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This library contains all cryptographic functions that are used by Cryptomator.

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package org.cryptomator.cryptolib.common;

import java.lang.ref.WeakReference;
import java.util.Queue;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.function.Supplier;

/**
 * A simple object pool for resources that are expensive to create but are needed frequently.
 * 
 * Example Usage:
 * 
{@code
 *     Supplier fooFactory = () -> new Foo();
 *     ObjectPool fooPool = new ObjectPool(fooFactory);
 *     try (ObjectPool.Lease lease = fooPool.get()) { // attempts to get a pooled Foo or invokes factory
 *         lease.get().foo(); // exclusively use Foo instance
 *     } // releases instance back to the pool when done
 * }
 *
 * @param  Type of the pooled objects
 */
public class ObjectPool {

	private final Queue> returnedInstances;
	private final Supplier factory;

	public ObjectPool(Supplier factory) {
		this.returnedInstances = new ConcurrentLinkedQueue<>();
		this.factory = factory;
	}

	public Lease get() {
		WeakReference ref;
		while ((ref = returnedInstances.poll()) != null) {
			T cached = ref.get();
			if (cached != null) {
				return new Lease<>(this, cached);
			}
		}
		return new Lease<>(this, factory.get());
	}

	/**
	 * A holder for resource leased from an {@link ObjectPool}.
	 * This is basically an {@link AutoCloseable autocloseable} {@link Supplier} that is intended to be used
	 * via try-with-resource blocks.
	 *
	 * @param  Type of the leased instance
	 */
	public static class Lease implements AutoCloseable, Supplier {

		private final ObjectPool pool;
		private T obj;

		private Lease(ObjectPool pool, T obj) {
			this.pool = pool;
			this.obj = obj;
		}

		public T get() {
			return obj;
		}

		@Override
		public void close() {
			pool.returnedInstances.add(new WeakReference<>(obj));
			obj = null;
		}
	}

}