambda. Attempting to use Unsafe.objectFieldOffset on an hidden class field will result in a UnsupportedOperationException preventing the UnsafeStrategy to evaluate correctly the memory used by the class. To avoid that problem UnsafeStrategy will rely on the SpecStrategy to measure hidden classes. This can lead to an overestimation of the object size as the SpecStrategy ignore some optimizations performed by the JVM

Field Summary

Fields inherited from interface org.github.jamm.MemoryMeterStrategy

MEMORY_LAYOUT

Constructor Summary

Constructors

Constructor and Description
UnsafeStrategy(MethodHandle isRecordMH, MethodHandle isHiddenMH, MemoryLayoutBasedStrategy strategy)

Method Summary

Modifier and Type	Method and Description
protected int	arrayBaseOffset() Returns the array base offset.
long	measureInstance(Object instance, Class<?> type) Measures the shallow memory used by objects of the specified class.

Methods inherited from class org.github.jamm.strategies.MemoryLayoutBasedStrategy

computeArraySize, measure, measureArray, measureArray, measureArray, measureArray, measureArray, measureArray, measureArray, measureArray, measureArray, measureArray, measureField, supportComputeArraySize

Methods inherited from class java.lang.Object

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

Constructor Detail

UnsafeStrategy

public UnsafeStrategy(MethodHandle isRecordMH,
                      MethodHandle isHiddenMH,
                      MemoryLayoutBasedStrategy strategy)

Method Detail

measureInstance

public long measureInstance(Object instance,
                            Class<?> type)

Description copied from class: MemoryLayoutBasedStrategy

Measures the shallow memory used by objects of the specified class.

Specified by:

measureInstance in class MemoryLayoutBasedStrategy

Parameters:

instance - the object to measure

type - the object type

Returns:

the shallow memory used by the object

arrayBaseOffset

protected int arrayBaseOffset()

Description copied from class: MemoryLayoutBasedStrategy

Returns the array base offset.

Array base is aligned based on heap word. It is not visible by default as compressed references are used and the header size is 16 but becomes visible when they are disabled.

Overrides:

arrayBaseOffset in class MemoryLayoutBasedStrategy

Returns:

the array base offset.