Java example source code file (DGCClient.java)
The DGCClient.java Java example source code/*
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package sun.rmi.transport;
import java.lang.ref.PhantomReference;
import java.lang.ref.ReferenceQueue;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.rmi.ConnectException;
import java.rmi.RemoteException;
import java.rmi.dgc.DGC;
import java.rmi.dgc.Lease;
import java.rmi.dgc.VMID;
import java.rmi.server.ObjID;
import sun.misc.GC;
import sun.rmi.runtime.NewThreadAction;
import sun.rmi.server.UnicastRef;
import sun.rmi.server.Util;
import sun.security.action.GetLongAction;
/**
* DGCClient implements the client-side of the RMI distributed garbage
* collection system.
*
* The external interface to DGCClient is the "registerRefs" method.
* When a LiveRef to a remote object enters the VM, it needs to be
* registered with the DGCClient to participate in distributed garbage
* collection.
*
* When the first LiveRef to a particular remote object is registered,
* a "dirty" call is made to the server-side distributed garbage
* collector for the remote object, which returns a lease guaranteeing
* that the server-side DGC will not collect the remote object for a
* certain period of time. While LiveRef instances to remote objects
* on a particular server exist, the DGCClient periodically sends more
* "dirty" calls to renew its lease.
*
* The DGCClient tracks the local reachability of registered LiveRef
* instances (using phantom references). When the LiveRef instance
* for a particular remote object becomes garbage collected locally,
* a "clean" call is made to the server-side distributed garbage
* collector, indicating that the server no longer needs to keep the
* remote object alive for this client.
*
* @see java.rmi.dgc.DGC, sun.rmi.transport.DGCImpl
*
* @author Ann Wollrath
* @author Peter Jones
*/
final class DGCClient {
/** next sequence number for DGC calls (access synchronized on class) */
private static long nextSequenceNum = Long.MIN_VALUE;
/** unique identifier for this VM as a client of DGC */
private static VMID vmid = new VMID();
/** lease duration to request (usually ignored by server) */
private static final long leaseValue = // default 10 minutes
AccessController.doPrivileged(
new GetLongAction("java.rmi.dgc.leaseValue",
600000)).longValue();
/** maximum interval between retries of failed clean calls */
private static final long cleanInterval = // default 3 minutes
AccessController.doPrivileged(
new GetLongAction("sun.rmi.dgc.cleanInterval",
180000)).longValue();
/** maximum interval between complete garbage collections of local heap */
private static final long gcInterval = // default 1 hour
AccessController.doPrivileged(
new GetLongAction("sun.rmi.dgc.client.gcInterval",
3600000)).longValue();
/** minimum retry count for dirty calls that fail */
private static final int dirtyFailureRetries = 5;
/** retry count for clean calls that fail with ConnectException */
private static final int cleanFailureRetries = 5;
/** constant empty ObjID array for lease renewal optimization */
private static final ObjID[] emptyObjIDArray = new ObjID[0];
/** ObjID for server-side DGC object */
private static final ObjID dgcID = new ObjID(ObjID.DGC_ID);
/*
* Disallow anyone from creating one of these.
*/
private DGCClient() {}
/**
* Register the LiveRef instances in the supplied list to participate
* in distributed garbage collection.
*
* All of the LiveRefs in the list must be for remote objects at the
* given endpoint.
*/
static void registerRefs(Endpoint ep, List<LiveRef> refs) {
/*
* Look up the given endpoint and register the refs with it.
* The retrieved entry may get removed from the global endpoint
* table before EndpointEntry.registerRefs() is able to acquire
* its lock; in this event, it returns false, and we loop and
* try again.
*/
EndpointEntry epEntry;
do {
epEntry = EndpointEntry.lookup(ep);
} while (!epEntry.registerRefs(refs));
}
/**
* Get the next sequence number to be used for a dirty or clean
* operation from this VM. This method should only be called while
* synchronized on the EndpointEntry whose data structures the
* operation affects.
*/
private static synchronized long getNextSequenceNum() {
return nextSequenceNum++;
}
/**
* Given the length of a lease and the time that it was granted,
* compute the absolute time at which it should be renewed, giving
* room for reasonable computational and communication delays.
*/
private static long computeRenewTime(long grantTime, long duration) {
/*
* REMIND: This algorithm should be more sophisticated, waiting
* a longer fraction of the lease duration for longer leases.
