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* accompanied this code).
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package java.lang.ref;
import jdk.internal.vm.annotation.DontInline;
import jdk.internal.HotSpotIntrinsicCandidate;
import jdk.internal.misc.JavaLangRefAccess;
import jdk.internal.misc.SharedSecrets;
import jdk.internal.ref.Cleaner;
/**
* Abstract base class for reference objects. This class defines the
* operations common to all reference objects. Because reference objects are
* implemented in close cooperation with the garbage collector, this class may
* not be subclassed directly.
*
* @author Mark Reinhold
* @since 1.2
*/
public abstract class Reference<T> {
/* A Reference instance is in one of four possible internal states:
*
* Active: Subject to special treatment by the garbage collector. Some
* time after the collector detects that the reachability of the
* referent has changed to the appropriate state, it changes the
* instance's state to either Pending or Inactive, depending upon
* whether or not the instance was registered with a queue when it was
* created. In the former case it also adds the instance to the
* pending-Reference list. Newly-created instances are Active.
*
* Pending: An element of the pending-Reference list, waiting to be
* enqueued by the Reference-handler thread. Unregistered instances
* are never in this state.
*
* Enqueued: An element of the queue with which the instance was
* registered when it was created. When an instance is removed from
* its ReferenceQueue, it is made Inactive. Unregistered instances are
* never in this state.
*
* Inactive: Nothing more to do. Once an instance becomes Inactive its
* state will never change again.
*
* The state is encoded in the queue and next fields as follows:
*
* Active: queue = ReferenceQueue with which instance is registered, or
* ReferenceQueue.NULL if it was not registered with a queue; next =
* null.
*
* Pending: queue = ReferenceQueue with which instance is registered;
* next = this
*
* Enqueued: queue = ReferenceQueue.ENQUEUED; next = Following instance
* in queue, or this if at end of list.
*
* Inactive: queue = ReferenceQueue.NULL; next = this.
*
* With this scheme the collector need only examine the next field in order
* to determine whether a Reference instance requires special treatment: If
* the next field is null then the instance is active; if it is non-null,
* then the collector should treat the instance normally.
*
* To ensure that a concurrent collector can discover active Reference
* objects without interfering with application threads that may apply
* the enqueue() method to those objects, collectors should link
* discovered objects through the discovered field. The discovered
* field is also used for linking Reference objects in the pending list.
*/
private T referent; /* Treated specially by GC */
volatile ReferenceQueue<? super T> queue;
/* When active: NULL
* pending: this
* Enqueued: next reference in queue (or this if last)
* Inactive: this
*/
@SuppressWarnings("rawtypes")
volatile Reference next;
/* When active: next element in a discovered reference list maintained by GC (or this if last)
* pending: next element in the pending list (or null if last)
* otherwise: NULL
*/
private transient Reference<T> discovered; /* used by VM */
/* High-priority thread to enqueue pending References
*/
private static class ReferenceHandler extends Thread {
private static void ensureClassInitialized(Class<?> clazz) {
try {
Class.forName(clazz.getName(), true, clazz.getClassLoader());
} catch (ClassNotFoundException e) {
throw (Error) new NoClassDefFoundError(e.getMessage()).initCause(e);
}
}
static {
// pre-load and initialize Cleaner class so that we don't
// get into trouble later in the run loop if there's
// memory shortage while loading/initializing it lazily.
ensureClassInitialized(Cleaner.class);
}
ReferenceHandler(ThreadGroup g, String name) {
super(g, null, name, 0, false);
}
public void run() {
while (true) {
processPendingReferences();
}
}
}
/* Atomically get and clear (set to null) the VM's pending list.
*/
private static native Reference<Object> getAndClearReferencePendingList();
/* Test whether the VM's pending list contains any entries.
*/
private static native boolean hasReferencePendingList();
/* Wait until the VM's pending list may be non-null.
*/
private static native void waitForReferencePendingList();
private static final Object processPendingLock = new Object();
private static boolean processPendingActive = false;
private static void processPendingReferences() {
// Only the singleton reference processing thread calls
// waitForReferencePendingList() and getAndClearReferencePendingList().
