uk.ac.starlink.fits

Class Unmapper

java.lang.Object

uk.ac.starlink.fits.Unmapper

public abstract class Unmapper
extends java.lang.Object

Knows how to map a region of a file in such a way that it is possible to attempt subsequent freeing of associated resources.

This is a tricky business. At Java 8 there is no good way to unmap the memory mapped by a MappedByteBuffer. The MappedByteBuffer javadocs say "A mapped byte buffer and the file mapping that it represents remain valid until the buffer itself is garbage-collected.", and there is no explicit unmap method. However, since the resources locked by memory mapping are separate from the JVM heap, there is no guarantee that garbage collection will happen even when mapped memory reaches crisis levels, and in practice this can lead to cache thrashing and the OS locking up even when there are many MappedByteBuffers without active references, at least on some platforms (I see it on Scientific Linux 6.5 with 24Gb RAM, but not SL 6.3 with 4Gb, but I'm not sure what the relevant differences are).

The best way around this is dependent on Java version. Before about Java 22, the only way was using implementation-specific classes in the sun.* namespace to do the unmapping. This is not guaranteed to work on all J2SE implementations, and moreover it risks a JVM crash if the buffer instance is used after the umapping has been done. So use it WITH EXTREME CAUTION.

At Java 22 and later (previewed in Java 19?), public classes in the java.lang.foreign package can be used for this purpose, which is safe.

Provision of the default Unmapper instance, available from the static getInstance() method, is therefore done using reflection to provide an implementation that depends on the current JRE. It is possible to specify a preference for how this is done among the available options (or providing a custom Unampper implementation) by use of the UNMAP_PROPERTY ("startable.unmap") system property, or programmatically using the setInstance method.

Since:

2 Dec 2014

See Also:

Java Bug #4724038, JEP 498, JEP 471

Field Summary

Fields

Modifier and Type	Field and Description
static Unmapper	NOP Unmapper instance that makes no attempt to reclaim resources.
static java.lang.String	UNMAP_PROPERTY Name of system property to control buffer unmapping ("startable.unmap").

Constructor Summary

Constructors

Modifier	Constructor and Description
protected	Unmapper() Constructor.

Method Summary

Modifier and Type	Method and Description
static Unmapper	getInstance() Returns the standard instance of this class.
abstract UnmappableBuffer	mapFile(java.nio.channels.FileChannel channel, long offset, int leng) Creates an object that represents a read-only mapped region of a file, and for which it is possible to attempt release of resources at a later date.
void	setInstance(Unmapper unmapper) Sets the instance of this class that will be returned by the getInstance() method.

Methods inherited from class java.lang.Object

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

Field Detail

UNMAP_PROPERTY

public static final java.lang.String UNMAP_PROPERTY

Name of system property to control buffer unmapping ("startable.unmap"). Possible values are currently:

• "sun": best-efforts sun.misc-based option
• "cleaner": sun.misc.Cleaner-based option (available in Oracle Java 6 through Java 8)
• "unsafe": sun.misc.Unsafe-based option (available in Oracle Java 9 through Java 25?)
• "memseg": java.lang.foreign.MemorySegment-based option (available in approx Java 22 and later)
• "none": no unmapping
• "default": tries to use memseg, then sun, then none, depending on what's available

You can also give the classname of an Unmapper concrete subclass with a no-arg constructor.

If the property is not set, behaviour is as "default".

See Also:

Constant Field Values

NOP

public static final Unmapper NOP

Unmapper instance that makes no attempt to reclaim resources. It is safe.

Constructor Detail

Unmapper

protected Unmapper()

Constructor.

Method Detail

mapFile

public abstract UnmappableBuffer mapFile(java.nio.channels.FileChannel channel,
                                         long offset,
                                         int leng)
                                  throws java.io.IOException

Creates an object that represents a read-only mapped region of a file, and for which it is possible to attempt release of resources at a later date. Whether such a release attempt is successful depends on the implementation.

No explicit checking of the parameters is done by this method; helpful messages to users about bad parameter choices should be generated by the caller.

Parameters:

channel - file channel

offset - offset into file of mapped region start

leng - number of bytes to map

Returns:

unmappable buffer object

Throws:

java.io.IOException

getInstance

public static Unmapper getInstance()

Returns the standard instance of this class. Under normal circumstances this will return a usable instance, but if the UNMAP_PROPERTY has asked for something specific that cannot be provided, a RuntimeException will be thrown.

Returns:

lazily constructed instance

Throws:

java.lang.RuntimeException - if an option has been explicitly chosen that cannot be instantiated

setInstance

public void setInstance(Unmapper unmapper)

Sets the instance of this class that will be returned by the getInstance() method.

Parameters:

unmapper - default instance