*/
return grantTime + (duration / 2);
}
/**
* EndpointEntry encapsulates the client-side DGC information specific
* to a particular Endpoint. Of most significance is the table that
* maps LiveRef value to RefEntry objects and the renew/clean thread
* that handles asynchronous client-side DGC operations.
*/
private static class EndpointEntry {
/** the endpoint that this entry is for */
private Endpoint endpoint;
/** synthesized reference to the remote server-side DGC */
private DGC dgc;
/** table of refs held for endpoint: maps LiveRef to RefEntry */
private Map<LiveRef, RefEntry> refTable = new HashMap<>(5);
/** set of RefEntry instances from last (failed) dirty call */
private Set<RefEntry> invalidRefs = new HashSet<>(5);
/** true if this entry has been removed from the global table */
private boolean removed = false;
/** absolute time to renew current lease to this endpoint */
private long renewTime = Long.MAX_VALUE;
/** absolute time current lease to this endpoint will expire */
private long expirationTime = Long.MIN_VALUE;
/** count of recent dirty calls that have failed */
private int dirtyFailures = 0;
/** absolute time of first recent failed dirty call */
private long dirtyFailureStartTime;
/** (average) elapsed time for recent failed dirty calls */
private long dirtyFailureDuration;
/** renew/clean thread for handling lease renewals and clean calls */
private Thread renewCleanThread;
/** true if renew/clean thread may be interrupted */
private boolean interruptible = false;
/** reference queue for phantom references */
private ReferenceQueue<LiveRef> refQueue = new ReferenceQueue<>();
/** set of clean calls that need to be made */
private Set<CleanRequest> pendingCleans = new HashSet<>(5);
/** global endpoint table: maps Endpoint to EndpointEntry */
private static Map<Endpoint,EndpointEntry> endpointTable = new HashMap<>(5);
/** handle for GC latency request (for future cancellation) */
private static GC.LatencyRequest gcLatencyRequest = null;
/**
* Look up the EndpointEntry for the given Endpoint. An entry is
* created if one does not already exist.
*/
public static EndpointEntry lookup(Endpoint ep) {
synchronized (endpointTable) {
EndpointEntry entry = endpointTable.get(ep);
if (entry == null) {
entry = new EndpointEntry(ep);
endpointTable.put(ep, entry);
/*
* While we are tracking live remote references registered
* in this VM, request a maximum latency for inspecting the
* entire heap from the local garbage collector, to place
* an upper bound on the time to discover remote references
* that have become unreachable (see bugid 4171278).
*/
if (gcLatencyRequest == null) {
gcLatencyRequest = GC.requestLatency(gcInterval);
}
}
return entry;
}
}
private EndpointEntry(final Endpoint endpoint) {
this.endpoint = endpoint;
try {
LiveRef dgcRef = new LiveRef(dgcID, endpoint, false);
dgc = (DGC) Util.createProxy(DGCImpl.class,
new UnicastRef(dgcRef), true);
} catch (RemoteException e) {
throw new Error("internal error creating DGC stub");
}
renewCleanThread = AccessController.doPrivileged(
new NewThreadAction(new RenewCleanThread(),
"RenewClean-" + endpoint, true));
renewCleanThread.start();
}
/**
* Register the LiveRef instances in the supplied list to participate
* in distributed garbage collection.
*
* This method returns false if this entry was removed from the
* global endpoint table (because it was empty) before these refs
* could be registered. In that case, a new EndpointEntry needs
* to be looked up.
*
* This method must NOT be called while synchronized on this entry.
*/
public boolean registerRefs(List<LiveRef> refs) {
assert !Thread.holdsLock(this);
Set<RefEntry> refsToDirty = null; // entries for refs needing dirty
long sequenceNum; // sequence number for dirty call
synchronized (this) {
if (removed) {
return false;
}
Iterator<LiveRef> iter = refs.iterator();
while (iter.hasNext()) {
LiveRef ref = iter.next();
assert ref.getEndpoint().equals(endpoint);
RefEntry refEntry = refTable.get(ref);
if (refEntry == null) {
LiveRef refClone = (LiveRef) ref.clone();
refEntry = new RefEntry(refClone);
refTable.put(refClone, refEntry);
if (refsToDirty == null) {
refsToDirty = new HashSet<>(5);
}
refsToDirty.add(refEntry);
}
refEntry.addInstanceToRefSet(ref);
}
if (refsToDirty == null) {
return true;
}
refsToDirty.addAll(invalidRefs);
invalidRefs.clear();
sequenceNum = getNextSequenceNum();
}
makeDirtyCall(refsToDirty, sequenceNum);
return true;
}
/**
* Remove the given RefEntry from the ref table. If that makes
* the ref table empty, remove this entry from the global endpoint
* table.