// These are separate operations to avoid a race with other threads
// that are calling waitForReferenceProcessing().
waitForReferencePendingList();
Reference<Object> pendingList;
synchronized (processPendingLock) {
pendingList = getAndClearReferencePendingList();
processPendingActive = true;
}
while (pendingList != null) {
Reference<Object> ref = pendingList;
pendingList = ref.discovered;
ref.discovered = null;
if (ref instanceof Cleaner) {
((Cleaner)ref).clean();
// Notify any waiters that progress has been made.
// This improves latency for nio.Bits waiters, which
// are the only important ones.
synchronized (processPendingLock) {
processPendingLock.notifyAll();
}
} else {
ReferenceQueue<? super Object> q = ref.queue;
if (q != ReferenceQueue.NULL) q.enqueue(ref);
}
}
// Notify any waiters of completion of current round.
synchronized (processPendingLock) {
processPendingActive = false;
processPendingLock.notifyAll();
}
}
// Wait for progress in reference processing.
//
// Returns true after waiting (for notification from the reference
// processing thread) if either (1) the VM has any pending
// references, or (2) the reference processing thread is
// processing references. Otherwise, returns false immediately.
private static boolean waitForReferenceProcessing()
throws InterruptedException
{
synchronized (processPendingLock) {
if (processPendingActive || hasReferencePendingList()) {
// Wait for progress, not necessarily completion.
processPendingLock.wait();
return true;
} else {
return false;
}
}
}
static {
ThreadGroup tg = Thread.currentThread().getThreadGroup();
for (ThreadGroup tgn = tg;
tgn != null;
tg = tgn, tgn = tg.getParent());
Thread handler = new ReferenceHandler(tg, "Reference Handler");
/* If there were a special system-only priority greater than
* MAX_PRIORITY, it would be used here
*/
handler.setPriority(Thread.MAX_PRIORITY);
handler.setDaemon(true);
handler.start();
// provide access in SharedSecrets
SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
@Override
public boolean waitForReferenceProcessing()
throws InterruptedException
{
return Reference.waitForReferenceProcessing();
}
});
}
/* -- Referent accessor and setters -- */
/**
* Returns this reference object's referent. If this reference object has
* been cleared, either by the program or by the garbage collector, then
* this method returns <code>null</code>.
*
* @return The object to which this reference refers, or
* <code>null</code> if this reference object has been cleared
*/
@HotSpotIntrinsicCandidate
public T get() {
return this.referent;
}
/**
* Clears this reference object. Invoking this method will not cause this
* object to be enqueued.
*
* <p> This method is invoked only by Java code; when the garbage collector
* clears references it does so directly, without invoking this method.
*/
public void clear() {
this.referent = null;
}
/* -- Queue operations -- */
/**
* Tells whether or not this reference object has been enqueued, either by
* the program or by the garbage collector. If this reference object was
* not registered with a queue when it was created, then this method will
* always return <code>false</code>.
*
* @return <code>true</code> if and only if this reference object has
* been enqueued
*/
public boolean isEnqueued() {
return (this.queue == ReferenceQueue.ENQUEUED);
}
/**
* Adds this reference object to the queue with which it is registered,
* if any.
*
* <p> This method is invoked only by Java code; when the garbage collector
* enqueues references it does so directly, without invoking this method.
*
* @return <code>true</code> if this reference object was successfully
* enqueued; <code>false</code> if it was already enqueued or if
* it was not registered with a queue when it was created
*/
public boolean enqueue() {
return this.queue.enqueue(this);
}
/* -- Constructors -- */
Reference(T referent) {
this(referent, null);
}
Reference(T referent, ReferenceQueue<? super T> queue) {
this.referent = referent;
this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
}
/**
* Ensures that the object referenced by the given reference remains
* <a href="package-summary.html#reachability"><em>strongly reachable</em></a>,
* regardless of any prior actions of the program that might otherwise cause
* the object to become unreachable; thus, the referenced object is not
* reclaimable by garbage collection at least until after the invocation of
* this method. Invocation of this method does not itself initiate garbage
* collection or finalization.
