Class CloseableThreadLocal<T>

java.lang.Object

net.imglib2.util.CloseableThreadLocal<T>

All Implemented Interfaces:

Closeable, AutoCloseable

Direct Known Subclasses:

CloseableThreadLocal.SuppliedCloseableThreadLocal

public class CloseableThreadLocal<T>
extends Object
implements Closeable

Java's builtin ThreadLocal has a serious flaw: it can take an arbitrarily long amount of time to dereference the things you had stored in it, even once the ThreadLocal instance itself is no longer referenced. This is because there is single, master map stored for each thread, which all ThreadLocals share, and that master map only periodically purges "stale" entries.

While not technically a memory leak, because eventually the memory will be reclaimed, it can take a long time and you can easily hit OutOfMemoryError because from the GC's standpoint the stale entries are not reclaimable.

This class works around that, by only enrolling WeakReference values into the ThreadLocal, and separately holding a hard reference to each stored value. When you call close(), these hard references are cleared and then GC is freely able to reclaim space by objects stored in it.

We can not rely on ThreadLocal.remove() as it only removes the value for the caller thread, whereas close() takes care of all threads. You should not call close() until all threads are done using the instance.

Nested Class Summary

Nested Classes

Modifier and Type	Class	Description
(package private) static final class	CloseableThreadLocal.SuppliedCloseableThreadLocal<T>

Field Summary

Fields

Modifier and Type	Field	Description
private final AtomicInteger	countUntilPurge
private Map<Thread,T>	hardRefs
private static int	PURGE_MULTIPLIER
private ThreadLocal<WeakReference<T>>	t

Constructor Summary

Constructors

Constructor	Description
CloseableThreadLocal()

Method Summary

Modifier and Type	Method	Description
void	close()
T	get()
protected T	initialValue()
private void	maybePurge()
private void	purge()
void	set(T object)
static <S> CloseableThreadLocal<S>	withInitial(Supplier<? extends S> supplier)	Creates a thread local variable.

Methods inherited from class Object

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

Field Details

t

private ThreadLocal<WeakReference<T>> t

hardRefs

private Map<Thread,T> hardRefs

PURGE_MULTIPLIER

private static int PURGE_MULTIPLIER

countUntilPurge

private final AtomicInteger countUntilPurge

Constructor Details

CloseableThreadLocal

public CloseableThreadLocal()

Method Details

withInitial

public static <S> CloseableThreadLocal<S> withInitial(Supplier<? extends S> supplier)

Creates a thread local variable. The initial value of the variable is determined by invoking the get method on the Supplier.

initialValue

protected T initialValue()

get

public T get()

set

public void set(T object)

maybePurge

private void maybePurge()

purge

private void purge()

close

public void close()

Specified by:

close in interface AutoCloseable

Specified by:

close in interface Closeable