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AspectJ tools most notably contains the AspectJ compiler (AJC). AJC applies aspects to Java classes during compilation, fully replacing Javac for plain Java classes and also compiling native AspectJ or annotation-based @AspectJ syntax. Furthermore, AJC can weave aspects into existing class files in a post-compile binary weaving step. This library is a superset of AspectJ weaver and hence also of AspectJ runtime.

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/*******************************************************************************
 * Copyright (c) 2004, 2005 IBM Corporation and others.
 *
 * This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License 2.0
 * which accompanies this distribution, and is available at
 * https://www.eclipse.org/legal/epl-2.0/
 *
 * SPDX-License-Identifier: EPL-2.0
 *
 * Contributors:
 *     IBM Corporation - initial API and implementation
 *******************************************************************************/
package org.eclipse.core.internal.registry;

/**
 * Monitor ensuring no more than one writer working concurrently.
 * Multiple readers are allowed to perform simultaneously.
 *
 * This class was borrowed from org.eclipse.jdt.internal.core.search.indexing.
 */
public class ReadWriteMonitor {

	/**
	 * <0 : writing (cannot go beyond -1, i.e one concurrent writer)
	 * =0 : idle
	 * >0 : reading (number of concurrent readers)
	 */
	private int status = 0;

	private Thread writeLockowner;

	/**
	 * Concurrent reading is allowed
	 * Blocking only when already writing.
	 */
	public synchronized void enterRead() {
		if (writeLockowner == Thread.currentThread())
			return;
		while (status < 0) {
			try {
				wait();
			} catch (InterruptedException e) {
				// ignore
			}
		}
		status++;
	}

	/**
	 * Only one writer at a time is allowed to perform
	 * Blocking only when already writing or reading.
	 */
	public synchronized void enterWrite() {
		if (writeLockowner != Thread.currentThread()) {
			while (status != 0) {
				try {
					wait();
				} catch (InterruptedException e) {
					// ignore
				}
			}
//			System.out.println(this + "lockowner:" + Thread.currentThread());
			writeLockowner = Thread.currentThread();
		}
		status--;
	}

	/**
	 * Only notify waiting writer(s) if last reader
	 */
	public synchronized void exitRead() {
		if (writeLockowner == Thread.currentThread())
			return;
		if (--status == 0)
			notifyAll();
	}

	/**
	 * When writing is over, all readers and possible
	 * writers are granted permission to restart concurrently
	 */
	public synchronized void exitWrite() {
		if (writeLockowner != Thread.currentThread())
			throw new IllegalStateException("Current owner is " + writeLockowner); //$NON-NLS-1$
		if (++status == 0) {
			//			System.out.println(this + "exitWrite:" + Thread.currentThread());
			writeLockowner = null;
			notifyAll();
		}
	}

	@Override
	public String toString() {
		StringBuilder buffer = new StringBuilder();
		buffer.append(this.hashCode());
		if (status == 0) {
			buffer.append("Monitor idle "); //$NON-NLS-1$
		} else if (status < 0) {
			buffer.append("Monitor writing "); //$NON-NLS-1$
		} else if (status > 0) {
			buffer.append("Monitor reading "); //$NON-NLS-1$
		}
		buffer.append("(status = "); //$NON-NLS-1$
		buffer.append(this.status);
		buffer.append(")"); //$NON-NLS-1$
		return buffer.toString();
	}
}