*
* <p> This method establishes an ordering for
* <a href="package-summary.html#reachability"><em>strong reachability</em></a>
* with respect to garbage collection. It controls relations that are
* otherwise only implicit in a program -- the reachability conditions
* triggering garbage collection. This method is designed for use in
* uncommon situations of premature finalization where using
* {@code synchronized} blocks or methods, or using other synchronization
* facilities are not possible or do not provide the desired control. This
* method is applicable only when reclamation may have visible effects,
* which is possible for objects with finalizers (See
* <a href="https://docs.oracle.com/javase/specs/jls/se8/html/jls-12.html#jls-12.6">
* Section 12.6 17 of <cite>The Java™ Language Specification</cite></a>)
* that are implemented in ways that rely on ordering control for correctness.
*
* @apiNote
* Finalization may occur whenever the virtual machine detects that no
* reference to an object will ever be stored in the heap: The garbage
* collector may reclaim an object even if the fields of that object are
* still in use, so long as the object has otherwise become unreachable.
* This may have surprising and undesirable effects in cases such as the
* following example in which the bookkeeping associated with a class is
* managed through array indices. Here, method {@code action} uses a
* {@code reachabilityFence} to ensure that the {@code Resource} object is
* not reclaimed before bookkeeping on an associated
* {@code ExternalResource} has been performed; in particular here, to
* ensure that the array slot holding the {@code ExternalResource} is not
* nulled out in method {@link Object#finalize}, which may otherwise run
* concurrently.
*
* <pre> {@code
* class Resource {
* private static ExternalResource[] externalResourceArray = ...
*
* int myIndex;
* Resource(...) {
* myIndex = ...
* externalResourceArray[myIndex] = ...;
* ...
* }
* protected void finalize() {
* externalResourceArray[myIndex] = null;
* ...
* }
* public void action() {
* try {
* // ...
* int i = myIndex;
* Resource.update(externalResourceArray[i]);
* } finally {
* Reference.reachabilityFence(this);
* }
* }
* private static void update(ExternalResource ext) {
* ext.status = ...;
* }
* }}</pre>
*
* Here, the invocation of {@code reachabilityFence} is nonintuitively
* placed <em>after</em> the call to {@code update}, to ensure that the
* array slot is not nulled out by {@link Object#finalize} before the
* update, even if the call to {@code action} was the last use of this
* object. This might be the case if, for example a usage in a user program
* had the form {@code new Resource().action();} which retains no other
* reference to this {@code Resource}. While probably overkill here,
* {@code reachabilityFence} is placed in a {@code finally} block to ensure
* that it is invoked across all paths in the method. In a method with more
* complex control paths, you might need further precautions to ensure that
* {@code reachabilityFence} is encountered along all of them.
*
* <p> It is sometimes possible to better encapsulate use of
* {@code reachabilityFence}. Continuing the above example, if it were
* acceptable for the call to method {@code update} to proceed even if the
* finalizer had already executed (nulling out slot), then you could
* localize use of {@code reachabilityFence}:
*
* <pre> {@code
* public void action2() {
* // ...
* Resource.update(getExternalResource());
* }
* private ExternalResource getExternalResource() {
* ExternalResource ext = externalResourceArray[myIndex];
* Reference.reachabilityFence(this);
* return ext;
* }}</pre>
*
* <p> Method {@code reachabilityFence} is not required in constructions
* that themselves ensure reachability. For example, because objects that
* are locked cannot, in general, be reclaimed, it would suffice if all
* accesses of the object, in all methods of class {@code Resource}
* (including {@code finalize}) were enclosed in {@code synchronized (this)}
* blocks. (Further, such blocks must not include infinite loops, or
* themselves be unreachable, which fall into the corner case exceptions to
* the "in general" disclaimer.) However, method {@code reachabilityFence}
* remains a better option in cases where this approach is not as efficient,
* desirable, or possible; for example because it would encounter deadlock.
*
* @param ref the reference. If {@code null}, this method has no effect.
* @since 9
*/
@DontInline
public static void reachabilityFence(Object ref) {
// Does nothing, because this method is annotated with @DontInline
// HotSpot needs to retain the ref and not GC it before a call to this
// method
}
}