For simple stop-the-world collectors this value is just the time * since the most recent collection. For generational collectors it is the * time since the oldest generation was most recently collected. Other * collectors are free to return a pessimistic estimate of the elapsed * time, or simply the time since the last full collection was performed. * *
Note that in the presence of reference objects, a given object that
* is no longer strongly reachable may have to be inspected multiple times
* before it can be reclaimed.
*/
public static native long maxObjectInspectionAge();
private static class Daemon extends Thread {
public void run() {
for (;;) {
long l;
synchronized (lock) {
l = latencyTarget;
if (l == NO_TARGET) {
/* No latency target, so exit */
GC.daemon = null;
return;
}
long d = maxObjectInspectionAge();
if (d >= l) {
/* Do a full collection. There is a remote possibility
* that a full collection will occurr between the time
* we sample the inspection age and the time the GC
* actually starts, but this is sufficiently unlikely
* that it doesn't seem worth the more expensive JVM
* interface that would be required.
*/
System.gc();
d = 0;
}
/* Wait for the latency period to expire,
* or for notification that the period has changed
*/
try {
lock.wait(l - d);
} catch (InterruptedException x) {
continue;
}
}
}
}
private Daemon(ThreadGroup tg) {
super(tg, "GC Daemon");
}
/* Create a new daemon thread in the root thread group */
public static void create() {
PrivilegedAction pa = new PrivilegedAction() {
public Object run() {
ThreadGroup tg = Thread.currentThread().getThreadGroup();
for (ThreadGroup tgn = tg;
tgn != null;
tg = tgn, tgn = tg.getParent());
Daemon d = new Daemon(tg);
d.setDaemon(true);
d.setPriority(Thread.MIN_PRIORITY + 1);
d.start();
GC.daemon = d;
return null;
}};
AccessController.doPrivileged(pa);
}
}
/* Sets the latency target to the given value.
* Must be invoked while holding the lock.
*/
private static void setLatencyTarget(long ms) {
latencyTarget = ms;
if (daemon == null) {
/* Create a new daemon thread */
Daemon.create();
} else {
/* Notify the existing daemon thread
* that the lateency target has changed
*/
lock.notify();
}
}
/**
* Represents an active garbage-collection latency request. Instances of
* this class are created by the {@link #requestLatency}
* method. Given a request, the only interesting operation is that of
* cancellation.
*/
public static class LatencyRequest implements Comparable {
/* Instance counter, used to generate unique identifers */
private static long counter = 0;
/* Sorted set of active latency requests */
private static SortedSet requests = null;
/* Examine the request set and reset the latency target if necessary.
* Must be invoked while holding the lock.
*/
private static void adjustLatencyIfNeeded() {
if ((requests == null) || requests.isEmpty()) {
if (latencyTarget != NO_TARGET) {
setLatencyTarget(NO_TARGET);
}
} else {
LatencyRequest r = (LatencyRequest)requests.first();
if (r.latency != latencyTarget) {
setLatencyTarget(r.latency);
}
}
}
/* The requested latency, or NO_TARGET
* if this request has been cancelled
*/
private long latency;
/* Unique identifier for this request */
private long id;
private LatencyRequest(long ms) {
if (ms <= 0) {
throw new IllegalArgumentException("Non-positive latency: "
+ ms);
}
this.latency = ms;
synchronized (lock) {
this.id = ++counter;
if (requests == null) {
requests = new TreeSet();
}
requests.add(this);
adjustLatencyIfNeeded();
}
}
/**
* Cancels this latency request.
*
* @throws IllegalStateException
* If this request has already been cancelled
*/
public void cancel() {
synchronized (lock) {
if (this.latency == NO_TARGET) {
throw new IllegalStateException("Request already"
+ " cancelled");
}
if (!requests.remove(this)) {
throw new InternalError("Latency request "
+ this + " not found");
}
if (requests.isEmpty()) requests = null;
this.latency = NO_TARGET;
adjustLatencyIfNeeded();
}
}
public int compareTo(Object o) {
LatencyRequest r = (LatencyRequest)o;
long d = this.latency - r.latency;
if (d == 0) d = this.id - r.id;
return (d < 0) ? -1 : ((d > 0) ? +1 : 0);
}
public String toString() {
return (LatencyRequest.class.getName()
+ "[" + latency + "," + id + "]");
}
}
/**
* Makes a new request for a garbage-collection latency of the given
* number of real-time milliseconds. A low-priority daemon thread makes a
* best effort to ensure that the maximum object-inspection age never
* exceeds the smallest of the currently active requests.
*
* @param latency
* The requested latency
*
* @throws IllegalArgumentException
* If the given latency is non-positive
*/
public static LatencyRequest requestLatency(long latency) {
return new LatencyRequest(latency);
}
/**
* Returns the current smallest garbage-collection latency request, or zero
* if there are no active requests.
*/
public static long currentLatencyTarget() {
long t = latencyTarget;
return (t == NO_TARGET) ? 0 : t;
}
}