*
* This method must ONLY be called while synchronized on this entry.
*/
private void removeRefEntry(RefEntry refEntry) {
assert Thread.holdsLock(this);
assert !removed;
assert refTable.containsKey(refEntry.getRef());
refTable.remove(refEntry.getRef());
invalidRefs.remove(refEntry);
if (refTable.isEmpty()) {
synchronized (endpointTable) {
endpointTable.remove(endpoint);
Transport transport = endpoint.getOutboundTransport();
transport.free(endpoint);
/*
* If there are no longer any live remote references
* registered, we are no longer concerned with the
* latency of local garbage collection here.
*/
if (endpointTable.isEmpty()) {
assert gcLatencyRequest != null;
gcLatencyRequest.cancel();
gcLatencyRequest = null;
}
removed = true;
}
}
}
/**
* Make a DGC dirty call to this entry's endpoint, for the ObjIDs
* corresponding to the given set of refs and with the given
* sequence number.
*
* This method must NOT be called while synchronized on this entry.
*/
private void makeDirtyCall(Set<RefEntry> refEntries, long sequenceNum) {
assert !Thread.holdsLock(this);
ObjID[] ids;
if (refEntries != null) {
ids = createObjIDArray(refEntries);
} else {
ids = emptyObjIDArray;
}
long startTime = System.currentTimeMillis();
try {
Lease lease =
dgc.dirty(ids, sequenceNum, new Lease(vmid, leaseValue));
long duration = lease.getValue();
long newRenewTime = computeRenewTime(startTime, duration);
long newExpirationTime = startTime + duration;
synchronized (this) {
dirtyFailures = 0;
setRenewTime(newRenewTime);
expirationTime = newExpirationTime;
}
} catch (Exception e) {
long endTime = System.currentTimeMillis();
synchronized (this) {
dirtyFailures++;
if (dirtyFailures == 1) {
/*
* If this was the first recent failed dirty call,
* reschedule another one immediately, in case there
* was just a transient network problem, and remember
* the start time and duration of this attempt for
* future calculations of the delays between retries.
*/
dirtyFailureStartTime = startTime;
dirtyFailureDuration = endTime - startTime;
setRenewTime(endTime);
} else {
/*
* For each successive failed dirty call, wait for a
* (binary) exponentially increasing delay before
* retrying, to avoid network congestion.
*/
int n = dirtyFailures - 2;
if (n == 0) {
/*
* Calculate the initial retry delay from the
* average time elapsed for each of the first
* two failed dirty calls. The result must be
* at least 1000ms, to prevent a tight loop.
*/
dirtyFailureDuration =
Math.max((dirtyFailureDuration +
(endTime - startTime)) >> 1, 1000);
}
long newRenewTime =
endTime + (dirtyFailureDuration << n);
/*
* Continue if the last known held lease has not
* expired, or else at least a fixed number of times,
* or at least until we've tried for a fixed amount
* of time (the default lease value we request).
*/
if (newRenewTime < expirationTime ||
dirtyFailures < dirtyFailureRetries ||
newRenewTime < dirtyFailureStartTime + leaseValue)
{
setRenewTime(newRenewTime);
} else {
/*
* Give up: postpone lease renewals until next
* ref is registered for this endpoint.
*/
setRenewTime(Long.MAX_VALUE);
}
}
if (refEntries != null) {
/*
* Add all of these refs to the set of refs for this
* endpoint that may be invalid (this VM may not be in
* the server's referenced set), so that we will
* attempt to explicitly dirty them again in the
* future.
*/
invalidRefs.addAll(refEntries);
/*
* Record that a dirty call has failed for all of these
* refs, so that clean calls for them in the future
* will be strong.
*/
Iterator<RefEntry> iter = refEntries.iterator();
while (iter.hasNext()) {
RefEntry refEntry = iter.next();
refEntry.markDirtyFailed();
}
}
/*
* If the last known held lease will have expired before
* the next renewal, all refs might be invalid.
*/
if (renewTime >= expirationTime) {
invalidRefs.addAll(refTable.values());
}
}
}
}
/**
* Set the absolute time at which the lease for this entry should
* be renewed.
*
* This method must ONLY be called while synchronized on this entry.
