co.mindus.ip2location

Class BufferUnmapper

java.lang.Object

co.mindus.ip2location.BufferUnmapper

public final class BufferUnmapper
extends Object

Forces immediate unmapping of a MappedByteBuffer.

Java provides no public API to unmap a MappedByteBuffer. On Linux this is merely a nuisance (the OS allows replacing mapped files), but on Windows the file remains locked until the mapping is released, preventing database file replacement during updates.

Two strategies are supported, selected once at class load time based on the JVM major version:

JDK	Strategy	JVM flags
8	DirectBuffer.cleaner().clean()	none
9–16	Not supported — throws exception
17–25	Unsafe.invokeCleaner()	--add-opens java.base/sun.misc=ALL-UNNAMED
26	Unsafe.invokeCleaner()	--sun-misc-unsafe-memory-access=allow
27+	Requires rewrite to FFM Arena-based mapping

JDK 27+ migration

When Unsafe.invokeCleaner is removed (expected after JDK 26, see JEP 471/498), the reader must be rewritten to use the FFM API's Arena-based file mapping:

   Arena arena = Arena.ofConfined();
   MemorySegment seg = channel.map(READ_ONLY, 0, size, arena);
   // ... use seg for reads ...
   arena.close();  // unmaps the file immediately
 

There is no drop-in replacement for unmapping an existing MappedByteBuffer after JDK 26.

Everything is via reflection so the code compiles on JDK 8+.

Author:

Christopher Mindus

Method Summary

Modifier and Type	Method and Description
static String	getStrategyDescription() Returns the detected unmap strategy for diagnostic/logging purposes.
static void	unmap(MappedByteBuffer buffer) Forces immediate unmapping of a MappedByteBuffer.

Methods inherited from class java.lang.Object

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

Method Detail

unmap

public static void unmap(MappedByteBuffer buffer)

Forces immediate unmapping of a MappedByteBuffer.

On success, the OS file lock is released and the underlying file can be replaced or deleted. The buffer must NOT be used after this call.

Parameters:

buffer - The MappedByteBuffer to unmap (must not be null).

Throws:

UnsupportedOperationException - if running on Java 9-16 (which is not supported for this operation).

getStrategyDescription

public static String getStrategyDescription()

Returns the detected unmap strategy for diagnostic/logging purposes.

Returns:

A human-readable description of the strategy in use.