/*
 
* Copyright (c) 1997, 2017, Oracle and/or its affiliates. All rights reserved.
 
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 
*
 
* This code is free software; you can redistribute it and/or modify it
 
* under the terms of the GNU General Public License version 2 only, as
 
* published by the Free Software Foundation.
  
Oracle designates this
 
* particular file as subject to the "Classpath" exception as provided
 
* by Oracle in the LICENSE file that accompanied this code.
 
*
 
* This code is distributed in the hope that it will be useful, but WITHOUT
 
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 
* FITNESS FOR A PARTICULAR PURPOSE.
  
See the GNU General Public License
 
* version 2 for more details (a copy is included in the LICENSE file that
 
* accompanied this code).
 
*
 
* You should have received a copy of the GNU General Public License version
 
* 2 along with this work; if not, write to the Free Software Foundation,
 
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 
*
 
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 
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* questions.
 
*/

package java.lang.ref;

import sun.misc.Cleaner;
import sun.misc.JavaLangRefAccess;
import sun.misc.SharedSecrets;

/**
 
* 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
     
*/

    
transient private Reference<T> discovered;
  
/* used by VM */


    
/* Object used to synchronize with the garbage collector.
  
The collector
     
* must acquire this lock at the beginning of each collection cycle.
  
It is
     
* therefore critical that any code holding this lock complete as quickly
     
* as possible, allocate no new objects, and avoid calling user code.
     
*/

    
static private class Lock { }
    
private static Lock lock = new Lock();


    
/* List of References waiting to be enqueued.
  
The collector adds
     
* References to this list, while the Reference-handler thread removes
     
* them.
  
This list is protected by the above lock object. The
     
* list uses the discovered field to link its elements.
     
*/

    
private static Reference<Object> pending = null;

    
/* 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 InterruptedException and Cleaner classes
            
// so that we don't get into trouble later in the run loop if there's
            
// memory shortage while loading/initializing them lazily.
            
ensureClassInitialized(InterruptedException.class);
            
ensureClassInitialized(Cleaner.class);
        
}

        
ReferenceHandler(ThreadGroup g, String name) {
            
super(g, name);
        
}

        
public void run() {
            
while (true) {
                
tryHandlePending(true);
            
}
        
}
    
}

    
/**
     
* Try handle pending {@link Reference} if there is one.<p>
     
* Return {@code true} as a hint that there might be another
     
* {@link Reference} pending or {@code false} when there are no more pending
     
* {@link Reference}s at the moment and the program can do some other
     
* useful work instead of looping.
     
*
     
* @param waitForNotify if {@code true} and there was no pending
     
*
                      
{@link Reference}, wait until notified from VM
     
*
                      
or interrupted; if {@code false}, return immediately
     
*
                      
when there is no pending {@link Reference}.
     
* @return {@code true} if there was a {@link Reference} pending and it
     
*
         
was processed, or we waited for notification and either got it
     
*
         
or thread was interrupted before being notified;
     
*
         
{@code false} otherwise.
     
*/

    
static boolean tryHandlePending(boolean waitForNotify) {
        
Reference<Object> r;
        
Cleaner c;
        
try {
            
synchronized (lock) {
                
if (pending != null) {
                    
r = pending;
                    
// 'instanceof' might throw OutOfMemoryError sometimes
                    
// so do this before un-linking 'r' from the 'pending' chain...
                    
c = r instanceof Cleaner ? (Cleaner) r : null;
                    
// unlink 'r' from 'pending' chain
                    
pending = r.discovered;
                    
r.discovered = null;
                
} else {
                    
// The waiting on the lock may cause an OutOfMemoryError
                    
// because it may try to allocate exception objects.
                    
if (waitForNotify) {
                        
lock.wait();
                    
}
                    
// retry if waited
                    
return waitForNotify;
                
}
            
}
        
} catch (OutOfMemoryError x) {
            
// Give other threads CPU time so they hopefully drop some live references
            
// and GC reclaims some space.
            
// Also prevent CPU intensive spinning in case 'r instanceof Cleaner' above
            
// persistently throws OOME for some time...
            
Thread.yield();
            
// retry
            
return true;
        
} catch (InterruptedException x) {
            
// retry
            
return true;
        
}

        
// Fast path for cleaners
        
if (c != null) {
            
c.clean();
            
return true;
        
}

        
ReferenceQueue<? super Object> q = r.queue;
        
if (q != ReferenceQueue.NULL) q.enqueue(r);
        
return true;
    
}

    
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 tryHandlePendingReference() {
                
return tryHandlePending(false);
            
}
        
});
    
}

    
/* -- 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
     
*/

    
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;
    
}

}