DatabaseRef.java

/*******************************************************************************
 * Copyright (c) 2015, 2016 Google, Inc 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:
 *   Stefan Xenos (Google) - Initial implementation
 *******************************************************************************/
package org.eclipse.jdt.internal.core.nd;

import java.util.function.Supplier;

/**
 * Holds a reference to a database entity that may be retained across read locks. In normal circumstances, it
 * is unsafe to retain a database address after a read lock is released since the object pointed to at that
 * address may have been deleted in the meantime. This class addresses this problem by remembering both the
 * address itself and enough information to determine whether that address is invalid and search for an
 * equivalent object if the original is lost.
 */
public class DatabaseRef<T extends NdNode> implements Supplier<T> {
	private final Nd nd;
	private T lastResult;
	private long writeCounter;
	private final Supplier<T> searchFunction;

	/**
	 * Constructs a new {@link DatabaseRef} that will search for its target using the given search function.
	 */
	public DatabaseRef(Nd nd, Supplier<T> searchFunction) {
		this.nd = nd;
		this.searchFunction = searchFunction;
		this.writeCounter = -1;
	}

	/**
	 * Constructs a new {@link DatabaseRef} that will search for its target using the given search function.
	 */
	public DatabaseRef(Nd nd, Supplier<T> searchFunction, T initialResult) {
		this.nd = nd;
		this.searchFunction = searchFunction;
		this.lastResult = initialResult;
		this.writeCounter = this.nd.getWriteNumber();
	}

	/**
	 * Returns the referenced object or null if the object is no longer present in the database.
	 */
	public T get() {
		long ndWriteNumber = this.nd.getWriteNumber();
		if (this.writeCounter == ndWriteNumber) {
			return this.lastResult;
		}

		T result = this.searchFunction.get();
		this.writeCounter = ndWriteNumber;
		this.lastResult = result;
		return result;
	}

	public Nd getNd() {
		return this.nd;
	}

	/**
	 * Acquires a read lock. Callers must invoke close() on the result when done.
	 */
	public IReader lock() {
		return this.nd.acquireReadLock();
	}
}