Java example source code file (GcFinalization.java)
This example Java source code file (GcFinalization.java) is included in the alvinalexander.com
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/*
* Copyright (C) 2011 The Guava Authors
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.google.common.testing;
import static java.util.concurrent.TimeUnit.SECONDS;
import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtIncompatible;
import com.google.j2objc.annotations.J2ObjCIncompatible;
import java.lang.ref.WeakReference;
import java.util.Locale;
import java.util.concurrent.CancellationException;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.TimeoutException;
/**
* Testing utilities relating to garbage collection finalization.
*
* <p>Use this class to test code triggered by finalization, that is, one of the
* following actions taken by the java garbage collection system:
*
* <ul>
* <li>invoking the {@code finalize} methods of unreachable objects
* <li>clearing weak references to unreachable referents
* <li>enqueuing weak references to unreachable referents in their reference queue
* </ul>
*
* <p>This class uses (possibly repeated) invocations of {@link java.lang.System#gc()} to cause
* finalization to happen. However, a call to {@code System.gc()} is specified to be no more
* than a hint, so this technique may fail at the whim of the JDK implementation, for example if
* a user specified the JVM flag {@code -XX:+DisableExplicitGC}. But in practice, it works very
* well for ordinary tests.
*
* <p>Failure of the expected event to occur within an implementation-defined "reasonable" time
* period or an interrupt while waiting for the expected event will result in a {@link
* RuntimeException}.
*
* <p>Here's an example that tests a {@code finalize} method:
*
* <pre> {@code
* final CountDownLatch latch = new CountDownLatch(1);
* Object x = new MyClass() {
* ...
* protected void finalize() { latch.countDown(); ... }
* };
* x = null; // Hint to the JIT that x is stack-unreachable
* GcFinalization.await(latch);}</pre>
*
* <p>Here's an example that uses a user-defined finalization predicate:
*
* <pre> {@code
* final WeakHashMap<Object, Object> map = new WeakHashMap();
* map.put(new Object(), Boolean.TRUE);
* GcFinalization.awaitDone(new FinalizationPredicate() {
* public boolean isDone() {
* return map.isEmpty();
* }
* });}</pre>
*
* <p>Even if your non-test code does not use finalization, you can
* use this class to test for leaks, by ensuring that objects are no
* longer strongly referenced:
*
* <pre> {@code
* // Helper function keeps victim stack-unreachable.
* private WeakReference<Foo> fooWeakRef() {
* Foo x = ....;
* WeakReference<Foo> weakRef = new WeakReference(x);
* // ... use x ...
* x = null; // Hint to the JIT that x is stack-unreachable
* return weakRef;
* }
* public void testFooLeak() {
* GcFinalization.awaitClear(fooWeakRef());
* }}</pre>
*
* <p>This class cannot currently be used to test soft references, since this class does not try to
* create the memory pressure required to cause soft references to be cleared.
*
* <p>This class only provides testing utilities. It is not designed for direct use in production
* or for benchmarking.
*
* @author mike nonemacher
* @author Martin Buchholz
* @since 11.0
*/
@Beta
@GwtIncompatible
@J2ObjCIncompatible // gc
public final class GcFinalization {
private GcFinalization() {}
/**
* 10 seconds ought to be long enough for any object to be GC'ed and finalized. Unless we have a
* gigantic heap, in which case we scale by heap size.
*/
private static long timeoutSeconds() {
// This class can make no hard guarantees. The methods in this class are inherently flaky, but
// we try hard to make them robust in practice. We could additionally try to add in a system
// load timeout multiplier. Or we could try to use a CPU time bound instead of wall clock time
// bound. But these ideas are harder to implement. We do not try to detect or handle a
// user-specified -XX:+DisableExplicitGC.
//
// TODO(user): Consider using
// java/lang/management/OperatingSystemMXBean.html#getSystemLoadAverage()
//
// TODO(user): Consider scaling by number of mutator threads,
// e.g. using Thread#activeCount()
return Math.max(10L, Runtime.getRuntime().totalMemory() / (32L * 1024L * 1024L));
}
/**
* Waits until the given future {@linkplain Future#isDone is done}, invoking the garbage
* collector as necessary to try to ensure that this will happen.
