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/*******************************************************************************
* Copyright (c) 2012, 2016 IBM Corporation and others.
* All rights reserved. This program and the accompanying materials
* are made available under the terms of the Eclipse Public License v1.0
* which accompanies this distribution, and is available at
* http://www.eclipse.org/legal/epl-v10.html
*
* Contributors:
* IBM Corporation - initial API and implementation
*******************************************************************************/
package org.eclipse.osgi.internal.container;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.locks.ReentrantLock;
/*
* Implementation note: This class does not pool ReentrantLocks for the objects
* that are being locked. This means that if the same object is locked and unlocked
* over and over then new ReentrantLocks are created each time. This set should be
* used with care. If the same object is going to be locked/unlocked over and over then
* consider using a different locking strategy.
*
* Previous implementations of this class attempted to use a WeakHashMap to cache
* the locks, but this proved to be a flawed approach because of the unpredictable
* timing of garbage collection, particularly with autoboxed types (e.g. bundle
* long ids).
*/
public class LockSet<T> {
static final class LockHolder {
private final AtomicInteger useCount = new AtomicInteger(0);
private final ReentrantLock lock = new ReentrantLock();
int incrementUseCount() {
return useCount.incrementAndGet();
}
int decremementUseCount() {
return useCount.decrementAndGet();
}
boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
return !lock.isHeldByCurrentThread() && lock.tryLock(time, unit);
}
void unlock() {
lock.unlock();
}
}
private final Map<T, LockHolder> locks = new HashMap<>();
public boolean tryLock(T t, long time, TimeUnit unit) throws InterruptedException {
final boolean previousInterruption = Thread.interrupted();
try {
LockHolder lock;
synchronized (locks) {
lock = locks.get(t);
if (lock == null) {
lock = new LockHolder();
locks.put(t, lock);
}
lock.incrementUseCount();
}
// all interested threads have the lock object and the use count is the number of such threads
boolean acquired = false;
try {
acquired = lock.tryLock(time, unit);
return acquired;
} finally {
if (!acquired) {
synchronized (locks) {
// If, after failing to acquire the lock, no other thread is using the lock, discard it.
if (lock.decremementUseCount() == 0) {
locks.remove(t);
}
}
}
}
} finally {
if (previousInterruption) {
Thread.currentThread().interrupt();
}
}
}
public void unlock(T t) {
synchronized (locks) {
LockHolder lock = locks.get(t);
if (lock == null)
throw new IllegalStateException("No lock found."); //$NON-NLS-1$
lock.unlock();
// If, after unlocking, no other thread is using the lock, discard it.
if (lock.decremementUseCount() == 0) {
locks.remove(t);
}
}
}
}
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