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package jdk.internal.ref;
import java.lang.ref.*;
import java.security.AccessController;
import java.security.PrivilegedAction;
General-purpose phantom-reference-based cleaners.
Cleaners are a lightweight and more robust alternative to finalization.
They are lightweight because they are not created by the VM and thus do not
require a JNI upcall to be created, and because their cleanup code is
invoked directly by the reference-handler thread rather than by the
finalizer thread. They are more robust because they use phantom references,
the weakest type of reference object, thereby avoiding the nasty ordering
problems inherent to finalization.
A cleaner tracks a referent object and encapsulates a thunk of arbitrary
cleanup code. Some time after the GC detects that a cleaner's referent has
become phantom-reachable, the reference-handler thread will run the cleaner.
Cleaners may also be invoked directly; they are thread safe and ensure that
they run their thunks at most once.
Cleaners are not a replacement for finalization. They should be used
only when the cleanup code is extremely simple and straightforward.
Nontrivial cleaners are inadvisable since they risk blocking the
reference-handler thread and delaying further cleanup and finalization.
Author: Mark Reinhold
/**
* General-purpose phantom-reference-based cleaners.
*
* <p> Cleaners are a lightweight and more robust alternative to finalization.
* They are lightweight because they are not created by the VM and thus do not
* require a JNI upcall to be created, and because their cleanup code is
* invoked directly by the reference-handler thread rather than by the
* finalizer thread. They are more robust because they use phantom references,
* the weakest type of reference object, thereby avoiding the nasty ordering
* problems inherent to finalization.
*
* <p> A cleaner tracks a referent object and encapsulates a thunk of arbitrary
* cleanup code. Some time after the GC detects that a cleaner's referent has
* become phantom-reachable, the reference-handler thread will run the cleaner.
* Cleaners may also be invoked directly; they are thread safe and ensure that
* they run their thunks at most once.
*
* <p> Cleaners are not a replacement for finalization. They should be used
* only when the cleanup code is extremely simple and straightforward.
* Nontrivial cleaners are inadvisable since they risk blocking the
* reference-handler thread and delaying further cleanup and finalization.
*
*
* @author Mark Reinhold
*/
public class Cleaner
extends PhantomReference<Object>
{
// Dummy reference queue, needed because the PhantomReference constructor
// insists that we pass a queue. Nothing will ever be placed on this queue
// since the reference handler invokes cleaners explicitly.
//
private static final ReferenceQueue<Object> dummyQueue = new ReferenceQueue<>();
// Doubly-linked list of live cleaners, which prevents the cleaners
// themselves from being GC'd before their referents
//
private static Cleaner first = null;
private Cleaner
next = null,
prev = null;
private static synchronized Cleaner add(Cleaner cl) {
if (first != null) {
cl.next = first;
first.prev = cl;
}
first = cl;
return cl;
}
private static synchronized boolean remove(Cleaner cl) {
// If already removed, do nothing
if (cl.next == cl)
return false;
// Update list
if (first == cl) {
if (cl.next != null)
first = cl.next;
else
first = cl.prev;
}
if (cl.next != null)
cl.next.prev = cl.prev;
if (cl.prev != null)
cl.prev.next = cl.next;
// Indicate removal by pointing the cleaner to itself
cl.next = cl;
cl.prev = cl;
return true;
}
private final Runnable thunk;
private Cleaner(Object referent, Runnable thunk) {
super(referent, dummyQueue);
this.thunk = thunk;
}
Creates a new cleaner.
Params: - ob – the referent object to be cleaned
- thunk –
The cleanup code to be run when the cleaner is invoked. The
cleanup code is run directly from the reference-handler thread,
so it should be as simple and straightforward as possible.
Returns: The new cleaner
/**
* Creates a new cleaner.
*
* @param ob the referent object to be cleaned
* @param thunk
* The cleanup code to be run when the cleaner is invoked. The
* cleanup code is run directly from the reference-handler thread,
* so it should be as simple and straightforward as possible.
*
* @return The new cleaner
*/
public static Cleaner create(Object ob, Runnable thunk) {
if (thunk == null)
return null;
return add(new Cleaner(ob, thunk));
}
Runs this cleaner, if it has not been run before.
/**
* Runs this cleaner, if it has not been run before.
*/
public void clean() {
if (!remove(this))
return;
try {
thunk.run();
} catch (final Throwable x) {
AccessController.doPrivileged(new PrivilegedAction<>() {
public Void run() {
if (System.err != null)
new Error("Cleaner terminated abnormally", x)
.printStackTrace();
System.exit(1);
return null;
}});
}
}
}