*
* @throws RuntimeException if timed out or interrupted while waiting
*/
public static void awaitDone(Future<?> future) {
if (future.isDone()) {
return;
}
final long timeoutSeconds = timeoutSeconds();
final long deadline = System.nanoTime() + SECONDS.toNanos(timeoutSeconds);
do {
System.runFinalization();
if (future.isDone()) {
return;
}
System.gc();
try {
future.get(1L, SECONDS);
return;
} catch (CancellationException ok) {
return;
} catch (ExecutionException ok) {
return;
} catch (InterruptedException ie) {
throw new RuntimeException("Unexpected interrupt while waiting for future", ie);
} catch (TimeoutException tryHarder) {
/* OK */
}
} while (System.nanoTime() - deadline < 0);
throw formatRuntimeException("Future not done within %d second timeout", timeoutSeconds);
}
/**
* Waits until the given latch has {@linkplain CountDownLatch#countDown counted down} to zero,
* invoking the garbage collector as necessary to try to ensure that this will happen.
*
* @throws RuntimeException if timed out or interrupted while waiting
*/
public static void await(CountDownLatch latch) {
if (latch.getCount() == 0) {
return;
}
final long timeoutSeconds = timeoutSeconds();
final long deadline = System.nanoTime() + SECONDS.toNanos(timeoutSeconds);
do {
System.runFinalization();
if (latch.getCount() == 0) {
return;
}
System.gc();
try {
if (latch.await(1L, SECONDS)) {
return;
}
} catch (InterruptedException ie) {
throw new RuntimeException("Unexpected interrupt while waiting for latch", ie);
}
} while (System.nanoTime() - deadline < 0);
throw formatRuntimeException(
"Latch failed to count down within %d second timeout", timeoutSeconds);
}
/**
* Creates a garbage object that counts down the latch in its finalizer. Sequestered into a
* separate method to make it somewhat more likely to be unreachable.
*/
private static void createUnreachableLatchFinalizer(final CountDownLatch latch) {
new Object() { @Override protected void finalize() { latch.countDown(); }};
}
/**
* A predicate that is expected to return true subsequent to <em>finalization, that is, one
* of the following actions taken by the garbage collector when performing a full collection in
* response to {@link System#gc()}:
*
* <ul>
* <li>invoking the {@code finalize} methods of unreachable objects
* <li>clearing weak references to unreachable referents
* <li>enqueuing weak references to unreachable referents in their reference queue
* </ul>
*/
public interface FinalizationPredicate {
boolean isDone();
}
/**
* Waits until the given predicate returns true, invoking the garbage collector as necessary to
* try to ensure that this will happen.
*
* @throws RuntimeException if timed out or interrupted while waiting
*/
public static void awaitDone(FinalizationPredicate predicate) {
if (predicate.isDone()) {
return;
}
final long timeoutSeconds = timeoutSeconds();
final long deadline = System.nanoTime() + SECONDS.toNanos(timeoutSeconds);
do {
System.runFinalization();
if (predicate.isDone()) {
return;
}
CountDownLatch done = new CountDownLatch(1);
createUnreachableLatchFinalizer(done);
await(done);
if (predicate.isDone()) {
return;
}
} while (System.nanoTime() - deadline < 0);
throw formatRuntimeException(
"Predicate did not become true within %d second timeout", timeoutSeconds);
}
/**
* Waits until the given weak reference is cleared, invoking the garbage collector as necessary
* to try to ensure that this will happen.
*
* <p>This is a convenience method, equivalent to:
* <pre> {@code
* awaitDone(new FinalizationPredicate() {
* public boolean isDone() {
* return ref.get() == null;
* }
* });}</pre>
*
* @throws RuntimeException if timed out or interrupted while waiting
*/
public static void awaitClear(final WeakReference<?> ref) {
awaitDone(new FinalizationPredicate() {
public boolean isDone() {
return ref.get() == null;
}
});
}
/**
* Tries to perform a "full" garbage collection cycle (including processing of weak references
* and invocation of finalize methods) and waits for it to complete. Ensures that at least one
* weak reference has been cleared and one {@code finalize} method has been run before this
* method returns. This method may be useful when testing the garbage collection mechanism
* itself, or inhibiting a spontaneous GC initiation in subsequent code.
*
* <p>In contrast, a plain call to {@link java.lang.System#gc()} does not ensure finalization
* processing and may run concurrently, for example, if the JVM flag {@code
* -XX:+ExplicitGCInvokesConcurrent} is used.
*
* <p>Whenever possible, it is preferable to test directly for some observable change resulting
* from GC, as with {@link #awaitClear}. Because there are no guarantees for the order of GC
* finalization processing, there may still be some unfinished work for the GC to do after this
* method returns.
*
* <p>This method does not create any memory pressure as would be required to cause soft
* references to be processed.
*
* @throws RuntimeException if timed out or interrupted while waiting
* @since 12.0
*/
public static void awaitFullGc() {
final CountDownLatch finalizerRan = new CountDownLatch(1);
WeakReference<Object> ref = new WeakReference
