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/*
* Copyright 1996-2001 Sun Microsystems, Inc. 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. Sun designates this
* particular file as subject to the "Classpath" exception as provided
* by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
* CA 95054 USA or visit www.sun.com if you need additional information or
* have any questions.
*/
package sun.rmi.transport;
import java.lang.ref.*;
import sun.rmi.runtime.Log;
/**
* WeakRef objects are used by the RMI runtime to hold potentially weak
* references to exported remote objects in the local object table.
*
* This class extends the functionality of java.lang.ref.WeakReference in
* several ways. The methods pin() and unpin() can be used to set
* whether the contained reference is strong or weak (it is weak upon
* construction). The hashCode() and equals() methods are overridden so
* that WeakRef objects hash and compare to each other according to the
* object identity of their referents.
*
* @author Ann Wollrath
* @author Peter Jones
*/
class WeakRef extends WeakReference {
/** value of the referent's "identity" hash code */
private int hashValue;
/** strong reference to the referent, for when this WeakRef is "pinned" */
private Object strongRef = null;
/**
* Create a new WeakRef to the given object.
*/
public WeakRef(Object obj) {
super(obj);
setHashValue(obj); // cache object's "identity" hash code
}
/**
* Create a new WeakRef to the given object, registered with a queue.
*/
public WeakRef(Object obj, ReferenceQueue q) {
super(obj, q);
setHashValue(obj); // cache object's "identity" hash code
}
/**
* Pin the contained reference (make this a strong reference).
*/
public synchronized void pin() {
if (strongRef == null) {
strongRef = get();
if (DGCImpl.dgcLog.isLoggable(Log.VERBOSE)) {
DGCImpl.dgcLog.log(Log.VERBOSE,
"strongRef = " + strongRef);
}
}
}
/**
* Unpin the contained reference (make this a weak reference).
*/
public synchronized void unpin() {
if (strongRef != null) {
if (DGCImpl.dgcLog.isLoggable(Log.VERBOSE)) {
DGCImpl.dgcLog.log(Log.VERBOSE,
"strongRef = " + strongRef);
}
strongRef = null;
}
}
/*
* Cache referent's "identity" hash code (so that we still have the
* value after the referent gets cleared).
*
* We cannot use the value from the object's hashCode() method, since
* if the object is of a remote class not extended from RemoteObject
* and it is trying to implement hashCode() and equals() so that it
* can be compared to stub objects, its own hash code could not have
* been initialized yet (see bugid 4102938). Also, object table keys
* based on server objects are indeed matched on object identity, so
* this is the correct hash technique regardless.
*/
private void setHashValue(Object obj) {
if (obj != null) {
hashValue = System.identityHashCode(obj);
} else {
hashValue = 0;
}
}
/**
* Always return the "identity" hash code of the original referent.
*/
public int hashCode() {
return hashValue;
}
/**
* Return true if "obj" is this identical WeakRef object, or, if the
* contained reference has not been cleared, if "obj" is another WeakRef
* object with the identical non-null referent. Otherwise, return false.
*/
public boolean equals(Object obj) {
if (obj instanceof WeakRef) {
if (obj == this)
return true;
Object referent = get();
return (referent != null) && (referent == ((WeakRef) obj).get());
} else {
return false;
}
}
}
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