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package com.sun.faces.util;
import java.util.concurrent.Callable;
import java.util.concurrent.CancellationException;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.FutureTask;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.faces.FacesException;
/**
* This class implements an abstract ConcurrentCache with objects in the cache potentially expiring.
* Only non-expired objects will be returned from the cache or considered to be contained in the cache
* The cache is self-managing, so no remove() method is defined
*/
public final class ExpiringConcurrentCache<K, V> extends ConcurrentCache<K, V> {
/**
* Interface for checking whether a cached object expired
*/
public interface ExpiryChecker<K, V>{
/**
* Checks whether a cached object expired
* @param key cache key
* @param value cached value
* @return true if the value expired and should be removed from the cache, false otherwise
*/
public boolean isExpired(K key, V value);
}
/**
* Public constructor.
* @param f used to create new instances of objects that are not already available
* @param checker used to check whether an object in the cache has expired
*/
public ExpiringConcurrentCache(Factory<K, V> f, ExpiryChecker<K, V> checker) {
super(f);
_checker = checker;
}
@Override
public V get(final K key) throws ExecutionException {
// This method uses a design pattern from "Java concurrency in practice".
// The pattern ensures that only one thread gets to create an object missing in the cache,
// while the all the other threads tring to get it are waiting
while (true) {
boolean newlyCached = false;
Future<V> f = _cache.get(key);
if (f == null) {
Callable<V> callable = new Callable<V>() {
@Override
public V call() throws Exception {
return getFactory().newInstance(key);
}
};
FutureTask<V> ft = new FutureTask<>(callable);
// here is the real beauty of the concurrent utilities.
// 1. putIfAbsent() is atomic
// 2. putIfAbsent() will return the value already associated
// with the specified key
// So, if multiple threads make it to this point
// they will all be calling f.get() on the same
// FutureTask instance, so this guarantees that the instances
// that the invoked Callable will return will be created once
f = _cache.putIfAbsent(key, ft);
if (f == null) {
f = ft;
ft.run();
newlyCached = true;
}
}
try {
V obj = f.get();
if (!newlyCached && _getExpiryChecker().isExpired(key, obj)) {
// Note that we are using both key and value in remove() call to ensure
// that we are not removing the Future added after expiry check by a different thread
_cache.remove(key, f);
}
else {
return obj;
}
} catch (CancellationException ce) {
if (_LOGGER.isLoggable(Level.SEVERE)) {
_LOGGER.log(Level.SEVERE,
ce.toString(),
ce);
}
_cache.remove(key, f);
} catch (ExecutionException ee) {
_cache.remove(key, f);
throw ee;
} catch (InterruptedException ie) {
throw new FacesException(ie);
}
}
}
@Override
public boolean containsKey(final K key) {
Future<V> f = _cache.get(key);
if (f != null && f.isDone() && !f.isCancelled()) {
try {
// Call get() with a 0 timeout to avoid any wait
V obj = f.get(0, TimeUnit.MILLISECONDS);
if (_getExpiryChecker().isExpired(key, obj)) {
// Note that we are using both key and value in remove() call to ensure
// that we are not removing the Future added after expiry check by a different thread
_cache.remove(key, f);
} else {
return true;
}
} catch (TimeoutException | ExecutionException ce) {
} catch (CancellationException ce) {
if (_LOGGER.isLoggable(Level.SEVERE)) {
_LOGGER.log(Level.SEVERE, ce.toString(), ce);
}
} catch (InterruptedException ie) {
throw new FacesException(ie);
}
}
return false;
}
private ExpiryChecker<K, V> _getExpiryChecker() {
return _checker;
}
private final ExpiryChecker<K, V> _checker;
private final ConcurrentMap<K, Future<V>> _cache = new ConcurrentHashMap<>();
private static final Logger _LOGGER = FacesLogger.UTIL.getLogger();
}