*/
private void setRenewTime(long newRenewTime) {
assert Thread.holdsLock(this);
if (newRenewTime < renewTime) {
renewTime = newRenewTime;
if (interruptible) {
AccessController.doPrivileged(
new PrivilegedAction<Void>() {
public Void run() {
renewCleanThread.interrupt();
return null;
}
});
}
} else {
renewTime = newRenewTime;
}
}
/**
* RenewCleanThread handles the asynchronous client-side DGC activity
* for this entry: renewing the leases and making clean calls.
*/
private class RenewCleanThread implements Runnable {
public void run() {
do {
long timeToWait;
RefEntry.PhantomLiveRef phantom = null;
boolean needRenewal = false;
Set<RefEntry> refsToDirty = null;
long sequenceNum = Long.MIN_VALUE;
synchronized (EndpointEntry.this) {
/*
* Calculate time to block (waiting for phantom
* reference notifications). It is the time until the
* lease renewal should be done, bounded on the low
* end by 1 ms so that the reference queue will always
* get processed, and if there are pending clean
* requests (remaining because some clean calls
* failed), bounded on the high end by the maximum
* clean call retry interval.
*/
long timeUntilRenew =
renewTime - System.currentTimeMillis();
timeToWait = Math.max(timeUntilRenew, 1);
if (!pendingCleans.isEmpty()) {
timeToWait = Math.min(timeToWait, cleanInterval);
}
/*
* Set flag indicating that it is OK to interrupt this
* thread now, such as if a earlier lease renewal time
* is set, because we are only going to be blocking
* and can deal with interrupts.
*/
interruptible = true;
}
try {
/*
* Wait for the duration calculated above for any of
* our phantom references to be enqueued.
*/
phantom = (RefEntry.PhantomLiveRef)
refQueue.remove(timeToWait);
} catch (InterruptedException e) {
}
synchronized (EndpointEntry.this) {
/*
* Set flag indicating that it is NOT OK to interrupt
* this thread now, because we may be undertaking I/O
* operations that should not be interrupted (and we
* will not be blocking arbitrarily).
*/
interruptible = false;
Thread.interrupted(); // clear interrupted state
/*
* If there was a phantom reference enqueued, process
* it and all the rest on the queue, generating
* clean requests as necessary.
*/
if (phantom != null) {
processPhantomRefs(phantom);
}
/*
* Check if it is time to renew this entry's lease.
*/
long currentTime = System.currentTimeMillis();
if (currentTime > renewTime) {
needRenewal = true;
if (!invalidRefs.isEmpty()) {
refsToDirty = invalidRefs;
invalidRefs = new HashSet<>(5);
}
sequenceNum = getNextSequenceNum();
}
}
if (needRenewal) {
makeDirtyCall(refsToDirty, sequenceNum);
}
if (!pendingCleans.isEmpty()) {
makeCleanCalls();
}
} while (!removed || !pendingCleans.isEmpty());
}
}
/**
* Process the notification of the given phantom reference and any
* others that are on this entry's reference queue. Each phantom
* reference is removed from its RefEntry's ref set. All ref
* entries that have no more registered instances are collected
* into up to two batched clean call requests: one for refs
* requiring a "strong" clean call, and one for the rest.
*
* This method must ONLY be called while synchronized on this entry.
*/
private void processPhantomRefs(RefEntry.PhantomLiveRef phantom) {
assert Thread.holdsLock(this);
Set<RefEntry> strongCleans = null;
Set<RefEntry> normalCleans = null;
do {
RefEntry refEntry = phantom.getRefEntry();
refEntry.removeInstanceFromRefSet(phantom);
if (refEntry.isRefSetEmpty()) {
if (refEntry.hasDirtyFailed()) {
if (strongCleans == null) {
strongCleans = new HashSet<>(5);
}
strongCleans.add(refEntry);
} else {
if (normalCleans == null) {
normalCleans = new HashSet<>(5);
}
normalCleans.add(refEntry);
}
removeRefEntry(refEntry);
}
} while ((phantom =
(RefEntry.PhantomLiveRef) refQueue.poll()) != null);
if (strongCleans != null) {
pendingCleans.add(
new CleanRequest(createObjIDArray(strongCleans),
getNextSequenceNum(), true));
}
if (normalCleans != null) {
pendingCleans.add(
new CleanRequest(createObjIDArray(normalCleans),
getNextSequenceNum(), false));
}
}
/**
* CleanRequest holds the data for the parameters of a clean call
* that needs to be made.
