io.grpc.internal.SharedResourceHolder Maven / Gradle / Ivy
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package io.grpc.internal;
import com.google.common.base.Preconditions;
import java.util.IdentityHashMap;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.TimeUnit;
import javax.annotation.concurrent.ThreadSafe;
/**
* A holder for shared resource singletons.
*
* Components like client channels and servers need certain resources, e.g. a thread pool, to
* run. If the user has not provided such resources, these components will use a default one, which
* is shared as a static resource. This class holds these default resources and manages their
* life-cycles.
*
*
A resource is identified by the reference of a {@link Resource} object, which is typically a
* singleton, provided to the get() and release() methods. Each Resource object (not its class) maps
* to an object cached in the holder.
*
*
Resources are ref-counted and shut down after a delay when the ref-count reaches zero.
*/
@ThreadSafe
public final class SharedResourceHolder {
static final long DESTROY_DELAY_SECONDS = 1;
// The sole holder instance.
private static final SharedResourceHolder holder = new SharedResourceHolder(
new ScheduledExecutorFactory() {
@Override
public ScheduledExecutorService createScheduledExecutor() {
return Executors.newSingleThreadScheduledExecutor(new ThreadFactory() {
@Override public Thread newThread(Runnable r) {
Thread thread = new Thread(r);
thread.setDaemon(true);
return thread;
}
});
}
});
private final IdentityHashMap, Instance> instances =
new IdentityHashMap, Instance>();
private final ScheduledExecutorFactory destroyerFactory;
private ScheduledExecutorService destroyer;
// Visible to tests that would need to create instances of the holder.
SharedResourceHolder(ScheduledExecutorFactory destroyerFactory) {
this.destroyerFactory = destroyerFactory;
}
/**
* Try to get an existing instance of the given resource. If an instance does not exist, create a
* new one with the given factory.
*
* @param resource the singleton object that identifies the requested static resource
*/
public static T get(Resource resource) {
return holder.getInternal(resource);
}
/**
* Releases an instance of the given resource.
*
* The instance must have been obtained from {@link #get(Resource)}. Otherwise will throw
* IllegalArgumentException.
*
*
Caller must not release a reference more than once. It's advisory that you clear the
* reference to the instance with the null returned by this method.
*
* @param resource the singleton Resource object that identifies the released static resource
* @param instance the released static resource
*
* @return a null which the caller can use to clear the reference to that instance.
*/
public static T release(final Resource resource, final T instance) {
return holder.releaseInternal(resource, instance);
}
/**
* Visible to unit tests.
*
* @see #get(Resource)
*/
@SuppressWarnings("unchecked")
synchronized T getInternal(Resource resource) {
Instance instance = instances.get(resource);
if (instance == null) {
instance = new Instance(resource.create());
instances.put(resource, instance);
}
if (instance.destroyTask != null) {
instance.destroyTask.cancel(false);
instance.destroyTask = null;
}
instance.refcount++;
return (T) instance.payload;
}
/**
* Visible to unit tests.
*/
synchronized T releaseInternal(final Resource resource, final T instance) {
final Instance cached = instances.get(resource);
if (cached == null) {
throw new IllegalArgumentException("No cached instance found for " + resource);
}
Preconditions.checkArgument(instance == cached.payload, "Releasing the wrong instance");
Preconditions.checkState(cached.refcount > 0, "Refcount has already reached zero");
cached.refcount--;
if (cached.refcount == 0) {
Preconditions.checkState(cached.destroyTask == null, "Destroy task already scheduled");
// Schedule a delayed task to destroy the resource.
if (destroyer == null) {
destroyer = destroyerFactory.createScheduledExecutor();
}
cached.destroyTask = destroyer.schedule(new Runnable() {
@Override public void run() {
synchronized (SharedResourceHolder.this) {
// Refcount may have gone up since the task was scheduled. Re-check it.
if (cached.refcount == 0) {
resource.close(instance);
instances.remove(resource);
if (instances.isEmpty()) {
destroyer.shutdown();
destroyer = null;
}
}
}
}
}, DESTROY_DELAY_SECONDS, TimeUnit.SECONDS);
}
// Always returning null
return null;
}
/**
* Defines a resource, and the way to create and destroy instances of it.
*/
public interface Resource {
/**
* Create a new instance of the resource.
*/
T create();
/**
* Destroy the given instance.
*/
void close(T instance);
}
interface ScheduledExecutorFactory {
ScheduledExecutorService createScheduledExecutor();
}
private static class Instance {
final Object payload;
int refcount;
ScheduledFuture destroyTask;
Instance(Object payload) {
this.payload = payload;
}
}
}