$ java -cp build Probe     (JDK 25 default)
got Unsafe instance
WARNING: A terminally deprecated method in sun.misc.Unsafe has been called
WARNING: sun.misc.Unsafe::allocateMemory has been called by Probe (file:<build>/c25/)
WARNING: Please consider reporting this to the maintainers of class Probe
WARNING: sun.misc.Unsafe::allocateMemory will be removed in a future release
read back 42
allocate/put/get/free all succeeded
second use read 7

$ java --sun-misc-unsafe-memory-access=allow -cp build Probe
got Unsafe instance
read back 42
allocate/put/get/free all succeeded
second use read 7

$ java --sun-misc-unsafe-memory-access=warn -cp build Probe
got Unsafe instance
WARNING: A terminally deprecated method in sun.misc.Unsafe has been called
WARNING: sun.misc.Unsafe::allocateMemory has been called by Probe (file:<build>/c25/)
WARNING: Please consider reporting this to the maintainers of class Probe
WARNING: sun.misc.Unsafe::allocateMemory will be removed in a future release
read back 42
allocate/put/get/free all succeeded
second use read 7

$ java --sun-misc-unsafe-memory-access=debug -cp build Probe
got Unsafe instance
WARNING: sun.misc.Unsafe::allocateMemory called by Probe (file:<build>/c25/)
	at Probe.main(Probe.java:12)
WARNING: sun.misc.Unsafe::putLong called by Probe (file:<build>/c25/)
	at Probe.main(Probe.java:13)
WARNING: sun.misc.Unsafe::getLong called by Probe (file:<build>/c25/)
	at Probe.main(Probe.java:14)
read back 42
WARNING: sun.misc.Unsafe::freeMemory called by Probe (file:<build>/c25/)
	at Probe.main(Probe.java:15)
allocate/put/get/free all succeeded
WARNING: sun.misc.Unsafe::allocateMemory called by Probe (file:<build>/c25/)
	at Probe.main(Probe.java:21)
WARNING: sun.misc.Unsafe::putLong called by Probe (file:<build>/c25/)
	at Probe.main(Probe.java:22)
WARNING: sun.misc.Unsafe::getLong called by Probe (file:<build>/c25/)
	at Probe.main(Probe.java:23)
second use read 7
WARNING: sun.misc.Unsafe::freeMemory called by Probe (file:<build>/c25/)
	at Probe.main(Probe.java:24)

$ java --sun-misc-unsafe-memory-access=deny -cp build Probe
got Unsafe instance
FAILED: java.lang.UnsupportedOperationException: allocateMemory
second use FAILED: java.lang.UnsupportedOperationException: allocateMemory

# the JVM also publishes the flag as a system property that libraries can read
$ java tools/Prop.java     (default)
sun.misc.unsafe.memory.access=null
$ java --sun-misc-unsafe-memory-access=allow tools/Prop.java
sun.misc.unsafe.memory.access=allow
$ java --sun-misc-unsafe-memory-access=warn tools/Prop.java
sun.misc.unsafe.memory.access=warn
$ java --sun-misc-unsafe-memory-access=debug tools/Prop.java
sun.misc.unsafe.memory.access=debug
$ java --sun-misc-unsafe-memory-access=deny tools/Prop.java
sun.misc.unsafe.memory.access=deny