*/
private static class CleanRequest {
final ObjID[] objIDs;
final long sequenceNum;
final boolean strong;
/** how many times this request has failed */
int failures = 0;
CleanRequest(ObjID[] objIDs, long sequenceNum, boolean strong) {
this.objIDs = objIDs;
this.sequenceNum = sequenceNum;
this.strong = strong;
}
}
/**
* Make all of the clean calls described by the clean requests in
* this entry's set of "pending cleans". Clean requests for clean
* calls that succeed are removed from the "pending cleans" set.
*
* This method must NOT be called while synchronized on this entry.
*/
private void makeCleanCalls() {
assert !Thread.holdsLock(this);
Iterator<CleanRequest> iter = pendingCleans.iterator();
while (iter.hasNext()) {
CleanRequest request = iter.next();
try {
dgc.clean(request.objIDs, request.sequenceNum, vmid,
request.strong);
iter.remove();
} catch (Exception e) {
/*
* Many types of exceptions here could have been
* caused by a transient failure, so try again a
* few times, but not forever.
*/
if (++request.failures >= cleanFailureRetries) {
iter.remove();
}
}
}
}
/**
* Create an array of ObjIDs (needed for the DGC remote calls)
* from the ids in the given set of refs.
*/
private static ObjID[] createObjIDArray(Set<RefEntry> refEntries) {
ObjID[] ids = new ObjID[refEntries.size()];
Iterator<RefEntry> iter = refEntries.iterator();
for (int i = 0; i < ids.length; i++) {
ids[i] = iter.next().getRef().getObjID();
}
return ids;
}
/**
* RefEntry encapsulates the client-side DGC information specific
* to a particular LiveRef value. In particular, it contains a
* set of phantom references to all of the instances of the LiveRef
* value registered in the system (but not garbage collected
* locally).
*/
private class RefEntry {
/** LiveRef value for this entry (not a registered instance) */
private LiveRef ref;
/** set of phantom references to registered instances */
private Set<PhantomLiveRef> refSet = new HashSet<>(5);
/** true if a dirty call containing this ref has failed */
private boolean dirtyFailed = false;
public RefEntry(LiveRef ref) {
this.ref = ref;
}
/**
* Return the LiveRef value for this entry (not a registered
* instance).
*/
public LiveRef getRef() {
return ref;
}
/**
* Add a LiveRef to the set of registered instances for this entry.
*
* This method must ONLY be invoked while synchronized on this
* RefEntry's EndpointEntry.
*/
public void addInstanceToRefSet(LiveRef ref) {
assert Thread.holdsLock(EndpointEntry.this);
assert ref.equals(this.ref);
/*
* Only keep a phantom reference to the registered instance,
* so that it can be garbage collected normally (and we can be
* notified when that happens).
*/
refSet.add(new PhantomLiveRef(ref));
}
/**
* Remove a PhantomLiveRef from the set of registered instances.
*
* This method must ONLY be invoked while synchronized on this
* RefEntry's EndpointEntry.
*/
public void removeInstanceFromRefSet(PhantomLiveRef phantom) {
assert Thread.holdsLock(EndpointEntry.this);
assert refSet.contains(phantom);
refSet.remove(phantom);
}
/**
* Return true if there are no registered LiveRef instances for
* this entry still reachable in this VM.
*
* This method must ONLY be invoked while synchronized on this
* RefEntry's EndpointEntry.
*/
public boolean isRefSetEmpty() {
assert Thread.holdsLock(EndpointEntry.this);
return refSet.size() == 0;
}
/**
* Record that a dirty call that explicitly contained this
* entry's ref has failed.
*
* This method must ONLY be invoked while synchronized on this
* RefEntry's EndpointEntry.
*/
public void markDirtyFailed() {
assert Thread.holdsLock(EndpointEntry.this);
dirtyFailed = true;
}
/**
* Return true if a dirty call that explicitly contained this
* entry's ref has failed (and therefore a clean call for this
* ref needs to be marked "strong").
*
* This method must ONLY be invoked while synchronized on this
* RefEntry's EndpointEntry.
*/
public boolean hasDirtyFailed() {
assert Thread.holdsLock(EndpointEntry.this);
return dirtyFailed;
}
/**
* PhantomLiveRef is a PhantomReference to a LiveRef instance,
* used to detect when the LiveRef becomes permanently
* unreachable in this VM.
*/
private class PhantomLiveRef extends PhantomReference<LiveRef> {
public PhantomLiveRef(LiveRef ref) {
super(ref, EndpointEntry.this.refQueue);
}
public RefEntry getRefEntry() {
return RefEntry.this;
}
}
}
}
}
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